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IR ANID SEDIMENTS. ;]'"
BOR, ST. CROIXM-;
AN U' SMU I N IT
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GOVERNMENT OF THE VIRGIN ISLANDS
DEPARTMENT OF HEALTH, DIVISION OF ENVIRONMENTAL HEALTH
WATER POLLUTION REPORT NO. 16
CARIBBEAN RESEARCH INSTITUTE
COLLEGE OF THE VIRGIN ISLANDS
ENVIRONMENT, WATER AND SEDIMENTS
CHRISTIANSTED HARBOR, ST. CROIX
,kTHE RALPH M. PAIEWONSKY LIBRARy
UNIVERSITY OF THE VIRGIN ISLANDS
TABLE OF CONTENTS i
LIST OF FIGURES ii
LIST OF TABLES v
P-I Conclusions P-1
P-II Recommendations P-7
P-III Future Research Needs and Studies for Students P-12
Statement of the Problem 1-1
Organizations and Objectives 1-3
Scope and Previous Studies 1-4
2. SUMMARY OF PROCEDURES
Field Methods 2-1
Station Locations 2-1
Sample Collection and Measurement 2-4
Laboratory Procedures 2-7
3. DESCRIPTION OF THE HARBOR
Aerial Photographic Studies 3-7
Shoreline Features 3-15
Tide Character 4-3
Seasonal Variations 4-4
Tidal Exchange 4-6
Circulati on 4-6
Dye Dispersal and Photography 4-10
Inflow from Land 4-17
5. WATER QUALITY DATA
Suspended Solids 5-1
Secchi Disk Depth 5-3
Dissolved Oxygen 5-3
Summary of Water Quality 5-5
6. ECOLOGY OF REEF COMMUNITIES
Prospective Stresses 6-2
Status of the Reefs 6-11
7. NOTES ON FISHERIES 7-1
8. BOTTOM GRASS AND ALGAE 8-1
9. BOTTOM SEDIMENTS
General Lithology 9-1
Grain Size Distribution 9-3
Cumulative Curves 9-5
Sand Composition 9-9
10. SEDIMENTARY ENVIRONMENTS
Sedimentation and Depth Changes 10-6
11. UTILIZATION OF THE HARBOR AND ITS ENVIRONMENTAL
12. REFERENCES CITED 12-1
APPENDIX I, ANCHOR STATION HYDROGRAPHIC DATA
Explanation of Data A-i
Hydrographic Data A-3
APPENDIX II, FIELD SEDIMENT DATA
APPENDIX III, PARTICLES SIZE PARAMETERS A-33
APPENDIX IV, CHARACTERISTICS OF SAND COMPONENTS A-34
APPENDIX V, PERCENTAGE COMPOSITION OF SAND COMPONENTS A-38
APPENDIX VI, WATER QUALITY DATA FROM CHRISTIANSTED
LIST OF FIGURES
1. Location of hydrographic stations. 2-2
2. Location of bottom sediment collecting stations. 2-3
3. Bathymetry of Christiansted Harbor. 3-4
4. Aerial Photograph of Christiansted Harbor, 1954. 3-9
5. Aerial Photograph of Christiansted Harbor, 1971. 3-13
6. Photographs of shoreline conditions, A,B,C,D. 3-17
7. Variations in the character of the tide displayed in time-height
8. Monthly change in mean sea level at Havanna for an annual
9. Vertical distribution of mean current speed in meters per
second at selected anchor stations. 4-5
10. Mean surface current, speed and direction, for harbor
currents and for littoral currents, August, 1971. 4-8
11. Circulation from dye patch dispersal at different release
points and various dates. 4-12
12. Suspended solids mean concentrations in surface water. 5-2
13. Secchi Disk depth. Mean values, May through August,
14. Index to coral reef observations. 6-6
15. Mounds built-up by burrowing shrimp or polychaete worms. 8-2
16. Distribution of turtle grass beds in 1954 and 1971. 8-4
17. Gross lithology of principal sediment types on the harbor
18. Distribution of median grain size. 9-4
19. Size distribution diagram relating to different sediment
20. Representative cumulative curves of grain size. 9-7
21. Relation of percent coral sand to Halimeda sand in differ-
environments of the harbor. 9-11
22. Distribution of sand components in percent by number. 9-12
23. Distribution of grey and black colored carbonate sand grains
in percent by number in samples throughout Christiansted
24. Core sections showing sediment variations with depth for
stations 64 (left), 18 and 78 (right). 9-19
25. View northeast from west Christiansted to Protestant Cay
showing sandy dredge spoil in foreground a source of tur-
bidity when acted on by waves. 9-19
26. Major sedimentary environments of deposition; Reef, Central
Harbor, Inner Harbor, Lagoon, and Gallows Bay, an embay-
ment of the inner harbor. 10-2
27. Photomicrographs of sand components from different sedi-
mentary environments in Christiansted Harbor. 10-5
28. Distribution of depth changes and landfill between 1924
and 1971. 10-7
29. Graphical Summary of the status of harbor activities and
shoreline use, 1971. 11-2
30A. View of eastern harbor from Protestant Cay to the Buccaneer
30B.Eastern harbor shoreline viewed toward Fort Louise Augusta
from Chandlers Marine. 11-5
31A.Air view of shoreline use and harbor conditions at the
cement plant. 11-7
3 IB.Air view of crowded waterfront at downtown Christiansted. 11-7
32. Areas of environmental sensitivity and problems, Christ-
iansted Harbor, 1971. 11-14
LIST OF TABLES
1. Anchor station observations and corresponding instruments
2. Summary of geographic and hydrographic data. 3-2
3. Distribution of area in depth intervals. 3-2
4. List of available charts. 3-6
5. Sources of aerial photo coverage. 3-7
6. Longshore transport data. 4-14
7. Coverage of grass beds between 1954 and 1971. 8-6
8. Summary of sand fraction composition in different environ-
ments of the harbor. 9-17
9. List of dredging permits. 11-4
10. Intercompatibility of multiple uses, Christiansted Harbor. 11-11
1. The harbor floor is shaped into broad shoals cut by a
deep winding channel in the eastern sector. The shoals are built
mainly of coral sand supplied from the barrier reef. The chan-
nel initially was cut during a period of lower sea level and its
course partly follows the local geologic structure. Presently,
erosion is deepening the channel floor near Barracuda Ground
whereas sedimentation is filling the western channel shoulder,
particularly near Sorensen Ground. Elsewhere the floor is rela-
2. Most harbor shores are stable. Attractiveness of beaches
is reduced by accumulations of marine grasses, by ledges of beach-
rock, abandoned boats, and by rubble washed from dredge fill.
Erosion is locally active at Turquoise Beach and on shores of
3. Water quality varies from excellent in the western harbor
and seaward reaches to poor in the eastern and inner parts. Poor
water quality is indicated by high turbidity and locally high fecal
coliforms. Suspended solids increase with distance toward the
shoreline source; from less then 2 mg/l near Long Reef to more than
10 mg/l near Christiansted and Gallows Bay. Greatest change occurs
in a "gradient" zone trending eastward from the Cement Plant to
Protestant Cay and Round Reef. High turbidity is traced to high
content of suspended material partly introduced by occasional
sewage and sediment discharge, but mainly generated by wave ero-
sion of unstabilized dredge spoil.
4. Spatially, water temperature, salinity and dissolved
oxygen vary within narrow limits. Spasmotic runoff from the
land and occasional "northerners" may alter the hydrographic
regime at times, but relatively uniform conditions prevail
most of the time. The salinity of harbor water is close to
ocean water and its uniform distribution at depth indicates
the water is well-mixed by wind and currents.
5. Harbor circulation is driven by the mass transport of )
ocean waves breaking on Long Reef. Water enters the harbor
through shallow channels in the reef. It flows eastward
through the harbor proper in an anti-clockwise direction with
an average speed of 0.07 m/sec (0.14 knots) and leaves the
harbor through the main entrance channel. The circulation is
regulated by the reef elevation, height of ocean waves and
height of the tide. The influence of offshore currents is
insignificant but local wind driven currents within the harbor
are important at times.
6. Although tidal exchange is insignificant, waters are
rapidly flushed and mixed by the over-reef flow supplemented
by wave motion. Mixing is greater in the western harbor, es-
pecially in seaward reaches and over shoals, than in inner
reaches and deeper parts. Thus, transport and dispersal of
materials, wastes and sediment is better in near-ocean areas
than near-shore or on the channel floors.
7. Bottom sediments are predominately coral sand with
admixtures of mud and Halimeda sand occurring in eastern and
inner reaches. The coral sand is derived from the barrier
reef whereas Halimeda was formerly produced on the inner
harbor floor. Supply of detrital sand from the land is rela-
8. Five sedimentary environments are recognized, each charac-
terized by distinct grain size characteristics, sand composition
and physical properties indicative of environmental quality.
These environments are: (1) reef, (2) central harbor, (3) inner
harbor, (4) lagoon, (5) Gallows Bay, an emerging new environment
of the inner harbor. Sediments are mainly transported landward
over the barrier reef by mass transport of breaking waves. Addi-
tionally, they are carried eastward toward the main harbor chan-
nel and westward of Scotch Bank by the predominate flow.
9. Several biological zones can be identified, the most
important of which are:
.. the fore reef, dominated by Acropora palmata, lush and
productive to the west but diminishing toward the east.
.. The shallow back reef area composed primarily of Thalassia,
Syringodium, and associated algae.
.. The inner reaches of the bay proper dominated now by
filamentous Enteromorpha, an indicator of eutrophic con-
ditions, with Thalassia only in shallower water where it
can receive sufficient light.
.. The nearshore, with plant cover varying with localized
conditions from Thalassia in the west to Enteromorpha and
Ulva in the center and east.
.. Large bare sand areas of the bay bottom produced by dred-
ging and natural sand shifting which prevents colonization
but supports large numbers of tube-building worms.
10. Reef communities of Long Reef are dominated by Acropora
palmata on the front-reef between 0-10 m depth, and by Porites
porites and small Acropora colonies on the back reef. The octo-
coral population has fewer species and lower density than in many
other Virgin Islands reefs. Long Reef is in relatively good
"health" except for the immediate sewage outfall environs and
Barracuda Ground which is under the direct stress of wave attack
and indirect stress of harbor dredging and pollution.
11. The area around the Long Reef sewer outfall displays
high concentrations of fecal material and paper wastes in addi-
tion to numerous dead corals and reduced species diversity. The
bulk of the sewage discharge is dispersed outside of the reef and
transported away from the harbor in the prevailing currents.
During the several months of observations only a very small frac-
tion was transported over the reef into the harbor. However, with
certain combinations of wind and sea directions, this general
situation is temporarily changed so that a much larger fraction
may be swept back into the harbor.
12. An estimated 46 percent of the harbor floor has been
directly or indirectly disturbed by dredging.
13. The environment shows signs of faltering under the com-
bined stress of pollution and utilization. The chief changes
since 1954 are:
.. A lowering of the harbor floor by dredging with a conco-
mitant increase of harbor volume by 14 percent, restric-
ted circulation of bottom water and locally increased sedi-
mentation. The stagnation effected is a stronger degrading
influence on quality of the deeper waters than the benefi-
cial effect of increased dilution which results from the
greater water volume.
.. A 34 percent loss of natural shoreline by conversion to
land, artificial beaches, or waterfront resulting in a 5
percent reduction of harbor area, restricted flushing of
Altona Lagoon and loss of natural habitats.
.. A 65 percent loss of the productive Thalassia-Syringodium
grass beds and invasion of the alga Enteromorpha.
.. Destruction of reef communities on Round Reef, Barracuda
Ground, north of Protestant Cay and about the Long Reef
.. Loss of sand by erosion along 20 percent of the shoreline.
.Loss of seasonal shellfish beds in the central and eastern
14. Pollution and disturbance of the inner and eastern harbor is by
.. Low water transparency with secchi disk depths reduced to 1 m
and high turbidity with suspended solids reaching 25 mg/l.
.. Almost complete extirpation of coral on Round Reef and the
death of reefs north and east of Protestant Cay.
.. Thick growths of benthic alga Enteromorpha which cover 41 per-
cent of the harbor floor.
.. Lush growths of Enteromorpha and Ulva at many places along
the shoreline in shallow water areas of Gallows Bay and the
central harbor. Growth is promoted by "pirate" waste/sewage
discharges and from the occasional discharge from the sewage
system bypass system.
.. Widespread contaminants in bottom sediments including small
percentages of poisonous copper paint chips, plastic and
paper packing fibers, vermiculite, coal and cinder particles,
glass fragments and metal shavings.
.. Accumulations of soft, poorly sorted organic-rich mud (Type
E) containing organic material in deeper areas of the harbor.
.. An abundance of grey-black carbonate sand grains, a product
of dredging older sediment.
