A Beginner's Guide to
Water Management Water Clarity
Information Circular #103
Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
^ FLORIDA F
teInstitute of eearood and
Institute of Food and Agricultural Sciences stu::,::',sT AWATC
This publication was produced by:
Florida LAKEWATCH 2001
University of Florida / Institute of Food and Agricultural Sciences
Department of Fisheries and Aquatic Sciences
7922 NW 71st Street
Gainesville, Florida 32653-3071
Web Address: http://lakewatch.ifas.ufl.edu
Copies are available for download from the LAKEWATCH web site:
A Beginner's Guide to
Water Management Water Clarity
Information Circular #103
Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Ecasystem n Research
Institute of Food and Agricultural Sciences LAK'WATCH
'-- ,--. 9Z7|
WT ater clarity, the clearness or transparency of water, is one of the most noticeable :: :
of a waterbody. It's also something of great importance to many people. The public
i::. 1 water i:, by what they can see, and so their evaluation is often based on this
standard. For example, lakes with very clear water may be perceived as good, ::::. .. i::i. i or pristine,
while lakes with limited t: .: ; : .. may be described as undesirable, polluted, or degraded.
Contrary to this popular perception, crystal clear water is NOT the ruler to which all lakes
should be compared. It is not true that lakes with lower ': ::. : ::. ::. are necessarily the result of
pollution or degradation. In Florida, lakes with a wide range of water clarity occur : :.:.::.:i even in
locations that are unaffected by human :".:: .; It is also not true that clearer water is safer to swim
in or to drink. On the contrary, clear water is just as likely as murky water to harbor ..:?:
bacteria, or other contaminants that could be harmful to human health.
In some instances, the standard for water clarity is often influenced by regional values or ideas
about how the waterbody is to be used. For. ;:::i.1. in Canada, the Canadian government recommends
that water should be ::ii :- :-:: :; clear so that a Secchi disc is visible at a minimum depth of 1.2
meters (about 4 feet). This recommendation stems from the fact that swimmers want their swimming
areas to be clear enough to see underwater obstacles. The 1.2-meter water clarity standard is one
reason many of the lakes in Canada, particularly those with an abundance of free-floating algae, do
not meet C.:::.:: : ::: standards for swimming and are deemed "undesirable." However, it should be
understood that many of these lakes have water clarity less than 1.2 meters :: ,j::: :'; and have not
been impacted by human activity.
Similar conditions exist in Florida, :: many people : :. :: .. that less clear water is undesirable.
However, one's preference for clear water is a value judgement, not a scientific measure, and should
be based on how people envision using the waterbody. For :::. 1. less clear waters \ i
support abundant populations of i: i 1,:::i birds, or other ..: :i:;. creating i*: : :::: i. for
popular outdoor activities such as i i: : ::.. and nature watching. In fact, some of Florida's
best fishing is found in murky, algae-rich waters.
In light of such popular misconceptions surrounding water clarity, one thing is clear all
Florida residents and visitors stand to benefit from a greater understanding of the dynamics and
S .. i .... of water clarity in Florida lakes. This circular i.:. ;.1. a first step by *: :: :::. a -.
important strategies used to manage water clarity. Basic information about water clarity, with an
: on its relationship to algal growth in lakes, is .: A. .1. .1 in the following segments:
1 Measuring Water Clarity
2 What Affects Water Clarity?
3 Water Clarity and Biological Productivity
4 Managing Lakes for Water Clarity
Before you begin however, we encourage you to review the definitions for commonly used
scientific terms provided in Appendix A, particularly for .::-:-..* and chlorophyll. More comprehensive
information may also be obtained by reading A P': :.:. ;.: i's Guide to i ::`.:.: i,.:i;:.: :: ,;.': .:: The
ABCs (Circular #101) and A Beginner's Guide to Water 3...:..: -.-, ..::.,:,. Nutrients (Circular # 102).
These publications can be downloaded for free from the Florida LAKEWATCH web site:
T here are several devices used by scientists disc is lowered into the water to find the depth at
to measure turbidity, light extinction, and which it first vanishes from the observer's sight.'
spectral analysis-all related to water The Secchi disc was named after Pietro Angelo
clarity. However, for the purposes of this publication Secchi, a scientific advisor to the Pope and head of the
we've decided to focus on use of the Secchi disc, Roman Observatory in the mid 1800s. Commander
one of the oldest, easiest, and most economical Cialdi, the commander of the Papal fleet, actually
methods for measuring water clarity, devised the Secchi disc. Secchi was asked by Cialdi to
The LAKEWATCH program uses Secchi experiment with this disc in the coastal waters of the
disc measurements because, aside from the fact Mediterranean. The first disc was lowered from the
that they're easy and inexpensive to use, they Papal yacht 'lmmacolata Concezione and used to
provide us with an indirect way to measure the measure water clarity in the Mediterranean Sea on
biological productivity of a lake an important April 20, 1865. Since that time, Secchi discs have been
component of lake management. But first things used to measure water clarity in tens of thousands of
first. We'll start with the Secchi disc. waterbodies around the world.
For more on biological productivity, see
Secchi discs are
Section 3 Water Clarity andBiological often colored with
Productivity on pages 8-11. alternating black
The Secchi Disc qu grants,
as shown here.
Obtaining a lake's Secchi depth involves the LAKEWATCHuses
use of a plate-sized device called a Secchi disc plain white Secchi
(pronounced with several variations, but usually discs as seen on the
SEH-key disk). Secchi discs of various sizes can be opposite page.
used, but customarily it is an 8-inch diameter disc 1 On occasion, the Secchi disc can still be seen as it rests on the
with alternating black and white quadrants. However, lake bottom, or it may disappear into thick submerged aquatic
some disks are solid white in color. Aline, rope or macrophyte growth. While the depth at which this happens
chain is attached through the center of the Secchi disc furnishes some information about the water clarity, it is not
considered to be a measurement ofthe waterbody Secchi depth.
and is marked offin intervals like a ruler, usually in Also, the word "Secchi is always capitalized because it refers to
feet or meters. To measure a lake's Secchi depth, the the name of the individual whofirst used it.
To measure a lake's Secchi depth, a Secchi disc is lowered into the water to find the depth at
which it first vanishes from the observer's sight
after clarity in Florida lakes ranges apparent color is the color of a water sample that
between 0.7 feet and 38 feet. has NOT had particulates filtered out of the water; and
Differences in water clarity are true color is the color of a water sample that
primarily caused by the presence (or lack) of HAS had all particulates filtered out of the water.
dissolved substances and/or suspended particles
The measurement of true color is the one
in the water. However, to fully understand the
most commonly used by scientists. To measure
dynamics of how dissolved substances and/or
true color, the color of a filtered water sample is
suspended particles affect water clarity, there are te coo e o a tee ate sa l
a few things to consider. matched to one from a range of standard colors.
e fia tor f conEach of the standard colors has been assigned a
Several factors can impact the abundance of
number on a scale of "platinum-cobalt units"
dissolved substances and/or particles in the water
consequently impacting water clarity. For (abbreviated as either "PCU" or "Pt-Co units").
t an o e dis s On the PCU scale, a higher value of true color
example, the abundance of dissolved substances
represents water that is more darkly colored.
and/or particles in the water can be influenced by eesens aer a is more d l
Because dissolved organic compounds (i.e., color)
the presence of aquatic plants and/or the location
absorb sunlight as the light passes through the
of the waterbody. Seasonal variations in climate
water, Secchi depth values decrease as the
can also impact water clarity. These factors are
discussed in the following section amount of color in the water increases. Color in
discussed in the following section.
Florida lakes ranges
Dissolved Substances from 0 to over 400 PCU.
Dissolved organic substances or compounds Particulates
can come from many types of terrestrial and Particulates include
aquatic plants, and can color the water reddish or free-floating algae, called
brown, sometimes even to the point of appearing phytoplankton, as well
black. When there is an abundance of dissolved
organic compounds in the water, scientists often Clrium her solids suspended in
the water. These include
refer to the water as being "colored" or sometimes sand, clay, or organic particles stirred up from
sand, clay, or organic particles stirred up from
they'll refer to the waterbody as being a "dark" lake. the bottom, washed in from the shoreline, washed
the bottom, washed in from the shoreline, washed
There are two types of color that are measured .
Sin from the surrounding land, or brought in by the
in waterbodies: wind and rain. Because particulates absorb and
scatter sunlight as the light passes through the bottom sediments that would otherwise be stirred up.
water, Secchi depth (water clarity) values decrease It's also thought that aquatic macrophytes
as the amount of particulates in the water increases, keep phytoplankton levels down due to the wave
While all particles are known to affect water buffering action of the plants. As a result, algal cells
clarity, studies throughout the world have shown settle and are prevented from being mixed into the
that free-floating algae are the dominant particles water column.
influencing water clarity in most lakes. All three of these mechanisms are probably in
Scientists often estimate the amount of free- action simultaneously, influencing the amount of
floating algae in a lake by measuring the amount of free-floating algae found in the water column. There's
chlorophyll2 in a water sample, measured in units even a formula of sorts that can be used to estimate
of micrograms per liter (pg/L). Lakes in the the impact that aquatic macrophytes may have on
Florida LAKEWATCH database analyzed prior to whole lake water clarity:
January 2000 have average chlorophyll concentra- Using the Florida LAKEWATCH database,
tions ranging from less it's been observed that if aquatic macrophyte
than 1 to over 400 pg/L. coverage is less than 30% of the bottom area of a
waterbody, the presence of plants does not
A. greatly influence the amount of free-floating
Algae in open-water. However, lakes with aquatic
macrophyte coverage over 50% or more of the
The presence or bottom area typically have reduced chlorophyll
absense of aquatic concentrations and clearer water.
H macrophytes3 in a In fact, in a lake with aquatic macrophyte
Swaterbody is especially coverage greater than 50%, chlorophyll and
Important in under- nutrient concentrations may become so low and
Coontail (,,.,. standing water clarity
and yet this relationship is sometimes overlooked. The size of individual
It's also a double-edged sword. While water
clarity can affect the growth of aquatic macrophytes, part w r
the reverse is also true: the presence of large amounts or other suspended
of aquatic macrophytes can influence water clarity. particles, has a strong
influence on water clarity.
There are several explanations for this: il e on w cl
One explanation is that submersed macrophytes, To visualize this effect,
consider putting a solid
or perhaps the algae attached to them, use available s consider pufing a so
C stick of chalk into a
nutrients in the water, depriving the phytoplankton
,. \ bucket of water. Upon
(i.e., free-floating algae) of these same nutrients. Micrasteriasbucket of water Upon
(ie. putting the chalk stick
Consequently, when there is less phytoplankton in p g te c
S Into the bucket, you will still be able to see
the waterbody, water clarity is usually increased. ,
through the water to the bottom of the bucket.
Another explanation for the water clarity/aquatic thoh the a e ott of the b t.
If, however, the same amount of chalk is
macrophyte relationship is that submersed macro- g
ground into fine particles and placed into the
phytes anchor nutrient-rich bottom sediments in e l ec t
water, the water will become so murky that the
place, buffering the action of waves, and depriving tt the et will n b In this
bottom of the bucket will not be visible. In this
the free-floating algae of nutrients contained in
manner, when smaller particulates such as
2 Chlorophyll is a green pigment found in all plants and small algae dominate an aquatic system, the
abundant in nearly all algae. water clarity is lower than in waterbodies
3 Aquatic macrophyte is the scientific term for large where larger particles dominate assuming the
aquatic plants. See page 23 ofAppendix A for more total amount of particulate matter is the same.
the water become so clear that it could mistakenly
Based on these observations, it becomes
be described as a biologically unproductive lake.
important for lake managers and/or residents
And yet, the presence of such large amounts of ortat for e aa resi
macrophytes tells us that the lake is extremely to be aware of the fact that removal of
productive. In such an instance, the practice of large amounts of macrophytes can result
characterizing a lake based on its water clarity alone in reduced water clarity.
Take Lake Brant in Hillsborough County
c See Water Clarity and Trophic for example. See below for details on how the
State in Appendix A. introduction of grass carp affected chlorophyll
concentrations in the lake and quickly reduced the
Based on these observations, it becomes lake's water clarity.
important for lake managers and /or residents to However, it may be reassuring to note that
be aware of the fact that removal of large aquatic plant control efforts conducted on lakes
amounts of aquatic macrophytes can result in with less than 30% aquatic plant coverage do not
reduced water clarity, produce major increases in chlorophyll concen-
For instance, it's been observed that aquatic trations even though they result in the removal of
plant management efforts that reduce a lake's significant amounts of aquatic plants. It should
plant coverage from high levels (greater than 50% also be noted that planting a fringe of aquatic
coverage) to low levels (less than 30%) can result in plants around a lake generally does little to
major increases in chlorophyll concentrations improve water clarity unless the plants grow to
(i.e., phytoplankton) and reduced water clarity, cover a major portion of the lake bottom.
Be careful what you ask for...
By May 7, 1993 a total of 325 grass carp
had been stocked into Lake Brant, a
60-acre lake, to remove large areas of
submersed macrophytes. Within three
months, chlorophyll concentrations more
than doubled, as evident in the bar graph
below. Water clarity was reduced by half.
May 7, 1993
FLORIDA LAKEWA TCH
ake Brant in Hillsborough County Brant/Hllsborough County
provides us with an example of how the j -....phi, uth,. (-40 H-0I
removal of large amounts of macrophytes
can affect water clarity. The chlorophyll graph
shown here tells the story: Grass carp, a
herbivorous species of fish, were stocked into
the lake to remove (eat) nuisance plants from
the lake. The fish did such a good job that within
three months a large portion of the plants were
gone and chlorophyll concentrations were on the I I .. I Ill 111iiII I
rise. Food for thought, if you're contemplating ---- ----'- --_ --
large-scale aquatic macrophyte removal.
The geology and physiography of a lake's
watershed influences many characteristics of a lake,
including algal levels and the true color of the water.
Consequently, the location of a waterbody is strongly
linked to its water clarity. Here's how it works: t ,
Water flowing through a watershed to a lake
picks up substances such as nutrients (required for
algal growth) and humic acids (that color the
water). If a lake is located in an area with nutrient-
poor or well drained soils, runoff or seepage water -
percolating up from underneath the lake has little
affect on its water clarity. There are simply fewer
nutrients and/or dissolved substances being carried
into the lake. Walden Lake in Hillsborough County is located
Lakes in northwestern Florida (in Washington, in the Lakeland Bone Valley UplandLake
Bay, Calhoun, and Jackson counties) provide a Region in central Florida. Chlorophyll
good example. LAKEWATCH data collected concentrations for this lake are typically high,
from this area show that these lakes tend to have ranging above 80 ,.g L. Water clarity is
chlorophyll concentrations below 3 8ig/L, color typically low, with Secchi depths of less than
values generally below 10 PCU, and Secchi three feet. This can be explained by the highly
r an phosphatic sands the lake is situated upon.
depths greater than 10 feet. This is documented phosphatic sands the lake is situated upon.
in Lake Regions of Florida4 (EPA/R-97/127).
"Lakes in the New Hope R,, g~, Greenhead Slope
Lake Region are clear low in nitrogen andphosphorus, This strong link between location and water
low in chlorophyll, and are among the mostoligotrophic clarity suggests there may be natural limits on
lakes in the United States (Canfield 1981). the level of water clarity that waterbody managers
In contrast, lakes in the Lakeland/Bone and users can expect in a specific location.
Valley Upland Lake Region in central Florida Consideration of the lake region in which the lake
(Polk and Hillsborough counties) tend to have is situated will provide a useful perspective and
chlorophyll concentrations above 80 fg/L, color help managers and users evaluate the feasibility
values above 20 PCU, and Secchi depths less of different management goals.
than 3 feet. This can be explained by the nutrient-
rich and poorly-drained soils of the region 4 Lake Regions are geographical areas in which lakes have
documented in Lake Regions of Florida similar geology soils, chemistry, hydrology, and biological
features. In 1997, using Florida LAKEWATCH data and
(EPA/R-97/127): other information, the United States Environmental
"... the Bone Valley Uplands and the Bartow ,,..,, ., g,y, designated 47 lake regions in Florida,
Embayment, i ilthin White s (1970) Polk Upland using these similarities as their criteria. The results of this
physiographic region, tend to be more poorly project were published in a report Lake Regions of Florida,
drainedflatwoods areas. All of these areas are u,. G.E. etal. 1997, US. Environmental Protection
Agency (EPA/R-97-127). For a copy write: US. EPA, 200 SW
covered by phosphatic sand or clayey sandfrom 35th Street, Corvallis, Oregon 97333. For more information,
the Miocene-Pliocene Bone Valley Member of the see Lake Regions in Appendix A. You may also call the
Peace River Formation in the Hawthorn Group LAKE WATCH office for a printout of a specific lake region
(Scott 1992; Scott and MacGill 1981). The region description (ofyour lake, for example) or for the
generally encompasses the area of most intensive LAKEWATCH information pamphlet, Florida Lake Regions
Sen Classification System. Call 1-800-LAKEWATCH (1-800-525-
phosphate mining..." 3928).
+ Lakes with high chlorophyll levels (hypereutrophic
lakes) tend to have highly fluctuating monthly levels
of chlorophyll for most of the year, but tend to have
Seasonal variations in weather conditions such lower levels in December, January, and February.
as temperature, wind, and amount of rainfall are also
closely linked with a lake's water clarity. These It's important to note that just like Secchi values,
seasonal changes can affect water clarity by influencing these patterns don't always apply to all lakes;
both algal levels and color levels within a lake. maximum or minimum chlorophyll values in Florida
lakes can occur at any time during the year.