.. Oil and asphalt spills and leechings along the western shore-
line of Gallows Bay.
15. Coastal shipping places the greatest demand on the harbor
environment. Its impact has produced a sequence of stresses:
dredging-filling- ecological destruction-shoreline erosion-
turbidity-sedimentation. Although each stress is relatively small,
successive stresses carry with them a potential for cumulative
effects with time, and for extending their effects throughout the
harbor via the active circulation.
16. Problems in the harbor are mainly produced by intense demands
concentrated in small areas and by disturbance of ecologically
sensitive areas. The prospects are that conflicting demands and
incompatible use will intensify in the future.
17. The harbor could be improved by holding pollution and
disturbance to a low level locally, by intelligent planning for
conservation areas in an effort to achieve an optimal balance of
environmental quality and utilization in a context of multiple use.
1. Waste discharges and bottom and shoreline alterations,
mainly dredging, must be held to a minimum within small areas
taking into account the circulation pattern and rates of mixing
that extend their effects. To minimize further deterioration of
water quality and ecological regression, correct and effective
regulations must be formulated for the various users of the har-
bor. A form of marine zoning is required in which areas are
designated for multiple use according to their intrinsic natural
A. For the central and western harbor, in the area
bounded by Long Reef, a north-south line from Barracuda Ground
to Protestant Cay to the Christiansted shoreline, and along the
shoreline westerly and north-westerly to Pelican Cove at the
junction with Round Reef, we recommend
... No further sand mining or bottom or shoreline alter-
ations be allowed for at least ten years after which
time a reassessment can be made.
... Industrial and commercial shipping traffic be cur-
tailed. Location of the cement plant/utility plant
complex at this site has proven to be extensively
damaging to the harbor and attempts at restoration
... Gradual phasing out over the next 2-3 years of opera-
tions at the cement plants so that eventually the
only shipping or industrial utilization of this sector
of the harbor will be that required for fuel transport
to the utility plant.
... Elimination of waste discharges on to Long Reef over
the next 2-5 years.
... If the existing outfall is to be kept as an emergency
bypass in the event of lift station failure, the out-
fall should be lengthened at least 200 meters.
... Limitation of the electric power generating and water
production capacities at this site to those already
installed or under contract. Continued expansion of
the utility capacities will negate the beneficial
effects to be derived from elimination of the cement
... Elimination of "pirate" waste discharges from water-
front businesses, hotels and restaurants, and better
control of operation of pumping stations so that
frequency of sewage bypass discharges is reduced.
... Revision of section 186-9, Legal Limits of the Water
Quality Standards for Coastal Waters of the Virgin
Islands to transfer central sector (Protestant Cay
to Golden Rock) from class "C" (Harbors) classifica-
tion to class "B" (Propogation of Marine Life and
Water Contact Recreation).
... Further reduction of the harbor area by landfill not
B. For the eastern harbor, including Gallows Bay and the
entrance channel, we recommend
... Very close supervision and control be exercised over
all bottom and shoreline alterations including sand
mining. These should be allowed only after adequate
expert review and study specifies that project con-
figuration and the control procedures which insure
minimum lasting degradation beyond the present con-
dition of the area.
... Any expansion of waterfront industrial, commercial,
or shipping activity in the Christiansted area be
allowed only in this eastern harbor sector.
... Cleanup of the eastern shoreline of Protestant Cay
and close supervision of the effluent discharges
from the Cay. Equipment and waste lines design,
including stand-by power, should be specified so that
effluents are not ever discharged into the harbor.
... Continuing development and future utilization of
Altona Lagoon be carefully controlled to the end
that there is no restriction of water flow through
... The present entrance to Altona Lagoon be reopened,
either by addition of 8-10 large culverts or by low
level bridging, which could allow skiff and outboard
motor boat passage into the Lagoon.
... No reduction of the harbor area be allowed. Addi-
tional small boat services, bulkheading and dockage,
should be made by cutting into the harbor shoreline
more than filling out from the existing shoreline.
2. Areas of unique value and high environmental sensitivity
should be designated as marine preserve or conservation areas.
They can alternatively support recreation or serve as open space
for residents and visitors because of esthetic appeal. Long Reef
acts as a baffle to ocean waves; it is a potential nursery ground
or spawning area and it supplies sand to the harbor.
A. For Long Reef from Barracuda Ground to Pelican Cove
... Designation of Long Reef as territorial marine preserve
... Section 186.9, Legal Limits of the Water Quality Standards
of the Virgin Islands be revised to add Long Reef to Class A
B. An opportunity exists for expanded recreational use in the
central and western harbor areas, after upgrading of the water quality
classification of the central harbor to Class B (Propogation of
Marine Life and Water Contact Recreation). We recommend promotion
of shoreline resorts, public beaches and water sports uses, outside
of the seadrome and channel to the dock at the cement plants/utility
C. For Altona Lagoon, after widening of the access water way,
we recommend full development of the recreational potential of the
area boating, fishing, picnicking, minor sports.
3. In the central harbor, much damage to the benthic ecology
and degradation of water quality has occurred. Investigation must
be made into possible restorative action. We recommend specifically
stabilization of actively eroding shorelines and submarine slopes
by planting with both terrestrial and marine grasses. The restoration
costs should be charged to users of the channels and waterways as
a necessary return payment to the harbor for the special use extracted.
4. To provide for the many diverse and conflicting future
demands on the harbor, both public and private, and still obtain
maximum long-term socio-economic benefits we recommend that a
comprehensive plan of development be formulated and implemented
through an appropriate agency of the Government of the Virgin
Islands. A set of planning objectives should be established,
data obtained regarding past and present utilization of the har-
bor, ecological values, etc., and these evaluated in alternative
multiple use plans.
5. Our limit of detection in estimating transport of sewage
effluent discharged from the outfall seaward of Long Reef was
grossly insensitive. To properly measure the effluent fractions
transported back over Long Reef into the harbor, we recommend
that a series of dye studies be conducted, under different con-
ditions of wind and sea, using a sensitive flourimeter to detect
the dispersal of the flourescein dye.
P-III FUTURE RESEARCH NEEDS AND STUDIES FOR STUDENTS
Results of this study have raised many questions, exposed
gaps in our knowledge that require further research. The harbor
environment must be further understood to achieve optimal quality
for future human use. The needs are:*
1. Monitoring of basic physical and chemical parameters
of the environment.!tide, salinity, temperature, dissolved oxygen
content, wind and stream discharge at several stations situated
in environmentally sensitive areas. These data should provide a
record of daily and seasonal changes that indicate the effects
of ecological disturbance and extreme events, stream flooding
2. An inventory of the harbor, its water and ecology,
during the winter season. Attention needs to be given to dissolved
oxygen content, plankton and pollutants in near-bottom water. The
mean circulation pattern and rate of flushing which predict the
fate of pollutants and sediment, needs to be defined especially
during extremes of flooding and northerners. Detailed field
studies are required to determine the character and rate of
over-reef flow as it relates to the reef elevation and tidal
heights as well as the harbor circulation and its capacity for
waste receival. Hydraulic model studies should be of use in
predicting effects of future engineering works, channel deepening
or alignment on circulation, salinity and sedimentation, as well
as on the ecology which depends on these factors. Because tides
are weak, an analysis of the effects of wind on harbor circulation
and mixing is of interest.
*The numerical order given does not imply priority.
3. An analysis of wave energy dissipation as it affects
(1) destruction of reefs and (2) sheltered anchorages in the re-
sponse of boats moored at anchor and at loading docks, including
the conditions of high swell during tropical storms and harbor
oscillations induced by mass transport of wave groups breaking
on the barrier reef.
4. An understanding of energy flow through the harbor and
effects of prospective changes that tie, or separate, the system
such as restricted circulation of bottom water and lagoon water.
Productivity measurements are needed in different environments of
the harbor and through a range of sun energy levels to prepare
an annual budget of organic matter and to integrate effects of
different processes throughout the harbor.
5. A knowledge of the distribution and ecology of fishes.
It is of special interest to know if the reef, lagoon or harbor
floor are spawning areas or nursery grounds important to the life
cycle of certain species.
6. A detailed knowledge of the composition and distribution
of the bottom fauna, flora and reef communities. There is a cri-
tical need for quantitative data concerning specific levels and
specific kinds of disturbance and pollution effects on the communi-
ties. We need to know the extent and degree of impact, its "side"
effects and "after" effects. How fast are communities eliminated
around outfalls or reestablished on dredged slopes and excavations?
And what is their capacity to tolerate stress? The long history
of dredging in the harbor provides an excellent opportunity for a
comparative study of populations in various stages of recovery.
Any quantitative information on adaptations and species sub-
stitutions also are of special interest, in addition to the
variety and mass of organisms that are supported or eliminated.
7. Land-borne sediment and pollutant sources have been
identified but the fate of these materials in the bottom sediments
is poorly understood. The possible cleansing action of burrowing
shrimp or polychaete worms should be investigated. The chemical
sequences involving trace metals and "sub-lethal" compounds in
the sediments require study, particularly as they interact with
early diagenetic changes. The probably diagenetic formation of
grey-black carbonate grains with increasing age and depth of
burial in the sediments is of considerable interest.
8. Although the modern sedimentary environments and rates
of deposition are partly known, their relation to older deposits
needs to be established through coring or drilling. Little is
known of the recent geologic history of harbor deposits as in-
fluenced by changing conditions during the post-glacial rise in
9. An understanding of how to achieve an optimal balance
of uses in the harbor. A basic inventory of harbor and shoreline
activities is urgently needed in addition to quantification of
esthetic (e.g. scenic) and ecological values. Possible archaeologic
and historic sites on the harbor floor and shoreline should be
identified. Intelligent management of the harbor will require at
least thorough evaluation of multiple socio-economic uses and
resource values to ensure optimum use and enhancement of the en-
ENVIRONMENT, WATER AND SEDIMENTS OF CHRISTIANSTED HARBOR, ST. CROIX
With t h e emerging demand for environmental quality, there is a
need to gain a new knowledge of the environment in which we work and play. Never
before has the public been so conscious of the necessity for protecting the quality
of water, for managing resources and for maintaining the balance for nature within
the limits of progress. The realization is growing that the environment is dynamic,
ever changing and interacting with forces imposed by man or nature. It is now recog-
nized for instance that pollution of coastal waters has a potential for extending its
effects over wide areas and for concentrating its products with time. Even engineering
works though small, often carry with them many side effects which permeate an entire
coastal environment and produce major, or at times catastrophic, changes in the natural
balance. Few more illustrative examples of such changes exist in a compact setting,
than in Christiansted Harbor.
Statement of the Problem
At Christiansted, where development has been historically linked to the
harbor, the shoreline and water have been put to a variety of different but inter-re-
lated uses. Renown as a sheltered anchorage, its waters serve as an airdrome for
amphibious planes and its channels as a transfer medium for coastal shipping and
recreational boating. The harbor floor is a local source of sand for landfill and
construction aggregate and its offshore reaches are a source of food. Additionally,
the harbor is a place in which to dump wastes and to disperse thermal effluents.
By contrast, its shores provide recreational beaches and sites for boat repair
and berthing. Not only does the harbor serve a variety of demands, but it pos-
sesses esthetic qualities and scenic values important both to tourists and to local
These diverse and often conflicting demands have exposed a multitude
of problems, some natural, others man-made or created by man interfacing
with nature. Beaches are reportedly eroding and parts of the coral reef destroyed
by violent winter storms. Sedimentation is smothering productive grass beds and
filling the shipping channel. Man has added to the complexity of these problems
through his efforts to excavate sand, to dredge shipping channels, and to bridge
contiguous waters like Altona Lagoon. All such activities have disrupted harbor
ecology and reduced the harbor's self-flushing capacity. Most striking is the
continual discharge of untreated sewage off the coral reef that protects the harbor.
As problems of development mount with growing industrial activity and an increasing
flow of tourists, new stresses are placed upon the harbor.
If we are to keep pace with the necessity to resolve the problems arising
from intensive use in addition to the emerging demand for environmental quality,
there is a need to gain new knowledge of the harbor environment. We need to learn
the nature of the place. What is the quality of the water and the character of the sedi-
mentary materials ? What processes are active in circulating materials and wastes ?
What is the "health" of the reef communities? Where are the ecologically sensitive
areas ? What changes have occurred in response to stresses of man and nature ? And
further, what problems require attention to improve the environment? Where are the
limitations to development --- the opportunities for use ?
Organization and Objectives
To address the questions posed by the harbor environment, and by custodians
concerned with its welfare, a study was brought to life through funds contracted from
the Division of Environmental Health, Department of Health, Government of the Virgin
Islands to the Caribbean Research Institute. A broad, but intensive research effort
was organized, objectives delineated and a team of scientists employed covering
different environmental disciplines. The team consisted of: R. vanEepoel, project
director; M. Nichols, principal investigator/oceanographer; R. Brody, reef ecologist;
D. Grigg, water chemist/marine ecologist; R. Crean and A. Sallenger, marine geo-
logists; and J. Olmon, marine biologist. Numerous summer students and laboratory
technicians were recruited for short-term assistance both in the field and in the labora-
tory. The team was directed to collect, chart and analyze all data on the harbor which
would aid in evaluation of the environment.