In Florida, it's been observed that water Color Levels
clarity in individual waterbodies varies in a pattern Changes in the true color of a waterbody seem
over the course of several years. For example, the to be strongly linked to the amount of seasonal
Florida LAKEWATCH database shows that: rainfall a watershed receives and the amount of
*Water clarity is greatest in lakes from December runoff into a waterbody. Runoff is the key factor to
through February. remember. During periods of drought, Florida
waterbodies tend to be clearer. Even though rainfall
Water clarity is lowest in lakes from March through May.
may be heavy as the drought abates, water color in
It should be noted that although these patterns the waterbody may not increase until there is sub-
are well documented, exceptions are common in stantial runoff When there is a lot of runoff, true
are well documented, exceptions are common in
that maximum or minimum Secchi depths can water color can increase quickly and substantially.
that maximum or minimum Secchi depths can
Sd a r m Studies of individual Florida lakes also show that
occur during any month. For more information
increases or decreases in color can significantly
on patterns in Florida lakes, see Brown et al.5 ..
influence a lake's water clarity. For example,
Grasshopper Lake in Lake County had Secchi depth
Algal Levels values greater than 12 feet during dry weather from
Similarly, chlorophyll concentrations (algae) in 1993-1994. Following heavy rains from 1995-1996,
Florida lakes can be highly variable over time and the same lake had Secchi depths of less than 3 feet.
have a direct effect on water clarity. Using the The corresponding chlorophyll concentrations
Florida LAKEWATCH database, a general seasonal averaged 1 [g/L during 1993-1994 and 4 pg/L
pattern of chlorophyll can be shown for many during 1995-1996.
Florida lakes. This pattern is described below: Although these data show an increase in chloro-
For lakes with low to moderately high chlorophyll phyll concentrations, the increase is not enough to
levels (oligotrophic to eutrophic), monthly chloro- account for such a drastic change in water clarity. So
phyll concentrations are typically lower than the how to explain such a drastic change in water clarity?
annual mean chlorophyll concentration from The difference in water clarity was related to
December to May. the additional color that washed into Grasshopper
Lake during the rainy years. Color concentrations in
During the months of August thru October,
SDuring the months of August thru October, the lake changed from 0 PCU to an observed tea
chlorophyll concentrations are typically higher than colored water approximatey 4 0 P ). Wi the
te a a m colored water (approximately 40-60 PCU). With the
the annual mean.
return of dry weather, water clarity increased as color
values fell below 2 PCU. This is why both chloro-
phyll concentrations and color should be monitored if
water clarity is a major lake management issue.
5 Brown, C.D., D.E. Canfield, Jr, R. W Bachmann, andM V
Hoyer 1998. Seasonal patterns ofchlorophyll, nutrient
concentrations and Secchi disc transparency in Florida lakes.
_________ Lake andReservoir Management 14: 60-76.
F loridians, like people throughout the insects, fish, etc.). Conversely, if algae are abundant
world, are concerned about water quality, in a lake, then we can generally estimate that
Determining the water quality of our there is the potential for more wildlife. In fact,
aquatic resources is a major responsibility of research in Florida lakes has shown that there is a
water managers and scientists. One way they direct correlation between chlorophyll concentra-
approach this task is to evaluate a waterbody's tions in a lake and the number of zooplankton,
biological productivity, fish, birds, and even alligators.
Biological productivity is defined as the Watr
ability of a waterbody to support life such as A o ,
As one might imagine, it's not always
plants, fish, and wildlife. However, measuring possible to sample lake water for chlorophyll
possible to sample lake water for chlorophyll
the ability of a waterbody to support all aquatic concentrations. (Not all research programs are
-. concentrations. (Not all research programs are
life is difficult and prohibitively expensive by fortunate enough to have dedicated LAKEWATCH
most standards. For this reason, many scientists volunteers collecting samples.) So how can we
volunteers collecting samples.) So how can we
try to estimate a lake's ability to support life by estimate the biological productivity of a lake
estimate the biological productivity of a lake
measuring a few basic parameters, namely .
measuring a few basic parameters, namely without collecting and analyzing water samples?
chlorophyll concentrations in water, water T t
Thanks to historical water chemistry data,
clarity, nutrient concentrations in water, and comparing
scientists noticed certain patterns when comparing
aquatic plant abundance. Read on to discover
aquatic plant abundance. Read on to discover chlorophyll and water clarity data. After looking
how these four parameters serve as important at hundreds of lakes, it became clear that, in most
clues to a lake's biological productivity lakes, as chlorophyll concentrations (phytoplankton)
Chlorophyll Concentrations increase, water clarity decreases.
Out of these four parameters, chlorophyll This led them to believe that, for the most
concentrations (i.e., phytoplankton) are used most part, they could begin to predict how biologically
often to estimate biological productivity because productive a lake is based on its water clarity. They
algae represent the actual base product of a lake's hypothesized that if lake water is not very clear, it's
food web. For example, if we know that chloro- more than likely due to an abundance of algae. The
phyll concentrations are low in a lake, then we presence of large amounts of algae suggests that
can generally estimate that the number of other the lake is a productive system providing an
aquatic organisms will be low especially those abundance of food for aquatic life.
that rely on algae for food (i.e., zooplankton, However, if a lake has clear water, it's more
likely to not be productive due to the small Whereas, if a lake has clear water, due to low
amounts of algae available to the food web. chlorophyll concentrations, but has large
This strong relationship between chlorophyll amounts of aquatic macrophytes, it can be stated
measurements and water clarity is why scientists that the lake is a biologically productive system.
have adopted use of the Secchi disc as an easy and But there's an additional twist to these
inexpensive way to determine a lake's biological relationships when considering the more biologi-
productivity. However, it should be noted that cally productive lakes. While the presence of
there are always exceptions; dissolved substances large amounts of aquatic macrophytes can affect
(color) in the water can greatly affect water water clarity,6 the reverse is also true; water clarity
clarity, as can suspended particles such as clay. can affect aquatic macrophyte growth. Picture
this: When lake water is turbid, sunlight can't
Nutnt Conentrtios penetrate as far into the water, limiting the
Just like the flowers in your garden or the
maximum depth at which aquatic macrophytes
grass in your lawn, algae and aquatic macrophytes
are also dependent upon nutrients for growth. Two Ti .
This inverse relationship between water
of the more important nutrients are phosphorus btenwe
of the more important es are phosphorus clarity and aquatic macrophytes suggests that the
and nitrogen. Both of these compounds are found
So biological productivity of a lake can shift between
naturally in rocks, soils, and even lake water.
Sp being a lake dominated with phytoplankton to a
While phosphorus and nitrogen concentra-
lake dominated by rooted aquatic macrophytes.
tions can certainly affect a lake's biological .Smiar to t and expense associate
Similar to the time and expense associated
productivity, the relationship between algae and h e s t
e. with collecting chlorophyll measurements, the
these nutrients can be somewhat complicated. collection of aquatic macrophyte data is not
collection of aquatic macrophyte data is not
For this reason, scientists often refer to other e n no
always feasible. Fortunately, now that we know
parameters such as chlorophyll concentrations or l. ate t
how closely linked water clarity is to aquatic
Secchi depth measurements to estimate a lake's c te ecci is a a
g macrophyte growth, the Secchi disc can be a
biological productivityuseful tool in predicting the potential for aquatic
plant growth. Water clarity or Secchi depth
c For more on nutrients and their relationship measurements can help scientists estimate the
to algal abundance, see Florida LAKEWATCH depth at which underwater aquatic macrophytes
Information Circular 102 A Beginner's Guide to w b r
will be expected to survive. A general rule of
Water Management Nutrients. .
Water Management Nutrientsthumb is that aquatic macrophytes can grow to a
depth of about 1.5 times the Secchi depth measure-
Aquatic Plant Abundance ment. For example, if a Secchi depth measurement
Aquatic plants are another indicator of a is three feet, the depth at which aquatic macro-
lake's biological productivity. If there are small phytes can grow is limited to about 4.5 feet.
amounts of aquatic macrophytes and algae, one
can generally state that the lake is unproductive. 6 For more on this, see Aquatic Plants in Part 2 on page 4.
When faced with the challenge of trying to
describe the various levels of biological productivity Oligotrophic (oh-lig-oh-TROH-fic) waterbodies
have the lowest level of biological productivity.
in a lake, scientists developed a system called the
Trophic State Classification System. Using Mesotrophic (mes-oh-TROH-fic) waterbodies
this approach, lakes are traditionally classified into have a moderate level of biological productivity.
four groups according to their level of biological Eutrophic (you-TROH-fic) waterbodies have a
productivity or trophicc state." high level of biological productivity.