The broad objective of the study was to determine the status of environmental
conditions in the harbor. More specifically:
(1) To describe and inventory the "status" or distribution of the basic
environmental elements in the harbor, its bathymetry, sediments, water
(2) To evaluate these elements in an interacting system. Inasmuch as
the harbor environment is dynamic and ever-changing in response to waves,
currents, light, etc., an attempt was made to understand how the basic
elements respond to environmental processes operating on them.
(3) To interpret the environmental processes and responses in terms
of potential changes that may affect continued use of the harbor and main-
tenance of its environmental quality. Specific problems are delineated
and recommendations offered for their solution with priorities for future
Scope and Previous Studies
The present study is part of a longer term investigation of the environmental
quality and pollution in bays, harbors and insular waters of the Virgin Islands. Com-
pared to previous studies of the Caribbean Research Institute, it is more comprehensive
and utilized more scientific and technical expertise and more sophisticated equipment
than here-to-fore available; yet, it was mobilized within a relatively short time.
The approach taken was to consider the harbor as an ecological unit. Spatially, it is
limited to the harbor proper, the entrance reaches, protecting coral reef and the shore-
line from Beauregard Bay on the east to Pelican Cove on the west. Upland sources of
water and contiguous waters offshore and in Altona Lagoona are included as appropriate.
Temporally, the study is limited to the period May through October 1971, essentially
the summer season. Time available did not permit observations during the winter
season, when certain environmental processes are more active than or different from
those of summer. Because environmental conditions in the harbor are so variable
with time and place, the investigation did not cover all local changes or permit an
examination of the full range of conditions, including extremes important to the life
of organisms. Observations necessarily were made at selected stations and along
a number of tracks; future studies utilizing a greater sampling density would un-
doubtedly show even greater variation than reported here. Also, as a status
report it is merely one "snapshot" out of the long history of changing environmental
conditions, and though we have attempted to document these conditions, they
will change with time as the harbor becomes more extensively utilized and altered.
Thus, this report does not provide final answers for all local development problems.
Rather, it is a baseline study of harborwide conditions within the scope outlined above.
Such basic studies are required because so little is known about the effects of pollu-
tion and ecological disturbance in tropical coastal waters.
A major difficulty confronting the formulation of the study was the lack of
previous quantitative data. Such background data are needed to assess trends in
the present conditions as well as to measure their rates of change, and to record
their history so that future trends can be predicted. No previous data on water
quality came to our attention except for the two week study by Jacobssen (1951) for
the Virgin Islands Department of Health and the reconnaissance sampling by the
Federal Water Pollution Administration (1967). The Division of Environmental
Health, Virgin Islands Department of Health collects water samples monthly for
enumeration of coliform and streptococci indicators, and records temperature
and chloride ion concentration of the surface water at selected stations. The
most recent bathymetric charts of the harbor are those of the U. S. Coast and
Geodetic Survey which date to 1924. Tides were recorded last in 1924 for a
limited period of 4 months. No inventory of fisheries or benthic ecology has
ever been attempted, except for the broad island-wide survey of Dammann (1970).
Despite the available aerial photographic coverage of federal agencies obtained
every 5 to 7 years no effort has been made to extract ecologically significant
information. Morever, there are no long-term stations in the harbor which
monitor basic environmental parameters like wind, stream discharge, salinity,
tide and temperature. These data are of use to provide a multiple "look" from
season to season, before and after changes are effected, and to evaluate extreme
The authors are grateful to many individuals who gave freely of their time.
Students in training at the Fairleigh Dickinson West Indies Laboratory assisted with
long hours of anchor station measurements on the "Reef Sampler", skippered by
Don Neil, amid squalls, rolling and occasional sea-sickness: M. Dong, D. Malaspina,
D. Jenner, M. Perrone, S. Whipple, W. Marchion, E. Clark, N. Samuel, L. Firth,
D. Jenus, J. Duerbig and J. Mikulak. Additionally, Mr. John Yntema of the Virgin
Islands Department of Conservation assisted with many aspects of field work and data
analyses including compilation of dredging activities, fisheries notes and basic tidal
data. Mr. J. Larsen, of CRI, installed tide gages and obtained aerial photographs
of dye releases; C. Gellert of CRI processed photography and typed parts of the
original manuscript. R. Galiber assisted with field and laboratory activity, and
E. Shatrosky with the assistance of N. Carlson, Wayne Neal, and P. Lierhaus,
accomplished most of the laboratory water analyses. Kaye Stubblefield and June
Hogman drafted the figures. K. Fiederer and V. Hodges cheerfully accomplished
the tedious task of typing and retyping the draft and final text material.. Ideas
expressed in this paper were shaped in discussions with harbor masters Captains
Grey and Conway, and with Dr. Grey Multer, Dr. John Adams, and Dr. John
2. SUMMARY OF PROCEDURES
Most of the field work was done from a runabout or skiff; but to measure
currents,12 "anchor" stations were occupied with the "Reef Sampler" an 11-meter
sportfishing vessel. The broad character of the study, and requirements of dif-
ferent investigations necessitated separate trips and different types of field sam-
pling. Altogether, a total of 94 sediment samples were collected, 12 anchor sta-
tions were occupied, 15 stations were repetitively occupied for water quality de-
termination, 7000 meters of "track" line observations run on coral reefs and
18, 000 meters of sounding lines run throughout the harbor. Field work was
accomplished between May and October, 1971, but was concentrated during tho
6-week period July 21 to September 3.
Stations were positioned in the field by ranging on buoys and landmarks
or by pelorus bearings. Positions were plotted on a hydrographic chart or
aerial photograph. The location of hydrographic stations, including water qua-
lity, anchor and littoral current stations is presented in Figure 1, and the
location of sediment sampling stations is given in Figure 2. Coordinates and
water depths are tabulated in the respective appendices. Water quality sta-
tions were located on a basic grid at 300 m intervals,. Anchor stations were
located primarily to delineate the harbor-wide circulation whereas sediment
stations were located to include a range of different sediment types, and dif-
ferent morphologic reaches of varying water depth and varying rates of fill,
scour and disturbance.
Location of hydrographic stations for measurements of tide, water quality, and
current ("anchor stations"). Littoral current stations are identified by roman
numerals. Depth contour at 6 meters and dredged channel is dashed.
PELICAN 6m 6m
6 l' ",-, ,' *84 *57
62" --. ---J e8L -.
*64 60 V-8a *86 5
6 64 277 LONG REEF 8659
63 +64a 6/ --8 87 R 85 58
65 70 93 92 .42 600 2 V
15 60 b*
72 *7 *76 020 470 48*
o69 91o0 41 40
24 11,022 37 35 I *80 AL:
75 86- 36
9 5 --" 23 90 4
960 3 t
SCALE 0m STATIONS
0 0.5mi 0 SURFACE SAMPLE
Figure 2. Location of bottom sediment collecting stations and location of cores. B G is
Barracuda Ground; P C is Protestant Cay; R R is Round Reef.
Sample Collection and Measurement
Most surface sediment samples were collected by forcing a 5 cm
diameter plastic coring tube into the bottom by hand. The top 2 cm of near-
surface sediment was removed from each core and the resulting equal-area
and equal volume sample retained for analyses. In water deeper than about
7 m, mainly in channels, a "Ponar" grab was used. Areas inaccessible by
boat were sampled by skin diving and "grabbing" the upper few centimeters
of sediment. Cores were obtained manually by driving a 2-3 m length of 3.5
cm diameter PVC pipe into the bottom. Cores were cut lengthwise on a table
saw and split with piano wire, photographed while fresh and sampled at 10 cm
depth intervals. An explanation of field data is given in Appendix II.
Observations of water characteristics and current measurements were
made at a number of fixed points or "anchor" stations, August 2-4 and August
10-17, 1971. As planned, the anchor station observations encompassed a period
of relatively stable weather, spring tide range and diurnal phase of the tide.
Observations were repeated every 1 to 2 hours for 1 tidal cycle of 25 hours
and 1 to 4 depth intervals throughout the water column. Stations in the eastern
and central harbor were occupied simultaneously two at a time, e.g. numbers
4 and 9, 5 and 10, 6 and 11. Table I summarizes the observations made and
corresponding instruments used. Further details are given in Appendix I.
TABLE 1. Anchor Station Observations and Corresponding Instruments Used.
Oceanographic Engineering Corp. Savonius Rotor;
TSK Propeller Meter
Oceanographic Engineering Corp. Vane;TSK Recording
Beckman RS5-3 conductivity-temperature indicator;
Water samples pumped by a Western Brass "Blue
Cascade" plastic submersible pump and titrated in
the lab with silver nitrate
Beckman RS5-3 conductivity-temperature indicator
Secchi, white-black disk, 20 cm diameter
Water samples pumped by a Western Brass "Blue
Cascade" submersible pump and filtered in the lab
By sounding line, Ross model 300-100 and Rathyeon
DE 119 fathometers
Recording portable automatic tide gage of the USC & GS
Water quality measurements were made and samples collected from surface
waters at 15 stations (Figure 1) from May through September between the hours
of 1000 and 1400. Temperature and dissolved oxygen were measured either
with a Precision Scientific model 10 or a YSI model 54 galvanic oxygen meter
calibrated in water at each station. All D. O. values are corrected to ambient
water temperatures. Salinities were determined on water samples returned to
the laboratory by Mohr titration and suspended solids were analyzed gravimetri-
cally using membrane filters with a 0.45 u pore size. Fecal coliforms were
determined on undiluted water samples by the membrane filter technique (APHA,
For surveying the bathymetry several different methods were used
according to water depth and equipment available. In shallow water of the
western harbor (Turquoise Bay) and nearshore areas, water depths were
measured by lead line at horizontal intervals less than 10 meters and posi-
tioned by reference to features shown on aerial photographs. In water depths
greater than about one meter encompassing the greater part of the harbor floor,
portable fathometers were used from small boats and depths were sounded con-
tinuously along previously surveyed "tracks" or lines marked with range poles.
Positioning was done underway by repeated ranging on landmarks and bouys
and by pelorus cross bearings. Accuracy of the fixes is better than 3 meters.
A Ross model 300-100 was used in the eastern harbor, a Rathyeon DC-119 in
the central and eastern harbor and a Ross "Fineline" was used in deeper water
of the entrance. A depth change less than 0.3 meter could be detected most of
Track lines were plotted on USC and GS hydrographic chart (#935), and
depths transferred to the charts after correction for transducer draft and reduc-
tion to mean low water. The distribution of track lines is given in Figure 6.
Interpolation of contours around dredge holes and isolated reefs was facilitated
by use of a 1971 US-SCS air photo.
In addition to the forgoing sampling and observations special efforts were
made to obtain tide-controlled aerial photography of dye releases. Procedural
details of these related efforts are given in subsequent report sections where they
Grain size of selected sediment samples was determined by a com-
bination of sieving and pipette analysis following procedures of Folk (1961).
Approximately 15-25 gm of sample was washed with tap water through a 4
phi (0. 0625 mm) sieve. The mud fraction collected in a 1500 ml dish was
decanted after settling 24 hours, and subsequently poured into one liter
containers. It was further analysed by pipette according to Folk (1961)
at time intervals corresponding to 4, 5,6, 8 and 10 phi. The mud fraction
was processed wet without adding a dispersant whereas the sand fraction
was dried, sieved into Wentworth size fractions and weighted.
Results of the size analyses were plotted as cumulative curves on
probability paper using a logrithmic phi scale for diameter, where 0 is log2
of the diameter in millimeters. The particle diameters at the 16, 50 and 84
percentile values were obtained from the curves and used to derive size para-
meters based on the relations of Inman (1952):
Median diameter (Md 0) = 0 50
Standard deviation (d0)= 0 84 0 16
The composition of different constituents in the sand fraction were exa-
mined under a binocular microscope in two size classes: (1) greater than 250 u
and from 62 to 250 u. Several aliquots were spread out on a gridded petri dish
and at least 200 grains in each fraction were counted along the grid lines. Per-
centages of different constituents were calculated and their distributions plotted
on chartlets following Shepard and Moore (1954).