The names of these four trophic states from
the lowest productivity level to the highest are Hypereutrophic (HI-per-you-TROH-fic) waterbodies
otro t i to hi a have the highest level of biological productivity.
oligotrophic, mesotrophic, eutrophic, and
Using average Secchi depth readings from more
Using Secchi Depth to than 500 Florida lakes in the LAKEWATCH
Determine Trophic State database (analyzed prior to January 2000), Florida
lakes were found to be distributed into the four
As discussed earlier in this section, overall
S. trophic states as follows:'
biological productivity is difficult to measure in trophic states as follows:
a lake. However, based on what we know about + approximately 7% of the lakes would be
the strong relationship between water clarity classified as oligotrophic (those with Secchi
(Secchi depth measurements) and chlorophyll depths greater than 13 feet);
concentrations, aquatic scientists often choose to about 22% of the lakes would be classified as
use Secchi depth measurements as an indirect mesotrophic(those with Secchi depths between 8
way of assessing biological productivity and its and 13 feet);
associated trophic state.
+ 45% of the lakes would be classified as
To do this, professionals may use the criteria ( w S d b
eutrophic (those with Secchi depths between 3
developed for lakes by two Swedish scientists, ad 8 feet;
and 8 feet); and
Forsberg and Ryding. There are other classification
systems available, but Florida LAKEWATCH uses 26% of the lakes would be classified as
the Forsberg and Ryding classification system hypereutrophic (those with Secchi depths less
because it seems to work well for Florida lakes. than 3 feet).
Forsberg and Ryding's trophic state classification
Using Algae to Determine Trophic State
system, using Secchi depth, is as follows: i e to Determine Trophic St
While Secchi depth readings can help us
Criteria for Determining Trophic State Based estimate a lake's biological productivity, at some
on Secchi Depth point, we may want to base a lake's trophic state
+ lakes with Secchi depths greater than 13 feet classification on algal levels (often measured as
are classified as oligotrophic; chlorophyll concentrations.)
+ lakes with Secchi depths ranging from 8 feet
to 13 feet are classified as mesotrophic;
S r7 This distribution oftrophic state is based solely on Secchi
* lakes with Secchi depths ranging from 3 feet depth values. It should be noted that trophic state
to 8 feet are classified as eutrophic; and determinations are more useful when scientists consider not
+ lakes having Secchi depths less than 3 feet are only Secchi depth but the concentrations of total nitrogen and
total phosphorus, chlorophyll concentrations, and aquatic
generally classified as hypereutrophic. macrophyte abundance.
Algae are the base product of a lake's food + 41% of the lakes would be classified as
web and give us a direct indication of a lake's eutrophic (those with chlorophyll concentrations
biological productivity. In other words, if algae are from 8 to 40 lg/L); and
abundant, then other forms of aquatic life will be 16% of the lakes would be classified as
abundant. Forsberg and Ryding's criteria for
hypereutrophic (those with chlorophyll concen-
chlorophyll concentrations (algae) are as follows': th
trations greater than 40 Clg/L).
Criteria for Determining Trophic State Based
on Chlorophyll Concentrations l l while Florida LAKEWATCH uses
* lakes with chlorophyll concentrations less than V criteria from the Forsberg and Ryding
or equal to 3 lg/L are classified as oligotrophic; trophic state classification system, it's
+ lakes with chlorophyll concentrations ranging important to know that other professionals in
from 4 to 7 lg/L are classified as mesotrophic; the water management arena may use a slightly
different set of criteria to determine trophic
* lakes with chlorophyll concentrations ranging state. Generally, the differences are not that
from 8 to 40 lg/L are classified as eutrophic; great, but non-professionals should be aware
* lakes having chlorophyll concentrations that they do occur.
greater than 40 [lg/L are generally classified as It's also important to understand that it
hypereutrophic. is a misuse of the trophic state classification
Using average chlorophyll concentrations system to use trophic categories as indicators
from more than 500 Florida lakes in the LAKE- of water quality. Each trophic state classification
WATCH database (analyzed prior to January has attributes that may be judged as having
2000), Florida lakes were found to be distributed "good" qualities or "poor" qualities.
into the four trophic states as follows:8 Judgements of quality depend largely
on how people want to use the waterbody.
* approximately 12% of the lakes would be For example, an oligotrophic waterbody
classified as oligotrophic (those wthh chlorophyll may be good for swimming because it will
concentrations less than 3 Clg/L); typically have clear water, but may not be a
* about 31% of the lakes would be classified as rewarding fishing site, because it does not
mesotrophic (those with chlorophyll concentrations support large fish populations.
ranging from 4 to 7 ig/L);
It's important to know that a lake may be
classified in more than one trophic state
depending on the criteria used. For example,
a lake with a chlorophyll concentration of
2 pg/L could be classified as oligotrophic based
on the amount of phytoplankton found in the
lake. However, the same lake, with a Secchi
depth of 4 feet could be classified as eutrophic,
based on its water clarity.
This inconsistency may seem troublesome 8 This distribution oftrophic state is based solely on chlorophyll
but it is, in fact, useful information. It tells us concentrations. Trophic state determinations are more useful when
scientists consider not only chlorophyll concentrations but also the
that the reduced Secchi depth could be related concentrations oftotal nitrogen and total phosphorus, Secchi depth,
to dissolved substances in the water (i.e., color) and aquatic plant abundance. For more on trophic states, see
or high sediment concentrations instead of LAKEWATCH information pamphlet entitled Trophic State:
htolankton abundanceA Waterbody's Ability to Support Plants, Fish, and Wildlife.
For aJfree copy call 1-800-LAKEWATCH (1-800-525-3928).
L ake management, or the management of The practice of managing water clarity by
any waterbody, should always begin with controlling algal growth has sparked an intense
the establishment of goals. And like interest in being able to predict how much change
anything else, lake management goals are often is likely to occur in water clarity, based on changes
as varied as the people who live on or use lakes. in phytoplankton abundance. Water managers
Some people are most interested in improving have a particular interest in being able to make
fishing, while others are concerned with an over- this type of prediction, because management
abundance of aquatic macrophytes, reducing boat strategies may only be considered successful
traffic, or preventing shoreline erosion. However, when water clarity is improved noticeably. For
because water clarity is such a noticeable attribute in example, it's been shown that even if a lake's
lakes, it could be listed as one of the top management chlorophyll concentration was reduced from 250
concerns for most lake users or residents. g/L to 50 .g/L (a five-fold reduction), Secchi
But how does one manage water clarity? Is depth measurements would most likely not
it necessarily good to have extremely clear water change noticeably. This is due to the hyperbolic
in a lake? When is there too much phytoplankton? relationship between water clarity and chlorophyll
These are questions that can only be answered concentrations.
based on our needs, activities, or expectations for
a particular lake. There are times when no matter & See hyperbolic relationships on pages 13-15.
what our preferences are for water clarity, nature
calls the shots and determines nutrient levels or In this instance, the cost-effectiveness and
phytoplankton concentrations, and thus water success of such a strategy may be questioned by
clarity, the citizenry. For this reason, managers and users
Hypothetically Speaking need a way to predict before-hand whether their
Let's say that our goal is to increase water proposed management strategy will produce signifi-
cant results or be worth the cost. The following
clarity on a hypothetical lake called My Lake. cant results or be worth the cost. The following
segment provides a mathematical approach for
Based on the large amount of data collected for p pp
-i making such predictions.
Florida lakes, it appears that potential strategies making such predictions.
for improving water clarity on My Lake would 9 This is generally true, except in waterbodies where
involve changing the abundance of phytoplankton.9 water clarity is influenced by other factors such as color
But is this always the case? or other non-algae particulates.
In their efforts to predict how specific Secchi Depth and Chlorophyll
management techniques will affect water clarity Figure 1 provides us with an excellent
in a lake, scientists and/or lake managers often example of how the relationship between Secchi
use mathematical techniques or models. Two of depth and total chlorophyll concentrations for
the more widely used mathematical techniques Florida lakes is a hyperbolic relationship. To
include the use of hyperbolic relationships and/ better understand the Secchi depth/chlorophyll
or empirical models. Read on to learn more relationship in Florida lakes, study Figure 1 to
about how these techniques can be used to see if you can recognize the following patterns:
predict water clarity. .
predict water clarity. Lakes with extremely low chlorophyll levels
Hare shown to potentially have high Secchi disc
readings (greater than 24 feet).
Science is often a matter of studying relationships + For lakes in the lower chlorophyll range,
among two or more variables. By observing the water clarity decreases rapidly as chlorophyll
way these variables relate to one another, scientists concentrations increase so rapidly that even
are able to spot relationships. small increases in chlorophyll levels produce
For example, when Secchi depth measurements substantial decreases in water clarity.
are plotted on a graph along with other lake variables + Once chlorophyll concentrations exceed 25 [pg/L
- such as phytoplankton abundance or the color (the chlorophyll value at the rounded comer of the
of the water patterns often emerge. graph), Secchi disc readings level off and change
Figure 1 on page 14 is an example of a little even when chlorophyll concentrations
hyperbolic relationship that emerges when Secchi increase significantly.
depth measurements were plotted with chlorophyll
concentrations on a graph. Figure 2 shows a Secchi Depth and Color
comparison between Secchi depth and color. There is a similar hyperbolic relationship
Notice the plotted points form distinctive "L" shapes between Secchi depth and true color (from dis-
or curves, also known as mathematical hyperbo- solved substances) in water. See Figure 2 for an
las, hence the phrase "hyperboolic relationships." illustration of this relationship. In this case, the
hyperbolic relationship has the following
+ Lakes with low color levels have a high
probability of having clear water.