3. DESCRIPTION OF THE HARBOR
Along the north coast of St. Croix, the shoreline is indented by nu-
merous bays and fringed by submerged coral reefs that give a distinctive
character to this part of the island. Christiansted Harbor, the largest em-
bayment on the north coast, is broadly lunate-shaped in plan view. It lies
behind a practically continuous intertidal coral reef, Long Reef, and is backed
by hills alternating with stretches of alluvial lowland. A small alluvial delta
protrudes into the harbor near the cement plant (Golden Rock). This is in
contrast with the shoreline elsewhere along the southwest shore where erosion
over long periods of time appears to have smoothed the shore and created low
bluffs. On the east side, a rocky headland, Fort Louise Augusta, is tied to
the upland by broad sand beaches which in turn, separate Altona Lagoon from the
harbor and the ocean. A geologic map of the area by Whetten (1961) gives some
indication that faulting has been responsible for the morphology along the south
and east shore.
Christiansted Harbor is 3.30 km long and 0.70km wide, Table 2.
Its total surface area is 2.31 km (570 acres). The harbor receives drainage
from an area of 7.25 km2 in addition to 0.48 km2 from Altona Lagoon and
2.71 km2 surrounding the lagoon. Its maximum depth is 23 meters (75 feet),
but because its depths are very irregular and include many broad shoals, its
overall average depth is only 1.80 meters.
Summary of Geographic and Hydrographic Data
Av age width
Mean depth overall
Mean tide range
Tidal pri m
439, 000 m
2.71 sq. km
Distribution of areas at various depth intervals in Christiansted
Area, sq. km
Percent of Total
TabUlation of the distribution of harbor area in different depth "zones",
Table 3, indicates the largest fraction of the harbor floor, 37.8 percent,
lies in the depth range of 0 to 2 m, whereas deeper portions, greater than
8 m are limited to less than 6 percent. Thus, the harbor is essentially
a shallow pan and its floor is susceptible to wave action as well as distur-
bance by boats and sea planes.
Bathymetry of the harbor in its present state is presented in Figure 3.
The floor of the eastern harbor is molded into a deep winding channel that is
surrounded by submerged shoals and shallow reefs. The channel sides are
relatively steep; especially in deeper central parts whereas the shoals are
relatively flat and extend over a broad area of the western harbor. The main
channel has only a few short tributaries and these enter the main channel
nearly at right angles. This pattern, and the sharply winding entrance
course, a course that is notoriously difficult to navigate, suggests the initial
channel course was controlled by geologic structure, such as faults or large-
scale joints. However, only the north-south channel axes parallel known faults
on land as mapped by Whetten (1961) and no direct connections have been found.
The outer entrance channel is distinguished by a V-shaped profile incised in the
shelf edge below the 16 m depth. Erosion most likely occurred during a period
of lower sea level and sub-aerial exposure in the Pleistocene, at a time when
Figure 3. Bathymetry of Christiansted Harbor, August 1971. Based on sounding lines, inset.
Depths in feet.
streams probably emptied into the ocean at the platform edge. Farther
inward, the entrance channel is U-shaped due to sedimentary infilling,
but inward between Barracuda Ground and Round Reef the channel is V-
shaped. In this reach, the channel attains its greatest depth, 23 m (75
feet) and the depth is increasing with time indicating sediment is being
eroded by currents.
An historical examination of old charts shows bathymetry has
substantially changed in different parts of the harbor. Table 4 lists
the charts examined. Comparison of depths on charts dated 1794 1799
(by Oxholm), and 1856 (by the British Admiralty) show that the harbor
floor was extensively altered prior to intensive development. In 1794-99,
a natural channel averaging about 4 m (12 feet) deep, extended to Christian-
sted west of Protestant Cay and a channel 6 to 8 m (18 to 24 feet) deep ex-
tended east of Protestant Cay landward almost to the Old Fort. Additionally,
in Sloop Channel depths averaged about 3 meters (18 feet), When these depths
are compared with those of 1856, the U. S. Coast and Geodetic Survey chart
of 1924 and the present survey, they show the inner harbor shoaled by 1 to 2 m
between 1794-99 and 1856. From 1856 to 1924, shoaling was small in the inner
harbor but reached more than 1 m in Sloop Channel. Filling of the inner
harbor may have been caused by a supply of sediment released by soil erosion
following early agricultural development of the island. Today, depths of 5 m
(16 feet), formerly the natural depth, are attained only by frequent dredging.
List of harbor charts that provide a source of historical
data on bathymetry.
P. L. Oxholm, Danish
J. Parsons, British Admiralty
U. S. Coast and Geodetic Survey
U. S. Army, Office of the District
Aerial Photographic Studies
Aerial photography is a relatively new tool that is proving very useful
for analyzing coastal environments. From an aerial or birds'-eye view di-
rected vertically downward one can see at a glance, variations of tone repre-
senting distributional patterns of biota and sediment. Such patterns are diffi-
cult to recognize or chart from on or below the water surface. Besides
showing environmental detail and physiographic features not shown on charts,
a sequence of photos of the same area taken from time to time reveals changes
resulting from environmental processes that is, processes like sediment
transport, recession of bottom communities, and shoreline erosion. In
some photographs, the processes are portrayed directly.
Aerial photographic coverage of the harbor is listed in Table 5. All
photos are vertically oriented, black and white, 9x9" size, and they were re-
duced to a common scale of 1: 15, 000 for comparative examination. Most
information was derived from coverage dated 1954 (U. S. G. S.), 1964 (U. S. C.
& G. S.), and 1971 (U.S. S.C. S.).
TABLE 5, Aerial photo coverage for Christiansted Harbor
Source Date Scale
U. S. Geological Survey 1954 1:23,600
U. S. Coast and Geodetic Survey 1959 1:30, 000
Mark Hurd Aerial Surveys (Minn.) 1962 1:24, 000
Aero Service (Phila. Pa.) 1962 1:12,000
U. S. Soil Conserv. Service 1971 1:20,000
National Ocean Survey (U. S. C. G. S.) 1971 1:30, 000
Most aerial mapping photography is flown with a 6" (0.5 ft.)
focal length camera; therefore, the flying altitude is close to
0.5 times the reciprocal of the scale; e.g., USGS 1954 photos were
flown at 0.5' x 23,600 or 11,800 ft.
Figure 4, a reproduction of an aerial photograph dated 1954,
shows the harbor more or less in its "natural" condition before
the intensive development and alterations which took place in the
1960's. Most conspicuous is the change between contrasting light-
dark tone areas of land which partly represent cultural features
such as streets and buildings in Christiansted (CH, in Figure 4)
or natural features such as grassland and trees between (CP) the
site of the present day cement plant and (LP) and Little Princess.
In the Harbor, dark-toned (almost black) and light-toned (almost
white) patches represent either areas of grass (dark) or sand
(light) on the floor.
Because of the high water transparency at the time of photography
in 1954, bottom features can be seen at a depth of about 5m in the
vicinity of Gallows Bay, and at about 15m near the entrance (E).
Wave crests are displayed by surface sun glint near bg, seaward
of Long Reef, and breaking surf is shown by overlapping and irre-
gular white bands.
Along the crest of Long Reef there is an inter-tidal ridge
which is recorded in the photography as a "diffuse line" landward
of the breaker zone, Figure 4. Composed of broken coral heads
and dead branches encrusted with calcareous algae, the ridge is
dis-continuously exposed 15 to 30 cm high.
FIGURE 4. Aerial photograph of Christiansted Harbor taken at 11, 800 foot altitude,
January 28, 1954. U.S.G.S. photo number GSYM 1-145 (AF 5577),
enlarged for reproduction 2 times.
This photograph shows the harbor in plain view prior to intense develop-
ment in the 1960's. Natural elements include Long Reef, an inter-tidal
and sub-tidal coral reef "ribbed" on its landward side with submerged
sand trains, st(white) interpersed with dark-toned coral
covered ridges, and "striated" on its submerged seaward side with
groove and buttress structures, bg. Wave crests displayed by sun
glint are seen approaching the reef at a slight angle and the breaking
surf line is represented by overlapping and irregular white bands.
The outer entrance E, is marked by dark landward extending tongues
showing the main channel with water depths greater than 12 meters.
An accumulation of coral sand flanks the entrance at s (light tone),
and extends landward in a tongue to the right side of the photo.
Farther landward, dense beds of turtle grass, t, cover the bottom
broken occasionally by lunate blow-outs, or sandy patches displayed
by light tones.
The dark-toned grass beds extend inward behind Round Reef (Rr), and
Fort Louise Augusta; a few irregular dark-toned grass patches are vi-
sible off the mouth of Altona Lagoon. Mangrove trees fringe the lagoon
shore and extend lagoonward at m. Near Barracuda Ground, coral sand
(white -toned) penetrates dense turtle grass beds in finger -like patterns
at t. Elsewhere, turtle grass is displayed in dense beds to the north of
Protestant Cay (t), as well as to the west in a broad zone (t2), and farther
westward-along the shore to Little Princess (LP).
The southwest shore between Little Princess and Christiansted is largely
undeveloped except for land fill at sites f and a shore facility with pier
extending seaward just southeast of the present-day cement plant (CP).
Most of the Christiansted waterfront is bulkheaded with wharves and
docks, just as today except in the Gallows Bay area (GB).
:'r.,~L' I c
~ ~F '
at low water, but is largely submerged at high water. Seaward of the breaker
zone the reef is "striated" with living coral that forms a groove and buttress
system (bg on Figure 4 and in the vicinity of 0, Figure 5). Although sun
glint in the photograph of Figure 4 limits observable grooves and buttresses
to the eastern part of Long Reef, they are found more or less continuously
distributed along the entire reef length such as shown in Figure 5. Addi-
tionally, the grooves cross more than three "bands", or depth zones of
algae and coral paralleling the reef.
Landward parts of the reef, or backreef, are "ribbed" by a series
of light-toned submerged shallow channels (st, Figure 4) alternating with dark-toned
ridges of massive finger coral (Porites sp.). Locally, these channels or "trains"
which are floored by coral, rubble or sand, and ridges penetrate the harbor over 100
meters from the reef crest in the form of a lobe, as st, Figure 4. Their landward
margin is fringed by a relatively steep sand slope 100 200, 15 -20 m wide, (white
in Figure 4) and this passes into the flat harbor floor represented by the large light-
toned area between st and t2, Figure 4. The light-tone areas are bare sand churned up
by burrowing organisms, whereas, the slightly darker tones are produced by a very
sparse grass cover, areas of less intense sediment reworking.
In the outer entrance, E, dark trough-like tongues reveal the main channel
at depths greater than about 15 m. A sill, or shoal of sandy sediment across the
channel at a water depth of 9 m, is visible in the channel bend northeast of Round Reef
(Rr). Additionally, a large accumulation of coral sand (s, in Figure 4) extends
westward (channelward) at two points as well as landward to the photo edge.
Turtle grass beds appear on the photograph as dark patches with
irregular boundaries (t, Figure 4). East of the outer entrance channel the
grass beds are interrupted by light-toned and lunate-shaped sandy patches
called blowouts. The dark-toned grass beds (t) extend into the harbor
behind Round Reef (Rr) and to the entrance of Altona Lagoon. Near Barracuda
Ground two "fingers" of coral sand penetrate, and partly bury, the large
grass bed at (tl). Elsewhere, dense turtle grass beds are displayed by
dark somewhat mottled, patches north of Protestant Cay, and westward
in a broad zone (t2) as well as along the southwest shore to Little Princess
South and east of Fort Louise Augusta, the shoreline is marked by
a narrow white band representing coral sand beaches more than 15m wide.
This is in contrast to the dark-toned (almost black) band of mangrove trees
bordering Altona Lagoon and occupying the Lagoon corner, m, Figure 4.
Between Little Princess (LP) and Christiansted (CH), beaches are generally
absent and the shoreline is undeveloped except for a rum distillery, at the
present-day cement and power plant side (CP), and landfill areas, f, Figure
4. Less than a dozen small vessels and boats occupy anchorages in
Gallows Bay (GB), and southwest of Protestant Cay.
By comparing the photograph dated 1954, Figure 4, with another
dated 1971, Figure 5, the environmental and cultural changes that have
taken place during a 17-year period are indicated. In tracing changes,
the slightly smaller scale of Figure 5 must be taken into account, as also
the sun glint over the surface of the eastern harbor, (a feature that prevents
complete examination of the bottom in this photo, but is absent in an adjoining
photo not reproduced here). The most striking change is displayed in the
blotchy dark-toned areas of the central harbor designated d in Figure 5.