+ For lakes in the lower color range (0 50 PCU),
water clarity decreases rapidly as color increases -
so rapidly that even small increases in color produce
substantial decreases in water clarity (Secchi depth).
+ Once color levels exceed 50 PCU (the color value
Sat the rounded portion of the graph), water clarity is
likely to be substantially reduced and remain
relatively constant for higher levels of color.
Figure 1 Secchi Depth and Total Chlorophyll
0 40 80 120 160 200 240 280 320
Total Chlorophyll (pg /L)
Figure 2 Secchi Depth and Color
0 200 400 600 800
Color (Pt-Co units)
The relationship between Secchi depth and total chlorophyll (Figure 1) and Secchi
depth and color (Figure 2) for Florida lakes are illustrated here as hyperbolic
These relationships are considered to be "hyperbolic" because the plotted points
form a curved "L" shape a mathematical hyperbola. While it may be difficult to
isolate individual data points on the graph, the overall image is what's important.
The knox\ledge that water clarity" is /Iperbolicull related to phytoplankton abundance"
and dissolved substances""" in lake water has significant implications for anyone interested in
managing a lake's water clarity
Graphing these relationships, as seen in Figures 1 and 2 (page 14) and below, provides
a quick way of interpreting or predicting how a lake's water clarity will "react" to increases
or decreases in phytoplankton abundance and or true color It all depends on where the water
clarity value for the lake is plotted on the graph whether it's above or below the rounded
corner of the hyperbolic "L" shaped curve Here's how it works
If the water clarity value for a lake, measured as Secchi depth, is plotted above (to the
left of) the rounded corner of the hyperbola, it means the lake is probably more susceptible to
dramatic changes in \water clarity if phytoplankton abundance or the color of the water should
happen to change Conversely. if a water clarity value for a lake is plotted below (to the right
of) the rounded corner of the graph, then the lake is less susceptible to change In other
words, lakes that already have lo\w water clarity will show negligible changes in \water clarity
when phytoplankton growth or color concentrations increase
There are exceptions to every rule. but these basic generalizations provide a good starting
point for managing lakes for water clarity
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If you happen to know what the average
E prc Mo s chlorophyll concentration and/or nutrient concen-
trations are for a lake over a given period of
More precise way of using the same water time, it's possible to plug those concentrations
chemistry data is to transform it into a into the equations and after doing a few calculations,
Mathematical format called an empirical estimate what the average water clarity should be.
model. An empirical model is an equation or a set of This can be taken one step further by plugging
equations derived from statistical analysis of a in hypothetical chlorophyll and/or nutrient concen-
specific set of data a chosen group of lakes. trations as a way ofpredicting what water clarity
should be. This type of exercise can be invaluable in
Using Empirical Models for Predicting determining whether or not a particular algae manage-
Water Clarity ment strategy is worth the cost of implementing. For
The following segment introduces four empirical example, is it worth a large expenditure of dollars to
models developed by Florida LAKEWATCH decrease phytoplankton levels through nutrient
staff, using the LAKEWATCH database. control if water clarity will only be increased from
The first model, the Secchi depth chlorophyll 0.5 foot of visibility to an estimated 1.0 foot?
empirical model on page 17 is used to identify
relationships between Secchi depth (water clarity) or step-by-step instructions on how to use
and chlorophyll concentrations (algae) in a lake. empirical models see page 17. Once
It can be used to predict water clarity, based on you've mastered the Secchi depth chlorophyll
chlorophyll concentrations. empirical model on page 17, try your hand at
The other three models, on pages 18 and 19, calculating each of the three chlorophyll nutrient
are chlorophyll- nutrient models. These models empirical models on pages 18-19.
are chlorophyll nutrient models. These models You may want to have your Florida LAKE-
relate chlorophyll concentrations to nutrients WATCH data packet handy so you can use your
(phosphorus, nitrogen, or both). Similar to the lake's average Secchi depth, chlorophyll, total
Secchi depth chlorophyll empirical model on phosphorus and/or total nitrogen concentrations
page 17, chlorophyll nutrient models can be used for the calculations. Or as mentioned earlier, you
can plug in hypothetical numbers to see how your
to predict how an increase or reduction of nutrients lake's phytoplankton levels might be expected to
might affect chlorophyll levels and thus water change.
clarity. In fact, chlorophyll nutrient empirical
models are now routinely used in conjunction i ril models
with the Secchi depth chlorophyll model to
develop lake management strategies for water
clarity. Here's how they work: In the empirical equations on pages 17-19,
you'll see the words "log" and antilogg." The
term log is an abbreviation for the
Surveys of lakes throughout the world and mathematical term logarithm. A logarithm is
Surveys of lakes throughout the world and
the "exponent that indicates the power to
whole-lake experiments have shown that which a number is raised to produce a given
chlorophyll concentrations in lakes are also number." [For the equation 102 = 100, the log
of 100 is 2. Using the equation 10 = 1000,
related to their nutrient concentrations, of 100 is singthe equation 10 = 1000
the log of 1000 is 3.]
especially phosphorus. Consequently, there has
The term antilog is an abbreviation for the
been a major effort to develop empirical models term an.
mathematical term antilogarithm. An
for chlorophyll phosphorus relationships, antilogarithm is "the number corresponding
chlorophyll nitrogen relationships, or to a given logarithm." [For the equation
102= 100, the antilog of 2 is 100. Using the
chlorophyll nutrient relationships
S- equation 103= 1000, the antilog of 3 is 1000.]
(using both phosphorus and nitrogen).
How To Use An Empirical Model
Consider that a hypothetical lake called My Lake has an average chlorophyll concentration
of 30 ILg/L and water clarity of 3.1 feet. Let's suppose that our lake homeowner's association is
interested in improving the water clarity by reducing the amount of algae in the lake. They decide
to decrease chlorophyll to 10 ftg/L. With the following empirical Secchi depth chlorophyll
model, developed from Florida LAKEWATCH data, we can plug in this hypothetical chlorophyll
concentration of 10 pg/L and "predict" what the water clarity is expected to be after reducing the
Log (Secchi) = 1.171 0.463 Log (Chlorophyll)
Where: Log is the common logarithm (base 10),
Secchi is the annual mean Secchi depth in feet, and
Chlorophyll is the annual mean chlorophyll concentration in [g/L.
To make this calculation...
use a calculator with a LOG button and follow these step-by-step instructions.
Step 1 Start by plugging in the hypothetical chlorophyll concentration of
10 pg/L into the equation (replace the word "chlorophyll" with the
number 10). Now find the Log of 10 on your calculator.
Tofind the log ofa number on your calculator type in the number on the key
pad (in this instance, type in the number 10), push the button marked "log, "
then push the "=" button. For this exercise, you should get an answer ofl.
Example: Log (Secchi) = 1.171 0.463 x Log (chlorophyll)
Log (Secchi) = 1.171 0.463 x Log (10)
Step 2 Multiply that number (1) by 0.463 (from the equation).
Example: Log (Secchi) = 1.171 0.463 x 1.0
Log (Secchi) = 1.171 0.463
Step 3 Now subtract 0.463 from 1.171.
Example: Log (Secchi) = 1.171- 0.463
Log (Secchi) = 0.708
Step 4 Find the antilog of your result. Tofind the antilog, leave the log (the
number from the right side of the equation) on the calculator You should
see the Number 0.708. While that number is on your screen, push the
antilog key, which is usually represented by the symbol 10f, then push the
= button. (Ifyour calculator doesn 't have the 10, button, check the instruction booklet.)
You should get an answer of 5.1, which is your predicted Secchi depth in feet.
For Florida lakes, the following empiricalchlorophyll phosphorus model has been
developed from the Florida LAKEWATCH database of 534 waterbodies. Using this model,
you can predict phytoplankton abundance (chlorophyll concentrations) by plugging in a
hypothetical total phosphorus concentration for a lake. [See the example How To UseAn Empirical
Model for step-by-step instructions on how to do the calculations.]
Log (Chlorophyll) = 0.369 + 1.053 Log (TP)
Where: Log is the common logarithm (base 10),
Chlorophyll is the annual mean chlorophyll concentration in [tg/L, and
TP is the annual mean total phosphorus concentration in pg/L.
Confidence Limit Statement:
Data analysis shows this model has a 95% confidence interval that ranges from 30% to 325%. For more
on confidence limits, see How Much Confidence Can You Have In An Empirical Model? on page 20.
Empirical chlorophyll nitrogen models can be derived in a manner similar to that
described for the chlorophyll phosphorus model above. For Florida lakes, the following
empirical chlorophyll nitrogen model has been developed from the Florida LAKEWATCH
database of 534 waterbodies. Using this model, you can predict chlorophyll concentrations
(phytoplankton levels) by plugging in a hypothetical total nitrogen concentration for a lake.
[See the example How to UseAn Empirical Model for step-by-step instructions. Apply the same steps to the
Log (Chlorophyll) = 2.42 + 1.206 Log (TN)
Where: Log is the common logarithm,
Chlorophyll is the annual mean chlorophyll concentration in [tg/L, and
TN is the annual mean total nitrogen concentration in [tg/L.
Confidence Limit Statement:
Data analysis shows this model has a 95% confidence interval ranging from 23% to 491% for
predicted chlorophyll concentrations (compared to 30% to 325%for the previous phosphorus-
chlorophyll model). For more on confidence limits, see How Much Confidence Can You Have In
An Empirical Model? on page 20.
The most reliable model is an empirical chlorophyll nutrient model that factors in both
phosphorus and nitrogen concentrations to predict chlorophyll levels. Using this model,
you can predict phytoplankton levels (chlorophyll concentrations) by plugging in hypothetical
total phosphorus and total nitrogen concentrations for a lake. For Florida lakes, the following
empirical nutrient-chlorophyll model has been developed from the Florida LAKEWATCH
database of 534 waterbodies. [See the example How to Use an Empirical Model for step-by-step instruc-
tions. Apply the same steps to the equation below.]
Log (Chlorophyll) = 1.10 + 0.91 Log (TP) + 0.321 Log (TN)
Where: Log is the common logarithm (base 10),
Chlorophyll is the annual mean chlorophyll concentration in [tg/L,
TP is the annual mean total phosphorus concentration in [tg/L, and
TN is the annual mean total nitrogen concentration in [tg/L.
Confidence Limit Statement:
Data analysis shows that this model is the best available model for Florida lakes. It has a 95%
confidence interval ranging from 33% to 312% for predicted chlorophyll concentrations. This is
the smallest confidence range for any published empirical chlorophyll- nutrient model that has been
tested for Florida lakes. The confidence interval is also smaller than those established for the simple
empirical phosphorus-chlorophyll (30% to 325%) or nitrogen-chlorophyll (23% to 491%) models.
For more on confidence limits, see How Much Confidence Can You Have In An Empirical
Model? on page 20.
Calculate this yourself
Using the chlorophyll phosphorus
empirical model example on page 18, we
Scientists often choose to answer this know that a chlorophyll concentration of 10 pg/L
question by calculating confidence limits for was predicted. We can use this predicted
their predictions. By doing a mathematical chlorophyll concentration of 10 pg/L along with
analysis from the same database used to create the 95% confidence limits of 30% (0.30) to
the empirical models, scientists can calculate these 325% (3.25), to do the following calculations:
confidence limits. 30% of 10 pg/L is 3 pg/L
A 95% confidence interval gives the range
of chlorophyll values that a measured chlorophyll 0.30 X 10 pg/L = 3 pg/L
should fall into 95% of the time. Confidence and
intervals can be smaller when the degree of 325% of 10 L is approx. 33 g
325% of 10pg/L is approx. = 33 pglL
certainty does not need to be as stringent (e.g., 90%
confidence, 85% confidence, etc. ). However,25 10 L = 33
water managers usually prefer to be more confident. In other words, the actual chlorophyll
Use of a 95% confidence interval reflects the value for this sample lake should be some-
desire of professionals to have their predictions where between 3 pg/L and 33 pg/L, 95% of
correct 95% of the time. the time.
To further explain this concept, let's use an
example of a lake with total phosphorus concentra-
tions of 20 pg/L. If we plug this lake's total Empirical Models and Their
phosphorus concentration of 20 pg/L into the Limitations
chlorophyll phosphorus empirical model (see While the confidence interval for this empirical
page 18), we find that the lake is predicted to model may seem large (30% to 325% is a rather
have a total chlorophyll concentration of approxi- expansive range), it's not unusual. The confidence
mately 10 pg/L. limits of even the most reliable empirical model
How much confidence can we have in this can yield a broad range of chlorophyll values.
prediction? The confidence limits provided with the
According to our calculations, the 95% three nutrient empirical models in this circular
confidence limits for that particular chlorophyll are based on Florida LAKEWATCH lakes and
phosphorus empirical model ranges from 30% to truly reflect the variability of chlorophyll con-
325%. In other words, there is a 95% confidence centrations found in waterbodies in this state.
that the actual chlorophyll concentration will fall (Look for confidence interval statements at the
somewhere between 3 pg/L and 33 pg/L. See bottom of each of the nutrient empirical models
Calculate this yourself (top right) for an explana- on pages 18 and 19.) Such variability makes
tion of how these percentages (30% 325%) were predictions from all empirical chlorophyll nutrient
translated into whole numbers (3 pg/L 33 pg/L). models somewhat uncertain, particularly when
only small changes occur in nutrient concentrations.
Also, keep in mind that when dealing with
real waterbodies, as opposed to hypothetical
ones, there is a broad range of possible chlorophyll
concentrations that can occur based on any
specific amount of nutrients in the system.
_. It's difficult to predict precise quantities when
S-- .dealing with real-world waterbodies and the
Probably the most important
lesson to be learned from
empirical models is that,
in Florida lakes, it's been found
that small changes in nutrient
concentrations will not
produce noticeable changes in
water clarity, except perhaps in
lakes with generally low
In other words,
if you want to decrease
(meaning algal levels) to the
point where people actually
see a change in water clarity,
you will have to dramatically
multitude of factors that can come into play. While there are several empirical models
Because other environmental factors such as currently being used throughout Florida, we
local climate, geology, and aquatic macrophytes strongly suggest that lake managers and citizens
can also influence phytoplankton levels, managers consider using the Secchi depth chlorophyll
may make their predictions more accurate by model (page 17) as well the three chlorophyll -
developing empirical models using data from nutrient empirical models provided in this
waterbodies within the same local geographic circular (pages 18 19). These models are based
region. When developing these empirical mod- on a large number of Florida lakes and offer a
els, a basic understanding of how waterbodies good starting point for determining the most
function in that area should be combined with appropriate management options for your lake or
the best available data. waterbody.
Of course, there are instances when an Lastly, remember that empirical models merely
individual lake may fall outside the predictions provide a framework for evaluating how changing
found while using any empirical model. When nutrient concentrations could affect phytoplankton
this happens, it's important for that lake to be levels in a lake, and thus water clarity. These
studied independently of others in its region to models provide a guide, not absolute answers.
find out what is "driving" the phytoplankton
productivity of the lake.
Algae only the phytoplankton. Chlorophyll concentra-
are a wide variety of tiny, often microscopic, plants tions are measured in units of micrograms per
(or plant-like organisms) that live both in water and Liter (abbreviated pg/L) or in milligrams per cubic
on land. The word "algae" is plural (pronounced meter (abbreviated mg/m3).
AL-jee), and algaa" is the singular form (pronounced + In certain cases, scientists prefer to count and
AL-gah). measure individual algal cells in a sample and use
One common way to classify water-dwelling their count to calculate the volume of the algae.
algae is based on where they live. Using this system, Most people consider algae to be unsightly,
three types of algae are commonly defined as particularly when it is abundant. For instance, a
follows: phytoplankton bloom can make water appear so
+ phytoplankton float freely in the water; green that it's described as "pea soup."
In Florida, when chlorophyll concentrations
+ periphyton are attached to aquatic vegetation reach a level over 40 g/L some scientists will
or other structures; call it an "algae bloom" or "algal bloom." The
* benthic algae grow on the bottom. public, however, usually has a less scientific
Algae may further be described as being approach. They often define an algal bloom as
Algae may further be described as being
sind, c l ( d t r in whenever more algae can be seen in the water
single-celled, colonial (grouped together in
than they are accustomed to seeing (even though
colonies), or filamentous (hair-like strands). The
Sr this may be a low concentration in some cases).
most common forms of algae are also described
,. Algal blooms may be caused by human
by their colors: green, blue-green, red, and Algal blooms may be caused by human
y t l eei,. e d activities, or they may be naturally occurring.
yellow. All these classifications may be used
yelw Al te .c i bSometimes, what seems to be an algal bloom is
together. For example, to describe blue-green, hair- Smete, wht s s be an algal boo i
merely the result of wind blowing the algae into a
like algae that are attached to an underwater plant, ere te re wind owing te agae into a
cove or onto a downwind shore, concentrating it in
you could refer to them as "blue-green filamen-
tous periphyton." a relatively small area. This is called "windrowing."
In addition to describing types of algae, it is The Role of Algae in Waterbodies:
useful to measure their quantity. The amount of Algae are essential to aquatic systems. As a
algae in a waterbody is often called algal vital part of the food web, algae provide the food
biomass. Scientists commonly make estimates necessary to support all aquatic animal life.
of algal biomass based on two types of measurements: + Filamentous algal blooms and benthic algal
+ Because most algae contain chlorophyll (the blooms have the potential to interfere with
green pigment found in plants), the concentration recreational uses like boating and fishing.
of chlorophyll in a water sample can be used to + An algal bloom can trigger a fish kill. In
indicate the amount of algae present. This method Florida, this is most likely to occur after several
however, does not include all types of algae, days of hot weather with overcast skies.