These are excavations, 4-8 m deep, dredged for sand, either construction
aggregate or landfilL .Growths of algae, Enteromorpha, now covering the
bottom of these holes, contribute to the dark-tone of the photograph. Ad-
joining these areas to the south and east are elongate dark-toned areas marked
c that represent segments of a shipping channel, 5.5 m deep, leading to the
power plant and the Cement Plant (CP). Sandy spoil dredged from the channel
is deposited as landfill in areas designated f, Figure 5 (white tone) in an effort
'to reclaim shoreward portions of the harbor floor. Channel dredging resulted
i iextrpatf6n-of grass beds along the channel course and landfill eliminated
* beds north of Protestant Cay as well as east of the Cement Plant (CP). Another
change,though of smaller magnitude, is visible near Barracuda Ground where
grass beds are eliminated by natural progradation of a sand lobe, tl, directed
southtoward the main channel. Additionally, there is less turtle grass coverage
on Round Reef (.Rr) and inward of Fort Louise Augusta (FL)than in 1954. Bridging
the lagoon entrance at b, with attendant restriction of flow to a 190 cm diameter
FIGURE 5. Aerial photograph of Christiansted Harbor taken at 10, 000-foot altitude,
February 17, 1971. U.S. -S. C. S. photo number PCFD-241, enlarged for
reproduction 1.45 times.
Natural elements include FL, Fort Louise Augusta, a rocky headland which
protects the harbor on the northeast; RR, Round Reef, is an isolated coral
reef in the inner harbor entrance; E is the outer entrance, a deep opening
between coral reefs to the west and east; s is Scotch Bank, and i, near
Gallows Bay (GB), is the main site of stream inflow.
Among the cultural features are CS, Christiansted business area and
waterfront; GB, Gallows Bay, a embayment dredged for shipping, White
patches designated f, are sandy areas of landfill and the dark patches, d,
are excavations in the floor left by sand dredges; LP is the waterfront re-
sidential area of Little Princess; o indicates site of outfalls which discharges
wastes and thermal effluents into the harbor and vicinity; g indicates guts
that discharge local runoff into the harbor.
'~F T~?Jur~>*~~T 4LT~~
~ I ~
*li tn ay .
l f A>- ,'
~tr '' g-
culvert constitutes a relatively small physical change, but it has had a
large effect in reducing exchange, water quality and biota of the lagoon.
Similarly, exchange in Gallows Bay (GB) is probably reduced by harborward
extension of a major wharf and by landfill (f). Since 1954, the number of
small vessels and boats occupying anchorages has increased more than three-
Although sources of pollution and waste disposal are not directly
shown in the photographs of Figures 4 and 5, information presented by
Jacobsen (1951) indicates that four sewers discharged directly into the harbor
at Christiansted. Additionally, nightsoil and garbage were dumped into the
harbor along the shore between Christiansted and the present-day cement plant.
And a rum distillery, located at the present-day site of the cement plant, re-
portedly discharged a substantial quantity of waste products directly into the
harbor. A municipal abattoir contributed wastes to Gallows Bay lowering the
dissolved oxygen content below 5 ppm. Today by contrast, most sewerage is
discharged into the ocean off Long Reef, 0 in Figure 5. But thermal effluents
are released from the power plant, 0 near CP, Figure 5; sewage wastes and
oil are discharged from anchored vessels and surface runoff from streets,
gutters and backyards of Christiansted carry miscellaneous wastes through
guts directly into the harbor in times of heavy rainfall. Municipal sewerage
discharge into the harbor is limited to intermittent periods of emergency opera-
A variety of different shoreline types are recognized according to
their natural character and utilization:
1. Wharves and bulkheaded waterfront
2. Rocky shoreline backed by bluffs
3. Artificial beaches fronting landfill
4. Natural beaches
Man-made structures consisting of wharves, sea walls, ramps, docks
and bulkheads, occupy 41 percent of the shoreline today, whereas in 1954, they
occupied only 16 percent. Built mainly for berthing, shipping, off-loading and
dockage, these structures secondarily serve to protect the shoreline from ero-
sion. However, such structures often have a deleterious effect on the water
quality and bottom sediments. By interfering with flow of natural currents,
they locally reduce circulation promoting anaerobic conditions. Additionally,
sediment is trapped and shoreline equilibrium upset, When waves attack
these structures they are reflected and sand is stirred up from the nearshore
bottom. This increases erosion, possibly undermines'the atrm~cturp,- ad con-
tributes to turbidity of the water.
Stretches of rocky and cobble-strewn shore backed by rock cliffs or
bluffs of alluvium, make-up 26 percent of the harbor shoreline. Located
chiefly around Fort Louise Augusta, Mt. Welcome and the east side of Pro-
testant Cay, this shoreline type was formerly much more extensive. Since
1954, 34 percent of the harbor shoreline formerly of this type has been eli-
minated or transformed to artificial beaches by landfill.
Although of limited practical use, rocky shoreline of headlands afford
sheltered anchorage and adds a great deal to the varied wealth of landscape
that enhances the natural setting of Christiansted.
Beaches make up 33 percent of the shoreline, of which 12 percent
are natural and 21 percent are artificial. The longest and most attractive
beach faces the ocean east of Fort Louise Augusta. It is relatively straight,
more than 15 m wide and firm to walk on. The sand is mainly coral, rich
in yellow-colored grains of Acropora palmata. It is medium to coarse-grained.
With increasing steepness toward the east, grain size increases and cobbles
are common at the east end (Figure 6 C). Because of seasonal variations in
intensity and direction of waves marked by "northerners", the beach is fre-
quently changed in width and steepness. Such changes reduce its stability.
Long-term recession of the beach is indicated by exposed tree roots and by an
exposure of beach rock 30 to 40 meters offshore (fb, Figure 6 C). Like beach
rock common to many shores of the Caribbean, it consists of calcareous sand
and shell debris that is most likely cemented in the vicinity of the water table
of a former beach (Russelland McIntire, 1965). Although the beach rock dis-
sipates wave energy and thus protects the present beach to some degree, it is
treacherous to boating and undesirable for bathing, especially when covered by
moss and coral.
The natural beach bordering the western harbor near Little Princess,
is stable, firm to walk on, and narrow, mostly less than 3 m except on the
westernmost point where it is more than 15 m wide. Despite the potential
Photographs of shoreline conditions at Christiansted Harbor, August, 1971.
A. Sandy beach on point of land at Little Princess, partly covered with
accumulations of dead grass ( t ) that is derived from shoals just
offshore and to the east. The beach is relatively wide (>20m), and
erosion is minimal. Easterly waves shown breaking at an angle re-
sult in a westward flowing littoral current and transport of sand.
B. Eroded bank (b) at Turquoise Beach fronted by rock debris (r), a fill
emplaced to arrest erosion. Note tree roots exposed by erosion.
C. Beach along Beauregard Bay, view southwest from the Bucaneer.
Beach rock (fb) marks a former beach seaward of the present beach
(pb). Exposed tree roots (t) indicate erosion is active at this poift.
D. Eroding beach along the eastern shore between Mt. Welcome (MW) and
Ft. Louise Augusta. Erosion has exposed several generations of landfill
consisting of white sandy sediment overlying dark clayey sediment. Tree
roots and stumps (t) indicate land extended farther seaward in the recant
%r- ~L :jllC: ~ IJYC~L~
Rab c. LL~C~-~Pn
supply of sediment from eroding bluffs of alluvium, particularly on the
east end at Turquoise Beach (Figure 6 B), most of the sediment consists
of the fine to medium-grained coral sand. Beachrock is intermittently
exposed 5-15 cm above the shallow nearshore bottom. Between Little
Princess and Pelican Cove erosion has reduced the beach to less than
3 meters width, but wave attack is retarded by the broad shoals of living
coral offshore and by short groins and rock fill. Attractiveness of the
beach is limited by thick accumulations of rotting grass derived from beds
just offshore and farther to the east (t, Figure 6 A).
Artificial beaches consist of sandy dredge spoil emplaced as land-
fill. Most of the material is derived from nearby channels and is rich in
white coral sand mixed with small proportions of shell and calcareous
nodules. It is distinguished by: (1) a substantial percentage of grey and
black sand grains which locally give it an overall grey color, and by (2)
its poor sorting, i.e., a range of grain size from fine to coarse sand,
plus calcareous cobbles.
The quality of the artificial beaches depends on the character of
the spoil and effectiveness of wave action in "reducing" the spoil to a
beach. On the west side of Protestant Cay where wave action is mini-
mal, spoil forms a firm, steeply sloping and relatively smooth beach.
But on the more exposed north side, and along the extensive landfill
east of the cement plant, wave action has effectively cut a scarp 2 m-high,
in the spoil and redistributed erodable materials offshore where they are
spread out on shoals of the nearshore bottom (e.g., s, Figure 2I). Coarse
material, shell and rubble, too heavy to be transported, are left on the beach
as a lag deposit whereas fine-grained silt and clay released from the spoil,
produce intense turbidity that is dispersed over a wide area (e.g., t, Figure
25). Unless the exposed landfill is stabilized these processes will continue
to a greater or lesser degree until the beach profile and plan configuration
reach a new equilibrium with the local wave energy and until the rate of
sediment supply balances the rate of loss.
Erosion is active along all the artificial beaches except the west
shore of Protestant Cay, and at Mill Harbor. At the public beach fronting
Altona Lagoon (Figure 6, D) several generations of cut and fill are super-
imposed on long-term recession which is evidenced by a prominent off-
shore ledge of beach rock and by eroded stumps of trees. Although groins
have been constructed at different places to arrest erosion, they are largely
ineffective. At.Turquoise Beach a groin-like structure is partly responsible
for accelerating erosion. Failure of groins attests to the lack of knowledge
of wave and current processes that they are intended to resist.
Rise and fall of the tide is relatively small being less than 35 cm
most of the time. The mean range in the harbor is 19 cm and the highest
extreme limit of tide is estimated at 60 cm, whereas the lowest tide is
-30 cm below mean low water. But what the tide lacks in magnitude is
made up in its variety and complex character.
Height of the tide was measured on continuously recording tide gages
installed at Chandlers Marina, Gallows Bay, June 29, 1971 and at the Cement
plant pier,, August 3, 1971. A tide gage installed by the U.S. Coast and
Geodetic Survey (NOS) at Gallows Bay for 4 months from April to July, 1924
provides data for secondary determination of mean low water as well as data
for predicting the approximate heights and times of the tide throughout the year.
These predictions are given in tide tables issued annually by the National
Ocean Survey (1971). Because the character of the tide at Christiansted is
like that at Galveston, Texas where the tide has been recorded since 1909,
predictions are made by reference to Galveston, Texas. For example, the
time of high water is 9 hours and 17 minutes earlier and heights are lower than
at Galveston by a factor of 0.57. Tides recorded in this study agree closely
with those of predicted tables as shown in Figure 7. Once reduced by com-
parison to the primary reference station, the new records will provide a new
preliminary tidal datum for the harbor.
Variations in the character of the tide displayed in time-height curves, from
predicted tables and from observed tides in the harbor from June 29 July 19,
Character of the tide changes from day to day during a month. As
shown in figure 7,at neap range June 29 July 1 and July 12 13 the tide is
mixed; there are two high and two low waters daily, each of unequal height.
By contrast at spring range, July 5 7 and August 17 18, the tide is of the
daily type; there is one high and one low water each day. Intermediate
periods have low waters of different height indicating the mixed tide is mainly
due to a diurnal inequality in the low waters. At certain times however, the
differences between lower high water and higher low water are so small, the
mixed tides merge into a daily type with one high and one low water each day.
This transformation, which occurs about once a week is known as a vanishing tide;
and results in a near still stand of water in the harbor for about 12 hours. This
sequence of variations arises from changing positions of the moon in relation
to the earth and sun as demonstrated by Marmer (1954). It is these tidal
changes which promote an exchange of water and which alternately flood and
expose the shore;
Although the tide changes character from time to time throughout a
month, it is predominately of the mixed type like other areas in the Antilles,
according to the U. S. Navy Oceanographic Atlas (U. S. Navy, 1965). In con-
trast to the regular daily variations, irregular fluctuations were noted in the
tide records, especially at neap range of the tide. When the records are com-
pared with the periodic rise and fall of predicted time-height curves as shown
in Figure 7 departures occur which relate to short-term effects of wind and
From place to place within the harbor tidal differences are small.
Differences in the time of high and low water between Chandlers Pier and
the Cement Plant Pier, a distance of 1.6 km, are less than 20 minutes and
the range is essentially the same. Thus, the tide rises simultaneously
throughout the harbor. Because the harbor entrance is relatively large and
unrestricted, tide levels in the harbor most likely follow those in the ocean
within relatively narrow limits.
Superimposed on the daily variations of range there are variations in
the average height of sea level or of mean low and high water from month to
month over an annual cycle. Although tide data at Christiansted are limited
to a few months of record, the magnitude of monthly variation over a year can
be inferred from data at Galveston, the reference station, as well as Havanna.
Figure 8 shows that mean sea level is more than 15 cm lower in winter than
in late summer. Likewise, monthly mean high water and mean low water may
be expected to follow a similar trend. Such variations have long been noted and
ascribed to the seasonal changes in wind and weather but quantitative data are
needed. Nonetheless, low levels of the tide are commonly observed by local
inhabitants during winter months, and the exposure of a large portion of Long
Reef must limit the transfer of ocean water across the reef.