Aquatic Macrophytes plant that is thought to be from Central and South
are aquatic plants that are large enough to be America, and hydrilla, an exotic submersed
apparent to the naked eye. In other words, they are aquatic plant that is thought to be from Asia.
larger than microscopic algae. The general phrase However, the term "weed" is not reserved for
"aquatic plants" usually refers to aquatic exotic aquatic plants only. In some circumstances,
macrophytes, but most scientists use it to mean natve aquatic plants such as cattails or Potamogeton
aquatic macrophytes and algae. (i.e., pondweed) can cause serious problems.
Aquatic macrophytes characteristically When assessing the abundance of aquatic
grow in water or in wet areas and are quite a plants in a waterbody, scientists may choose to
diverse group. For example, some are rooted in measure or calculate one or more of the following:
the bottom sediments, while others float on the + PVI (Percent Volume Iifct\,cJ or Percent
water's surface and are not rooted to the bottom. Volume Inhabited) is a measure of the percentage
Aquatic plants may be native to an area, or they of a waterbody's volume that contains aquatic
may have been imported (referred to as "exotic"). plants;
Most aquatic macrophytes are vascular + PAC (Percent Area Covered) is a measure of
plants, meaning they contain a system of fluid- the percentage of a waterbody's bottom area that
conducting tubes, much like human blood has aquatic plants growing on or over it;
vessels. Cattails, waterlilies, and hydrilla are
examples. Large algae such as Nitella, Lyngbya, frequency of occurrence is an estimate of the
and Chara are often included in the category of abundance of a specific aquatic plant; and
aquatic macrophytes. + average plant biomass is the average weight
Even though they are quite diverse, aquatic of several samples of fresh, live aquatic plants
macrophytes have been grouped into four general growing in a given amount of a lake's area.
The Role of Aquatic Macrophytes in Waterbodies:
* emergent aquatic plants are rooted in the
Aquatic macrophytes perform several
bottom sediments and protrude up above the
t r functions in waterbodies, often quite complex
waters surface; ones. A few are briefly described below.
* submersed aquatic plants primarily grow
submerged aquatic plants primarily grow Aquatic macrophytes provide habitat for fish,
completely below the water's surface; and ii r ic
wildlife, and other aquatic animals.
+ floating aquatic plants float on the water with A c ms p h a
+ Aquatic macrophytes provide habitat and food
roots suspended down into the water; .
roots suspended down into the water; for organisms that fish and wildlife feed on.
+ floating-leaved aquatic plants can be rooted to
bottom sediments and have leaves that float on Aquatic macrophytes along a shoreline can protect
bottom sediments and have leaves that float on
the water's surface. the land from erosion caused by waves and wind.
Aquatic macrophytes are a natural part of + Aquatic macrophytes can stabilize bottom
waterbodies, although in some circumstances sediments by dampening wave action.
they can be troublesome. The same plant may be + The mixing of air into the water that takes
a "desirable aquatic plant" in one location and a place at the water's surface can be obstructed by
"nuisance weed" in another. When exotic aquatic the presence of floating plants and floating-
plants have no natural enemies in their adopted leaved plants. In this way, they can cause lower
area, they can grow unchecked and may become oxygen levels in the water.
Slda f e m o d Floating plants and floating-leaved plants create
In Florida for example, millions of dollars
shaded areas that can cause submersed plants
are spent each year to control two particularly ae ae t s ersed
beneath them to grow slower and even die.
aggressive and fast-growing aquatic macrophytes
- water hyacinth, an exotic floating aquatic When submersed aquatic plants become abun-
dant, these plants can cause water to become clear. In Florida:
Conversely, the removal or decline of large Waterbodies in the Florida LAKEWATCH
amounts of submersed aquatic plants can cause database analyzed prior to January 2000, had
water to become less clear, average chlorophyll concentrations which ranged
from less than 1 to over 400 pg/L. Using these
+ When aquatic macrophytes die, the underwater
average chlorophyll concentrations from this
decay process uses oxygen from the water. If average chlorophyll concentrations from this
same database, Florida lakes were found to be
massive amounts of plants die simultaneously, a
fish kill can result due to low oxygendistributed into the four trophic states as follows:
fish kill can result due to low oxygen.
+12% of the lakes would be classified as olig-
+ Decayed plant debris (dead leaves, etc.) contrib-
utes to the buildup of sediments on the bottom otrophic (those with chlorophyll values less than
utes to the buildup of sediments on the bottom.
or equal to 3 pg/L);
Biological Productivity + about 31% of the lakes would be classified as
is defined conceptually as the ability of a waterbody mesotrophic (those with chlorophyll values
to support life (such as plants, fish, and wildlife), greater than 3 and less than 7 pg/L);
Biological productivity is defined scientifically + 41% of these lakes would be classified as
as the rate at which organic matter is produced. eutrophic (those with chlorophyll values greater
eutrophic (those with chlorophyll values greater
Measuring this rate directly for an entire waterbody than 7 and less than or equal to 40 g/); and
than 7 and less than or equal to 40 ug/L); and
is difficult and prohibitively expensive.
For this reason, many scientists base esti- nearly 16% of the lakes would be classified as
mates of biological productivity on one or more hypereutrophic(those with chlorophyll values
quantities that are more readily measured. These greater than 40 pg/L).
include measurements of concentrations of In Florida, characteristics of a lake's
nutrients in water, concentrations of chlorophyll geographic region can provide insight into how
in the water, aquatic plant abundance, and/or much chlorophyll may be expected for lakes in
water clarity. The level of biological productivity that area. For example, water entering the water-
in a waterbody is used to determine its trophic bodies by stream flow or underground flowage
state classification, through fertile soils can pick up nutrients that
can then fertilize the growth of algae and aquatic
Chlorophyll plants. In this way, the geology and physiography
is the green pigment found in plants and in nearly of a watershed can significantly influence a
all algae. Chlorophyll allows plants and algae to waterbody's biological productivity.
use sunlight in the process of photosynthesis for Health Concerns:
growth. Thanks to chlorophyll, plants are able to Chlorophyll poses no known direct threat to
provide food and oxygen for the majority of human health. There are some rare cases where
animal life on earth, algae can produce toxins in high enough abundance
Scientists may refer to chlorophyll a, which to cause concern. However, toxic algae are
is one type of chlorophyll, as are chlorophyll b generally not a problem.
and chlorophyll c. Measurements of total chloro-
phyll include all types. Chlorophyll can be Eutrophic
abbreviated CHL, and total chlorophyll can be is an adjective used to describe the level of
abbreviated TCHL. biological productivity of a waterbody. Florida
The Role of Chlorophyll in Waterbodies: LAKEWATCH and many professionals classify
Measurements of the chlorophyll concentra- levels of biological productivity using four trophic
tions in water samples are useful to scientists. For state categories (oligotrophic, mesotrophic,
example, they are often used to estimate algal eutrophic, and hypereutrophic). Of the four
biomass in a waterbody and to assess a trophic state categories, the eutrophic state is
waterbody's biological productivity, defined as having a high level of biological
productivity, second only to the hypereutrophic category. Geologic Region
The prefix "eu" means good, well, or sufficient. is an area that has similar soils and underlying
A eutrophic waterbody is capable of pro- bedrock features. The characteristics of the
during and supporting an abundance of living geologic region in which a waterbody is located
organisms (plants, fish, and wildlife). Eutrophic may be responsible for the water's chemical
waterbodies generally have the characteristics characteristics and trophic state. Geology can also
described below: have a significant influence on the shape of a
* Eutrophic lakes are more biologically productive waterbody's basin, a factor that affects many of
than oligotrophic and mesotrophic lakes and are features of a waterbody.
often some of Florida's best fishing lakes. They
usually support large populations of fish, including Hypereutrophic
sportfish such as largemouth bass, speckled is an adjective used to describe the level of
perch (black crappie), and bream (bluegill). biological productivity of a waterbody. Florida
LAKEWATCH and many professionals classify
* Typically, eutrophic waters are characterized .
S i ,. e levels of biological productivity using four trophic
as having sufficient nutrient concentrations to
state categories oligotrophic, mesotrophic,
support the abundant growth of algae and/or eutrophic, and hype
Sp eutrophic, and hypereutrophic.
aquatic plants. .
Of the four trophic state categories, the
* When algae dominate a eutrophic waterbody, hypereutrophic state is defined as having the
its water will have high chlorophyll concentrations highest level of biological productivity. The prefix
(i.e., greater than 7 pg/L). The water will be less "hyper" means over abundant. Hypereutrophic
clear, causing Secchi depth readings to be low. waterbodies are among the most biologically
In contrast, when instead of algae, aquatic plants productive in the world. Hypereutrophic
dominate a eutrophic waterbody, its water will waterbodies generally have the characteristics
have lower chlorophyll concentrations and often described below.
lower nutrient concentrations and clearer water.