MEAN MONTHLY HEIGHT
I I I IM J
J F M A M J
I I1 I I
J A S 0 N D
Monthly change in mean sea level at Havanna
for an annual cycle, 1947-1950. Height in
centimeters above mean low water. Modified
from Marmer (1954).
Vertical distribution of mean current speed in
meters per second at selected anchor stations
2, 7, 8, August 1971.
- MSL -
The periodic rise and fall of the tide is one of the important elements
of water exchange. Vertical range of the tide determines the extent of shore
that is alternately flooded and exposed, and the range determines the amount
of water entering and leaving the harbor each tidal cycle, i. e. the tidal prism
or volume of water between the mean low and high water planes. In Christian-
sted, the mean range, 19 cm times the harbor area, 2.31 sq. km. gives an
immediate estimate of the volume of tidal flow, or 4.4 x 105 m3, a figure which
indicates 10 percent of the harbor volume is exchanged each tide. This com-
pares with a figure of 3.78 x 10 m per hour determined by direct measurement
of volume transport through the entrance. Thus, tidal transport contributes
only a small part to the exchange of water, less than an estimated 10 percent
of the total water exchange. Wind drift and mass transport are much more
Special effort was applied to study of the circulation because movement
of water is of great significance in determining the character of the environment;
it is essential to problems of pollution and sediment transport. Additionally,
little is known about circulation in the harbor or in similar tropical environments
of the West Indies. Without a study of circulation, few logical conclusions could
be drawn from corollary geologic, ecologic and water quality data.
Because of the many current producing forces active in an harbor -- tide,
wind, waves and mass transport -- the measured current at any one place and
time is a resultant of components having several driving forces. These forces
vary in strength with time and interact with bottom geometry and shore con-
figuration from place to place to produce complicated patterns of convergence
and divergence. For this reason coastal circulations are studied by continuous
observations and analyzed statistically.
Circulation of water follows a broad counterclockwise pattern directed
eastward through the central and eastern harbor. The pattern is illustrated
in Figure 10 and is derived from current speed and direction measurements at
12 fixed "anchor" stations occupied over a 24-hour period. In areas where currents
were not metered, directions are inferred from the continuity of flow. The
measurements indicate currents are generally faster in central reaches and about
the entrance than elsewhere. Instanteous peak speeds reached more than 0.25 m/sec
in near-surface water of the entrance (Station 7) whereas in inner harbor reaches,
speeds are generally less than 0. 06 m/sec. In shallow water less than 3 m deep,
both surface and bottom water flow in a similar direction and at like speeds but in
deeper water of the channels speed is much reduced, and direction follows the channel
configuration. For example, the mean surface current exceeds bottom current more
than two-fold at Station 7, as shown in Figure 9. Only at Station 10, Sorensen Ground,
are bottom currents of greater strength than surface currents, a condition probably
controlled by channel configuration.
Mean surface current, speed and direction for harbor currents and for littoral
currents, August 1971. Length and width of arrow indicates speed in meters per
second. Harbor currents based on anchor station measurements; littoral currents
on dye patch measurements. indicates two alternating predominate directions.
Although current measurements were made near spring range of the
tide, a time when tidal currents are most active, no tidal reversals are in-
dicated except at Station 9, Long Reef, offshore. At this station, there is
an indistinct reversal in the near-surface water, from flood to ebb every 10
to 12 hours. In near-bottom water, there is a weak and incomplete rotary
tidal current superimposed on an erratic coastal flow. It is possible that
during the period of current measurements wind-driven currents obscured
the tidal currents. During calm periods, however, there may be a flood
flow through the main entrance channel and an ebb flow through Sloop Chan-
The harbor-wide counterclockwise current is, undoubtedly, driven by
large-scale mass transport of waves breaking over Long Reef. Over-reef
transport is partly substantiated by drift of dye patches and by the distribu-
tional patterns of backreef sediments. Such a transport builds up a head of
water in the central and western harbor. Local accounts indicate that the
eastward harbor flow is speeded up during "northerners", when high surf
overtops the reef. Excess water driven into the harbor over the reef must
leave it through the main entrance to the east. The current measurements
leave no doubt that flow through the entrance is predominately seaward.
The stress of easterly trade winds on harbor water locally alters the
predominate easterly flow and may reinforce the set-up of mass transported
rIc wailer ;aloIg tliu southwest shore. Wind drift locally mn:sks the easterly
harbor flow at station 4, with speeds up to 0. 15 m/sec. And when the wind
gains strength during the day, it temporarily reversed the predominate flow
at Station II between 0900 and 1300. In exposed waters of Scotch Bank Channel,
wind drift is the chief current. Farther seaward in the outer entrance, this
wind drift merges with outflowing harbor water and the two currents flow seaward
with a mean velocity of 0.16 m/sec.
A westerly flowing oceanic current, part of the prevailing North Equatorial
Current, reportedly sweeps offshore areas. Established in the region by the U. S.
Navy Oceanography Office, and summarized in vanEepoel, et al. (1971), this current
has a mean speed of about 0.35 m/sec. It is generally assumed (Dammann, 1970)
this current flows shoreward and eastward as a large counterclockwise loop between
White Horse Shoal and the Scotch Bank Buck Island area. But the extent to which
this flow impinges on coastal water or influences its movement is unknown. The
easterly flow detected off Long Reef, Station 9, has a reciprocating and rotary tidal
character rather than a component of permanent flow. Moreover, measurements
at Station 8, Pelican Cove, offshore, indicate a westerly flow in both surface
and bottom waters, a direction opposite to the supposed easterly flowing loop.
Dye Dispersal and Photography
On three occasions, August 3, 5 and September 9, a series of dye releases
were made simultaneously with aerial photo coverage. The dye consisted of 1 to 2
pounds of fluorescein and it was released as a slug at mid-depth. The photographs
were taken single-handed by Jean Larsen flying at about 300 m altitude in a Cessna
150 equipped with a Hasselblad 500 EL camera having a 50 mm lens, in addition to,
35 mm Nikon and Contarex Super cameras. Different types of film were used: Ektacolor,
Kodacolor and Plus-X. Winds blew east to northeast on August 3-5th,
and east to southeast September 9th. The photographs show the path
and dispersal of dye under the combined action of current, wind stress
and wave action. Additionally, some of the photographs reveal gross di-
rectional trends of sewerage effluents and turbid plumes as well as a
number of short-term irregularities and variations in the local circulation.
Release points, and tracks of the dye which were determined from a
sequence of photographs at approximately 15-20 minute intervals, are plotted
in Figure II. Dye released off the Turquoise Beach on August 3rd drifted
northwestward, whereas on August 5th, dye released from the same point
drifted southeastward. Elongation of these patches, like others, is presumably
due to current-shear, being more or less along the wind. Northwesterly drift
from the Long Reef and Pelican Cove outfalls is consistent with offshore current
measurements. But, at station 5 near the site of dredging, the dye drifted slowly
west to southwest, a direction quite different from the northerly flow measured at
this site. Dispersal trends of dye, as well as of turbid plumes for August 3-5th
in the eastern harbor indicate a convergence of ocean water, driven harborward
through Sloop Channel,with harbor water flowing seaward east of Protestant Cay.
From the point where the flows meet in the vicinity of Station 5, the current passes
westward and thence seaward through the main channel. It cannot be presumed,
however, that these dye paths and turbid patterns represent the mean circulation.
Instead they show the variations that may be expected from time to time as a result
of changes in winds, waves and mass transport.
Figure 11. Circulation from dye patch dispersal at different release points and various dates.
+ values indicate time after release. Current direction derived from analysis of
turbid plumes shown in aerial photographs taken at the same time as dye studies
is also indicated by arrows.
Longshore currents and littoral drift
In a narrow zone dominated by swash and backwash of waves,
there is a local current that carries sand alongshore. Since waves
approach the harbor and its inner shores from an easterly direction,
they break on the shore at a slight angle as shown for the eastern harbor
in Figure 5, and at Little Princess, Figure 6 A. The forward motion
of the water keeps moving in the direction of wave approach with a com-
ponent parallel to shore. Sand trapped around beach structures and minor
irregularities in the shoreline give evidence that longshore currents are
active, but direct measurements are often difficult.
A series of dye experiments were conducted daily between August
31 September 3 to estimate the direction and rate of sand transport at
selected sites (Fig. 1). The significant wave height was measured from
trough to crest at the breaker line; the wave angle between approaching
wave crests and the shore (or reef) was measured by pelorus; the wave
period was determined by counting the number of waves arriving in one
minute; the beach slope was measured by clinometer; and water depth
at the breaker line was measured with a meter stick. Longshore current
velocity was determined from the travel of dye introduced at the breaker
line. Northeasterly winds blew 6-9 m/sec during the period. Appropriate
data are summarized in Table 6.
Longshore transport data for selected sites, August 31 September 3, 1971.
* Q/F (A) is the relative littoral transport.function
along ocean shores, assuming Ho/ Lo is constant.
N. B. Winds were 7-10 m/sec from 450 550 T during
** Q/F (B) is the relative littoral,X tRapoport
,function along harbor shores, assuming
Ho/ Lo is constant.
The littoral drift of sand was estimated according to a relation of
LeMehaute and Brebner (1967):
Q 112I f(s,aL m, Ho / Lo)
wherr s is the beach slope, oi is the angle of wave approach, m is the material
characteristics, Ho and Lo are the deep water wave height and length, and Q is
the rate of littoral sand transport. Because Ho / Lo and m were not measured,
they are assumed constant, and for comparative purposes, the functions f(A) and
f(B) are substituted; F(A) is designated ocean shores and f(B) is designated for
inner harbor shores.
The measured velocities and inferred directions illustrated in
Figure 10 indicate relatively strong westerly currents, greater than 08 m/sec,
along the outer reef except near Fort Louise Augusta where a local easterly
flow was detected. Interestingly, at three sites on Long Reef the flow was
directed harborward across the reef. Currents along the inner harbor shores
are weak and variable. On the northwest side of Protestant Cay sand is partly
transported westward whereas eroding landfill just east of the Cement Plant
(site VIII) is carried eastward. At Mill Harbor, near Turquoise Beach
large quantities of beach fill are carried westward and trapped on the east
side of a groin-like structure. Farther to the west near Little Princess,
a westward current appears to converge with a slight east flow resulting in
a stable beach .
There are many ways in which the current data of this study are
of practical use. Inasmuch as water is introduced into the harbor mainly
by mass transport over Long Reef, and flows easterly through the harbor
and out the main entrance, areas of the eastern harbor that lie "downstream"
are vulnerable to disturbance or loading from the west. For example, if
material were released by dredging off the Cement Plant, it would spread
eastward toward downtown Christiansted and Protestant Cay. By contrast,
if surface-borne wastes were discharged seaward off Fort Louise Augusta,
they would disperse away from shore in the northwesterly flow. In this
way, the basic pattern of circulation can serve to delineate areas vulnerable
to disturbance and to determine the effectiveness of waste dispersal.
The pattern of near-surface circulation predicts where sites of local
convergence or divergence may occur; such a feature may concentrate flota-
bles in surface water. Similarly, opposing flows that meet near station 5,
as indicated by dye dispersal produce a high horizontal gradient of turbidity
that should lead to an accumulation of suspended materials. Refraction of
waves on Barracuda Ground may result in building up a local head,of water
resulting in a diverging flow away from the Ground, partly channelward.
A predominate westerly current offshore off Pelican Cove, and at times, off
Long Reef predicts a weak upwelling. And local convergence of longshore
currents near Little Princess, favors accumulation of beach sand, whereas
farther east at Turquoise Beach erosion predominates.
The distribution of current speeds indicates water movement is
faster in entrance reaches and offshore than in inner reaches like Gallows
Bay. Additionally, near-surface water of the channel moves faster than
near-bottom water. Thus, mixing and diffusion which are active in seaward
areas and over shoals promote rapid dispersal of wastes and transport of
sediment. Whereas in inner reaches and deeper parts of channels, these
processes are so slow that waste is retained and sediment is trapped. Current
velocities also indicate that flushing of the harbor is relatively good. For
example, a waste introduced in surface water at the Cement plant would be
expected to pass out the entrance via the Protestant Cay-Christiansted passage
in less than 24 hours. Flow measurements of this study, indicate 8 percent
of the water in the harbor passes out the entrance every hour. But with long-
term enlargement of the harbor floor and deepening of the main channel, the
proportion of bottom water (by volume) actively mixed and exchanged becomes
Inflow from Land
Direct measurements of stream inflow and surface runoff are not avail-
able. However, occasional flooding is observed following rain squalls and
showers especially during the months of August through November. Although
the mean annual rainfall is 104 cm, mean annual potential evapotranspiration is
149 cm and the loss consistently exceeds the monthly rainfall throughout the year,
except for perhaps one or two months. Inasmuch as the harbor lies outside the
main paths of most tropical storms and hurricanes, major storm runoff and
tidal flooding are infrequent. There are less than 3 hurricanes per 100 years
(Mills et al. 1970).
5. WATER QUALITY DATA
Water temperature throughout the harbor on a given day reveals that
the surface waters are well mixed. Spatial variation in water temperature on
individual days was 1.0 to 1.30 C. Generally, the inner reaches of the harbor,
particularly, Gallows Bay, tend to be warmer than ocean water. Water tempera-
ture at individual stations varied 2.2 3.50 C during the study period and were
higher in August than in June or September.
From May to September salinities ranged from 34.7 ppt. to 38.3 ppt.
throughout the harbor and from 2.4 to 3.4 ppt. at individual stations between
June and September. There was no significant gradient of salinity within
the harbor; the range of values among 15 stations on any given day was 0.4 to
2.3 ppt., most of which is likely due to local freshwater discharge.
The concentration of suspended matter in surface waters was higher in
the nearshore waters of the southern harbor (Figure 12). Here the concentrations
were 5.2 to 12.9 mg/L. Seaward to the northwest and northeast, suspended solids
decrease to intermediate levels (3.6 4.8 mg/L), and in the clear water just
behind and outside of the reef, mean concentration were 1.3 3.2 mg/L. This
distribution parallels the distribution of Secchi Disk depth (Figure 13).
L.D *5. .
.. . .. :.... :.
Figure 12. Suspended solids, mean concentrations in surface water,
May through August, 1971.
Fig re 2. Sus end d oli s, ea co cen raion in sufac wa er
May through August, 1971.:~:I:
Secchi Disk Depth
The water in Gallows Bay and the southeast area of the harbor are
always very turbid. Visibility is lowest in the south central section of the
harbor, especially east of the Cement Plant. Measurements at station numbers
12 and 16 were 1.5 to 2.5 meters. Water depth at these points is 2 to 6 meters.
North of Protestant Cay most of the bottom can be seen from a boat,
except over the deeper dredged channel. In the western half of the harbor, and
over the fore and back reef water is clear enough to allow visibility to the bottom
at depths up to 16 meters (Figure 13).
Water from each of the fifteen stations was analyzed six times between
June and August for the presence of fecal coliform organisms. Most stations were
negative or had only very low counts. No fecal coliform were isolated from
Stations 4,5, and 9. Station 2, near the sewage outfall had the most consistently
high densities. There was only one positive test at the two stations inside the reef
closest to the outfall (5 and 6). High densities were encountered, once each, at
Stations 8 and 13. Relatively low fecal coliform densities (1-64 cells/100ml)
were regularly found at Stations 15 and 16 in Gallows Bay.
The surface waters are adequately oxygenated throughout most of the harbor
during mid-day hours of measurement. Average concentrations less than 6. 0 mg/L
were found only at Stations 2, 13, and 15. The highest levels of dissolved oxygen,
6.0 8.0 mg/Ls, were measured at Station 5, an area of shallow, clear water
Secchi Disk Depth in meters.
Mean values, May through August, 1971
overlying a lush Thalassia bed which must produce high oxygen. Generally,
distribution of oxygen varies within narrow limits throughout the harbor.
Summary of Water Quality
Turbidity increases more than 5 times with distance landward from the
ocean and barrier reef to the inner harbor. With this broad trend of turbidity
there is a corresponding inward decrease of disk depth or transparency. The
most striking gradient in water clarity was found in a band running from the
cement plant to the north end of Protestant Cay, and to Fort Louise Augusta.
Locally high turbidity east of the cement plant is mainly produced by carbonate
fines released by wave action from dredge spoil of the landfill, area. Added to
this, terrigenous fines are intermittently discharged from guts by land runoff
during times of storm. And growth of phytoplankton promoted by nutrients of
intermittent sewage discharge also contributes to the high turbidity level of inner
During the course of this work it was noted that the color of the water
south of Protestant Cay often suggested large numbers of planktonic algae. The
lush growths of filamentous green algae along the bulkheads and piers in this
area support the inference that the waters are highly enriched. The occurrence
of fecal bacteria in these waters indicates the source of this enrichment, but
because of the transient nature of these organisms the present data probably
underestimate the degree of sewage contamination to which the area is subjected.
According to present Government designation, stated in Title 12 of
V. I. C., Subchapter 186, all of Christiansted Harbor from Fort Louise Augusta
to Golden Rock (Cement Plant) is Class C water "for harbor and docking faci-
lities". By exclusion, that part of the bay west of Golden Rock is Class B "for
propagation of marine life and for water contact recreation". Pertinent quality
criteria for these classes are:
Class B Class C
Dissolved Oxygen not less than not less than
5.5 mg/L 5. 0 mg/L
Fecal Coliform monthly mean not monthly mean not
to exceed 70/100 ml to exceed 1000/100 ml
For harbors criteria are relaxed "for areas immediately adjacent to outfalls
or drainage ditches" to allow for "admixture of waste effluent with harbor
waters". In this context, all of the waters of Christiansted Harbor meet the
local quality criteria for both classes. However, current trends in development
and use of the harbor will certainly lead to further degradation of water quality.
The territorial water quality standards include a non-degradation clause designed
to protect water quality existing at the time the standards were adopted (1969).
This report establishes several baseline descriptions which can be used in
assessing the quality of the harbor water in the future. To minimize further
destruction of water quality, different areas of the harbor should be designated
for particular uses and the uses should have priorities. Along with such a plitn,
strict enforcement of pollution practices is required. Christiansted Harbor
currently serves two distinctly different uses: commercial traffic in the eastern
two-thirds, and tourist oriented water contact recreation in the west. This
dichotomy of uses is recognized by explicit classification of the harbor for
these two uses in the Water Quality Standards, but planning is needed now to
ensure this separation in the future.
(6. ECOLOGY OF REEF COMMUNITIES
A short survey of the reef communities of Christiansted, St. Croix w;is
made to determine the "state of health" of the reef populations, and to assess
damage which the reef reportedly sustained as a result of development-asso-
ciated projects in the Christiansted area. It must be understood that there are
no previous data as to the "healthiness" of reef communities in this area, and
that all judgements made about the Christiansted reefs stem from comparisons
with reef communities elsewhere in the Virgin Islands (e.g. Brody et al, 1969,
1970). This in itself is a somewhat specious line of reasoning because no two
biotic communities are precisely the same and the physical, chemical, and bio-
logical interactions of any one community are probably never equivalent to those
of any other community. In addition, there is a great paucity of experimental data
from which correct extrapolations can be made about either acute or chronic ill-
effects which reef organisms may suffer as a result of input of sewage, suspended
particulate matter, and marine traffic with their corollary changes in light pene-
tration, oxygen and nutrient concentrations, BOD, COD, pH, etc. Thus, this
section does not describe a biological study of the area which pin-points the
problems facing the reef communities, but rather presents an educated "impression"
of the most general types of problems easily noticed by the trained observer.
It was undertaken with several preconceptions as to the effects of human
development on coral reef communities. In Christiansted Harbor there are two
major types of environmental modifiers (indeed pollutants) which give cause for
The area inside the reef has been the site of several dredging projects
for improving or maintaining navigable channels, for stockpiling of calcium
carbonate sediments for concrete production, and for development of re-
creational areas as beaches. Dredging produces a variety of pollution pro-
blems, some of which are obvious and others more subtle. The distinct,
short-term effects are the obvious ones: some benthic organisms are re-
moved outright, others are damaged and die as a result of abrasion or la-
ceration, still others are smothered by the fraction of sediments moved but
not retained within the dredge pipe. In addition, some of the organisms in
the immediate vicinity of the dredging work may be exposed to a radically
different chemical regime when newly exposed anaerobic sediments are
uncovered, perhaps causing their death or severely impaired metabolism.
There are several probable effects of large-scale dredging which are
more subtle and thus more likely to be overlooked. A portion of the sediments
disturbed in any dredged area will be too small in size (or too far away from
the suction of the dredge) to be effectively pumped up and will become suspended
in the water column. These small size particles can produce either or both of
two negative effects. Where wave energies or current velocities are sufficiently
high these particles will be carried out of the dredging area and into lower energy
areas where they may fall out of suspension and abrade the surface tissues,
olog Ith poues (or mouths, or gut cavities) or even smother the sessile organisms.
And while these particles are in suspension they are responsible for a very signi-
ficant increase in turbidity. This turbidity increase effects a decrease in
radiant energy which reaches the sessile primary producers, and may cause
a serious reduction in the carbon fixation and oxygen production associated
with the photosynthetic activity of the plant cells. This is particularly true
in a typical coral reef community where photosynthetic productivity is
the result of the combined activity of marine grasses and algae at the margins
of the reef and in the back-reef areas, calcareous algae on the reef and the
algal symbionts of the coral animals. The ultimate health of the coral colonies
and the calcareous algae is an especially important consideration near dredging
areas because these are often the very organisms which produce the sediments
being dredged. The observed consequences of dredging to the Christiansted reef
and calcareous alga communities are discussed later.
The population of Christiansted has grown rapidly in the past decades and
of course, so has the sewage output. The effects of the various chemical and bio-
logical qualities of human sewage on coral reef organisms are poorly known, but there
are at least a few statements which can be made. Excessive BOD and COD are dele-
terious to most natural'communities and it can probably be assumed that the reef
complex is no exception. If these oxygen demand levels are coupled with low oxygen
production as a result of turbidity, it is quite possible that community health may
suffer. The physical agitation of the water column as it crosses the reef with breaking
surf may not be sufficient to deter a serious problem with the reef community's oxygen
balance. It is important to note that it is the extremes (eg. low tide, low circulation,
high BOD) which cause the mortalities, not the average of daily variations.
We have very little data on the nutrient requirements of reef systems,
but there is certainly abundant evidence that the reef does not have an infinite
capacity for nutrient enrichment. The reef system can (and has) undergone
drastic changes in some areas where eutrophication occurs. In addition,
changes in the composition of the reef community's species structure are a
probable step in nutrient enrichment and these changes may have severe
The biological changes caused by dredging and sewage outfalls cannot
easily be separated from the consequences of several other forms of pollution.
Thus, turbidity produced by erosional runoff, storm drainage, and boat traffic,
the hydrocarbons from engine exhausts, and myriad lesser forms of pollution
are not considered separable from the highly visible forms within the context
of a brief ecological survey such as this one. Such work is important and should
be pursued as separate research projects in the future. Until the interactions
of specific levels of specific kinds of pollution has been quantified, we are forced
to speak only in generalizations.
A series of observational dives were made with snorkel and SCUBA tech-
niques over the entire length of Long Reef and Round Reef, Christiansted, St.
Croix. The fore-reef and back-reef environments were studied with an eye
to distinct changes from the normal patterns of faunal association and organism
healthiness. Normal is defined in terms of characteristics of other similar
reef systems which have been observed rather than as a comparison to an ear-
lier, "pre-pollution" state of the ecosystem; healthiness is primarily based upon
variations in color pattern, polyp expansion, colony shape, and obvious new
growth in the scleractinian corals; the same characteristics apply in the octo-
coral groups with the added one of colonial mesoglial condition. The echinoderm,
crustacean, poriferan, and other macro-invertebrate groups were casually
examined when encountered. The fish populations were likewise noted, but
their mobility and desirability as food make fish a poor pollution indicator
without careful study. In the base-reef and back-reef environments, the abun-
dance, color, growth, and depth-light level patterns notable in the marine algae
(particularly the Siphonalean genera Caulerpa, Halimeda, Penecillus, and Udotea)
and the abundant marine spermatophytes (Thalassia and Syringodium) were used
as indicators of the normalcy of the benthic community.
The bulk of the area was observed during late October, 1971, by divers
towed slowly behind a small boat; where warranted, a more detailed reconnaissance
was made. Figure 14 shows the areas observed; place-names and compass bearings
were taken from USC & GS chart #935 and from local knowledge.
64 43' 64042'
I u n L o n g R e e f
t- \t ; ----- -
S-s- -x-_--- Ground
\ ''' < ''~.-~--';''- TBT----
--- TRANSECT SCALE 0.8K
Ci AREA 0,
_ _ _ --I
Figure 14. Index to coral reef observations. For details see text.
. U.V l l.
Area I is a front reef area composed almost exclusively of Acropora
palmata which forms substantial colonies (height up to three meters) in close
proximity to one another. Other prominent corals comprise only about 5% of
the population. This community extends seaward from the breaking reef off
Pelican Cove Beach to depths of seven-fifteen meters. Acropora in this area
were seemingly undisturbed with new growth evident in most colonies. The
octocoral population was minimal; presumably, high wave energy and the dense
areal coverage of Acropora limit colonization. The fish population seemed typi-
cal of many similar Virgin Islands reefs; the obvious piscivores were few and
coral feeders predominated. This population structure is perhaps encouraged
by high fishing pressure.
Area II is an adjacent front reef environment. Acropora palmata pre-
dominated with Diploria and Montastrea as genera of secondary prominence. As
one proceeded from west to east along the reef the variety of scleractinian corals
increased with Acropora accounting for approximately 70% of the sessile growth,
Montastrea and Diploria (in that order) about 15%, and other scleractinians and some
species of gorgonaceans (notably Plexauriids of the genera Eunicea, Pseudoplexaura
and scattered Gorgoniids, eg: Gorgonia, Pseudopterogorgia). The remaining bottom
cover, about 10%, was made up of Poriferans, colonial anemones, etc. There were
scattered clumps of coralline algae. The area tended to grade away from the closely
packed stands of Acropora to a more broken-up, cosmopolitan community with sandy
patches and "surge channels". There was no noticeable die-back or particularly un-
usual pattern of growth, but it was estimated that the sessile biomass decreased along
a west-east gradient.
The vicinity of the sewer outfall itself is an exception to the generalities
presented above. The area about 20 meters east and west of the pipe and about
100 meters north of the pipe's end was a distinctly barren one. The area is
generally uniform in depth with the open end of the pipe at a depth of about six meters.
The density of sessile organisms was markedly decreased although there was no
radical change in species diversity. There was a 200 to 300% increase in the
fish population (much greater in the case of the black durgon, Melichthys niger,
and surgeonfishes, Acanthurus coeruleus. These fishes had modified their
normal diet to include pieces of toilet paper and feces. Although one may decry
the sight of a gorgoniid axis festooned with pieces of all sorts of sewage waste
materials, it was not observed that a very large area had been severely affected by
discharge. On the day of the observational dive, the wind was blowing from the ESE
at a speed of 4-6 knots, and the sea was calm at State 1-2 on the Beaufort scale.
There was rapid dispersal of the effluent to the West outside of the reef, but the
observed dispersal pattern cannot be presumed to be a fixed one. Several residents
of the Christiansted area have reported distinctly seeing "slicks" emanating from
the outfall and spreading south across the reef into the harbor.
Area III includes a transitional zone from the eastern section of Area II,
and an obviously disturbed zone eastward. Visibility decreased markedly from west
to east from 15+ meters at Area II to less than five meters at the east end of Barracuda
Ground. Acropora palmata was noted to be far less dominant in this area, representing
less than 20% of the bottom cover off eastern Barracuda Ground; total coral growth
covered less than 50% of the bottom with uncolonized coarse sand over reef rubble
comprising the rest. There was a noticeable reduction in the active growth of
new polyps in Acropora when comparison is made to Area I. This reduction in
growth-rate was not detectable in the gorgonaceans in depths of five to ten meters
to the north of the reef. With increasing depth, Acropora disappeared at about
five meters and was replaced by scattered gorgonaceans (Pterogorgia in sand, then
Pseudopterogorgia and Eunicea). Also, the community increasingly was comprised
of mixed gorgonaceans (about 75%) and small scleractinians (Diploria, Montastrea)
with scattered small Acropora cervicornis visible; scleractinians represented about
5% of the sessile macro-invertebrate growth at ten meters.
Area IV, including Round Reef was investigated to complete the survey of
the fore-reef environment off Christiansted. The area between Barracuda Ground and
Round Reef (Great Middle Ground) was virtually devoid of coral growth to depths of about
six meters, the limit of visibility. The ship channel cut into the natural western slope
of Round Reef had not yet been re-colonized. The coral kill on Round Reef is about 90%;
the dead colonies of Acropora, Montastrea, Diploria, and Porites could still be
recognized. Visibility was reduced to about two meters with scattered colonies
of Porites porites, P. asteroides, and an occasional fragment of Acropora palmata
noticeable. These live corals were not laying down new skeletons at as rapid a
rate as elsewhere off Christiansted; several colonies were obviously being rapidly
buried by shifting sediment.
Area V is the most active area of the back-reef due to its proximity to the
channel and to current flow. A series of observations were made on tracks across
Sorensen Ground, Figure 15, south and west of Barracuda Ground and across the
back-reef of Long Reef as far west as the western boundary of Area III; The zone
immediately south of Barracuda Ground and adjacent to the ship channel was
obviously undergoing rapid change and had a very low sessile biomass. West of
this there were bands or patches of Porites porites (with occasional P. asteroids and
small Acropora palmata colonies) alternating with Thalassia or Syringodium mixed
with calcareous algae. Several patches of Sargassum ( as a sessile plant) were
also noted. This back-reef seems typical of similar reef areas throughout the
Virgin Islands; these organisms are among the hardiest and are adapted to occasional
high wave energy, high turbidity, and rapid sedimentary variations. Syringodium
was noted in water somewhat shallower than is perhaps typical; it was an unusually
long and luxuriant growth. This depth-distribution may be correlable with the higher
turbidity at the western margins of the recently dredged channel to the west of Protestant
Area VI is a small area along the north shore of Protestant Cay. A short
track was run to assess possible damage to the patch reefs. The turbidity was
quite high throughout the area; visibility was less than water depth except shallower
than 1 meter. The Thalassia-algal assemblage at the edge of the dredge-cut was
obviously a new community but shows a typical speciation pattern. The coral
at the edge of the old reef was virtually all dead; there were scattered live Porites
colonies amongst dead ones. The dead reef was populated by scattered colonies
of brown algae (Dictyota, Ectocarpus). The sessile community changed abruptly
with the change in depth at the easternmost point of Protestant Cay; at two meters
there was a dead reef with patchy algae; at 20 meters depth ( only
a few meters away) the Thalassia-algae community was established. A
distinct increase in water clarity could also be noted.
Status of the Reefs
The front-reef areas of Long Reef, Christiansted, were in good con-
dition except (1) Barracuda Ground, Area III at the eastern end, and (2) the
section of Area II about the sewer outfall. The community is composed
primarily of Acropora palmata in the zone from ten meters to the surf, with
Montastrea annularis, Diplora, and Porites as other important scleractinians.
The octocoral population is somewhat reduced from the numbers of species and
densities normally found on similar Virgin Islands' reefs. The back-reef areas
of Long Reef with the exception of eastern segments of Sorensen Ground are
typical of most back-reef communities with Porites, Acropora, and Thalassia-
Syringodium algae assemblages present. This area is populated by organisms
best adapted to higher stress resulting from sedimentary changes.
Barracuda Ground is under stress from a combination of several factors
and it is difficult to assess the differences observed between the western and
eastern sections of Area III. Density of bottom cover was less than 50 percent
in the eastern section with large, open, coarse sand patches uncolonized by the
usual benthic biota; the scleractinian coral cover was less than 10 percent in
parts of the section. There were very few large dead coral colonies. The pattern
of growth and the absence of large dead coral colonies suggest that the eastern
end of Long Reef has historically been an area of relatively rapid sedimentary
change because of high incident wave energy, and the bottom environment
is somewhat hostile to potential colonization. If there had been large numbers
of dead colonies, a recent radical change resulting in extirpation of the com-
munities would have been suggested. -Other stress factors are (1) lowered
light penetration because of increased suspended particulate matter (turbidity).
One result of the higher turbidity nearshore is a turbidity increase above that
which formerly obtained in the waters over Barracuda Ground in the new gradient
now established to the still clear waters in the western section of Area III.
(2) Polluting substances, including those from the bypass sewage discharges
and hydrocarbon leaches and spills, etc., carried seaward in the prevailing
harbor current. Without baseline data, or long time series observations it
is not possible to establish if there had been in the recent past more abundant
The section of Area II around the sewage outfall is obviously under
severe stress. Numerous dead corals, reduced species diversity and concen-
trations of fecal matter, paper and other suspended wastes were observed. The
status suggests to the reef ecologist that increasing the volume of sewage dis-
charged much above the present level could oveistep a limit beyond which much
more widespread destruction would be effected;
Round Reef has been almost completely extirpated as a result of develop-
ment and use of the harbor. Only scattered colonies of Porites and occasional
Acropora were noted alive. Turbidity was high, and visibility was less than
two meters. It is impossible to assign a single cause for this particular reef kill.
All of the stress factors discussed above for Barracuda Ground are to be found here
and at much higher levels, except that the level of wave energy and the resultant
coarse sand transport rate are lower. It is probable that the most severe of recent
stresses was dredging alongside the reef with the resultant sediment fall out onto
the reef. It is unlikely that the area will repopulate unless there is drastic change
in the harbor use pattern. Uses now promote high turbidity, fine sediment erosion
into the waters and low light levels, all of which make the Barracuda Ground hostile
to re-colonization by corals and coraline algae.
7. NOTES ON FISHERIES
Inasmuch as lagoons, bays and shallow harbors support fisheries
and shell fisheries of major economic value and are among the most productive
environments in the world (Odum, 1971), a cursory inquiry was made into the
status of fisheries of Christiansted. A comprehensive study of fisheries
potential throughout the Virgin Islands is given by Dammann (1969).
Fishes dependent on the reefs and banks include parrot fishes, grunts,
groupers, butterfly fishes, sharpnose puffers, and moray eels. In areas of
grassy bottom there are halfbeak, porgies, rays, and probably small tarpon.
Lagoon areas of mud or grass have black and white mullet. Among the fishes
that inhabit offshore waters, and occasionally are found in the harbor either in
search of food or to spawn, are mackerel, barracuda, snappers and kingfish.
Besides fishes, certain shellfish like conchs are reported, as also shrimp,
lobsters and green turtles.
As a result of blasting rock in the cement plant channel near bouy 4
on Sept. 18, 1971, John Yntema of the Virgin Islands Department of Conservation
reports: "I was surprised there were so many fish in so barren a spot. There
was one mutton snapper (Lutjanus analis) one immature french angelfish
(Pomocanthus paru), 20 to 30 french grunts (Haemulon flavolineatum), 15 to 20
mojarras (probably Gerres cinereus) and 6 to 8 squirrel fish (Holocentridae).
..... we also spotted a green turtle of about 30 to 50 lbs."
Reports of local fishermen indicate a sport and commercial fishery
thrived in the harbor more than ten years ago. Degradation of the industry
was blamed on a distillery in the southwest corner of the harbor which
occasionally discharged effluents, causing massive fish kills. Although the
distillery was closed about 1965 the number of fish have not returned.
Similarily, Altona Lagoon, which formerly was very productive, has been
restricted at its entrance from a natural width of 15 m, to a 1. 9 m diameter
culvert, causing a near-stagnant environment unsuited to many fish. In a
report by F. J. Mvather (1957) it was noted:
"Some inshore fishing was done in Christiansted Harbor. In early
April, a 23 1/2 lb. tarpon was caught from the Comanche dock, and a 31 lb.
barracuda was taken by trolling in the harbor. A short casting trip was made
in Altona Lagoon. Although no fish were caught, many were seen jumping or
swimming close to the surface. It is reported that there are some large jacks
and many bonefish in this lagoon. It is recommended that the entrance to this
lagoon be kept open to provide a good fishing ground for inshore fishermen..."
These notes suggest the harbor had a fisheries potential but what remains
needs to be fully evaluated and compared to that of other coastal fisheries about
St. Croix. before its importance, future trends, and the impact of environmental
status can be assessed.
8. BOTTOM GRASS AND ALGAE
More than 60 percent of the harbor floor and contiguous reef is
covered with marine grass and algae. The chief rooted plants are broad-
bladed turtle grass, Thalassia testidinum and round bladed grass Syringodium
filiforme. Species of Chaetomorpha and Diplanthera and occasionally Sargassum
are often interdispersed in minor amounts. Blades of turtle grass reach heights
of 30-40 cm above the bottom and act as a "baffle" in trapping current-borne
suspended material (Multer, 1969). Near the bottom matted stems and roots
form a firm carpet with the sediments and stabilize the bottom. Animals of
great diversity and beauty live in the "meadows" including sea cucumbers, tube
worms, molluscs, fishes, urchins, and welks plus polychaete worms and the
burrowing shrimp that churn up the bottom into depressions and conical mounds
such as shown in Figure 15. In addition to these grass-consumers, filter feeders
keep the water clear by processing decomposed grass. As a result, organic
material buried beneath the meadows is scarce and plant detritus throughout
the harbor is remarkably low (Figure 22C) less than 2 percent except for one
station, despite the potentially high gross production of Thalassia beds (Odum
and Wilson, 1962).
The filamentous algae Enteromorpha sp, mixed occasionally with
Cladophora sp., forms a thin bright pistachio green carpet over turbid reaches
of the inner and eastern harbor. Additionally, it covers the floor of less turbid
Mounds built up 20-30 cm high by burrowing shrimp or polychaete
worms such as common to turtle grass beds of Christiansted Harbor.
Photo reproduced from Multer (1969).