Hypereutrophic waterbodies have extremely
The resulting water clarity will be reflected in hih utriet c nations
,. high nutrient concentrations.
Secchi depth readings that are greater than in
eutrophic waterbodies that have few aquatic +While hypereutrophic waterbodies can be
plants. dominated by non-sportfish species (gizzard
Despite being classified as eutrophic, these shad or threadfin shad), they can also support
plant-dominated waterbodies display the clear large numbers and large sizes of sportfish includ-
water, low chlorophyll concentrations, and low ing largemouth bass, speckled perch (black
nutrient concentrations that are more characteristic crappie) and bream (bluegill).
of mesotrophic or oligotrophic waterbodies. A hypereutrophic waterbody has either an
* Regardless of whether eutrophic waterbodies abundant population of algae or an abundant
are plant-dominated or algae-dominated, they population of aquatic macrophytes and
generally have a layer of sediment on the bottom sometimes it will support both.
resulting from the long-term accumulation of Hypereutrophic waterbodies that are dominated
plant debris. In some eutrophic lakes, however, by algae are characterized by having high chlo-
the action of wind and waves can create beaches rophyll concentrations (greater than 40 pg/L).
or sand-bottom areas in localized places. These waterbodies will have reduced water
* Eutrophic waterbodies can have occasional clarity, causing Secchi depth readings to be less
algal blooms and fish kills. However, fish kills than 1 meter (about 3.3 feet). In contrast, when
generally occur in hypereutrophic lakes when aquatic macrophytes instead of algae dominate a
chlorophyll concentrations exceed 100 pg/L. hypereutrophic waterbody, its water can have
lower chlorophyll concentrations. The resulting
water clarity will be reflected in higher Secchi The lake regions are mapped and described
depth readings (clearer water), mimicking those of in Lake Regions of Florida (EPA/R-97/127).
less biologically productive waterbodies. The Florida LAKEWATCH Program can provide
+ Regardless of whether a waterbody is plant- you with a free handout describing (1) how and
dominated or algae-dominated, typically it will why the lake regions project was developed; (2)
have organic bottom sediments as the decaying how to compare your lake with others in its Lake
plant and/or algal debris accumulates. Region; and (3) how the Lake Region Classifica-
tion System can be useful to you.
+ Hypereutrophic waterbodies may experience
frequent algal blooms. Limnology
+ Oxygen depletion may also be a common is the scientific study of the physical, chemical,
cause of fish kills in these waterbodies. and biological characteristics of inland (non-
marine) aquatic systems. A limnologist is a
Lake region scientist who studies inland aquatic systems.
is a geographic area in which lakes have similar
geology, soils, chemistry, hydrology, and biological Macrophytes
features. In 1997, using Florida LAKEWATCH See Aquatic Macrophytes.
data and other information, the United States Mean Depth
Environmental Protection Agency divided Florida
into 47 lake regions using these similarities as their is another way of saying "average water depth."
criteria. The mean water depth is measured in either feet or
Lakes in an individual lake region exhibit meters and is designated in scientific publications
remarkable similarities. However, lakes in one by the letter "z."
lake region may differ significantly from those in Mean depth can be estimated by measuring
a different lake region. For example, most lakes the water depth in many locations and averaging
in the New Hope Ridge/Greenhead Slope lake those values. Individual depth measurements
region in northwestern Florida (in Washington, may be taken by using a depth finder (fathometer)
Bay, Calhoun, and Jackson counties) tend to have or by lowering a weight, at the end of a string
lower total nitrogen, lower total phosphorus, lower or rope, into the water and measuring how far it
chlorophyll concentrations, and greater Secchi sinks below the surface until it rests on the bottom.
depths when compared to other Florida lakes. If more accuracy is needed, mean depth
While lakes in the Lakeland/Bone Valley should be calculated by dividing a waterbody's
Upland lake region in central Florida (in Polk volume by its surface area. This method will
and Hillsborough counties) tend to have higher often result in a different value than if measured
total nitrogen, higher total phosphorus, higher depths are averaged.
chlorophyll concentrations, and reduced Secchi Mesotrophic
depths when similarly compared.
thsin sii f a re. is an adjective used to describe the level of
Using descriptions of lake regions, water-
g dn of le r biological productivity of a waterbody. Florida
body managers can establish reasonable, attain- ioloial prodigy of a ae y
LAKEWATCH and many professionals classify
able water management goals for individual lakes.
l levels of biological productivity using four trophic
Lake region characteristics can also be used to ees o ioo roci strophic
state categories oligotrophic, mesotrophic,
help choose management strategies that are trophic, mesot
likely to be effective in achieving management etophic, and hypereutrophic.
Of the four trophic state categories, the
goals. In addition, lakes with water chemistry te or state te
that differs markedly from that of other lakes in esotrophic state is defined as having a moderate
g level of biological productivity. The prefix "meso"
the same lake region can be identified and
means mid-range. A mesotrophic water-body is
investigated to determine the cause of their being
atypical capable of producing and supporting moderate
populations of living organisms (plants, fish, and farms, yards, and streets anywhere in the
wildlife). Mesotrophic waterbodies generally have: watershed.
+ moderate nutrient concentrations; Oligotrophic
+ moderate growth of algae, aquatic plants or both; is an adjective used to describe the level of
+ water that is clear enough (visibility between biological productivity of a waterbody. Florida
8 and 13 feet) that most swimmers are not LAKEWATCH and many professionals classify
repelled by its appearance and can generally see levels of biological productivity using four trophic
any potential underwater hazards. state categories oligotrophic, mesotrophic,
eutrophic, and hypereutrophic.
Nitrogen Of the four trophic state categories, the
is an element that, in its different forms, stimulates the oligotrophic state is defined as having the lowest
growth of aquatic plants and algae. level of biological productivity. The prefix
Nutrients "oligo" means scant or lacking.
An oligotrophic waterbody is capable of
are chemicals that algae and aquatic plants need for producing and supporting relatively small populations
their growth. Nitrogen and phosphorus are the of living organisms (plants, fish, and wildlife).
two most influential nutrients in Florida waterbodies. The low level of productivity in oligotrophic
Nutrients can come from a variety of sources. waterbodies may be caused by there being a low
In most cases, nutrients are carried into a level of a limiting nutrient in the water, particularly
waterbody primarily when water drains through nitrogen or phosphorus, or by limiting environ-
the surrounding rocks and soils, picking up mental factors other than nutrients.
nitrogen and phosphorus compounds along the Oligotrophic waterbodies generally have
way. For this reason, knowledge of the geology the following characteristics:
and physiography of the area can provide insight
~ ~ h ntiet e i cn i Because nutrients are typically in short supply,
into how much nutrient enrichment can be
into how uc nutrient enric ent can be aquatic plants and algae in oligotrophic water-
reasonably expected in an individual waterbody bodies are in low abundance.
bodies are in low abundance.
from this natural source.
For example, lakes in the New Hope Ridge/ An oligotrophic waterbody typically will have
Greenhead Slope lake region in northwestern little plant debris accumulated on the bottom
Florida (in Washington, Bay, Calhoun, and since aquatic plants and algae are in low abundance.
Jackson counties) can be expected to have low + Oligotrophic waterbodies will often tend to
nutrient levels, because they are in a nutrient- have clear water, because the clarity is not
poor geographic region. While lakes in the Lake- diminished by the presence of free-floating algae
land/Bone Valley Upland lake region in central in the water. The clarity may be decreased,
Florida (in Polk and Hillsborough counties) can be however, by the presence of color, stirred-up
expected to have high nutrient levels, because bottom sediments, or washed-in particulate matter.
the land surrounding the lakes is naturally
te n rrnin te e Fish and wildlife populations will generally be
Snutrient-ri. small, because food and habitat are often scarce.
There are many other sources of nutrients.
hare geeraly not as sus l as nutrient Oligotrophic waterbodies usually do not support
that are generally not as substantial as nutrient a -
abundant populations of sportfish such as large-
contributions from surrounding rocks and soils.
mouth bass and bream, and it usually takes longer
Some occur naturally, and some are the results of f i g my
for individual fish to grow in size. Fishing may be
human activity. For example nutrients are conveyed. .
an acivit For example nutrients are co good initially if the number of anglers is small, but
in rainfall, stormwater runoff, seepage from g y g
can deteriorate rapidly when fishing pressure
septic systems, bird and animal feces, and the air
increases and fish are removed from the waterbody.
itself. Most nutrients can move easily through the
environment. They may come from nearby woods, A waterbody may have oligotrophic charac-
teristics even though it has high nutrient levels. Phosphorus
This can occur when a factor other than nutrients is an element that, in its different forms, stimulates the
is limiting the growth of aquatic plants and algae. growth of aquatic plants and algae in waterbodies.
For example, where a significant amount of
suspended sediments (stirred-up sediments or Physiographic region
particles washed in from the watershed) or darkly is a geographic area whose boundaries enclose
colored water is retarding plant growth by territory that has similar physical geology (i.e., soil
blocking sunlight. types, land formations, etc.).
is an abbreviation for percent area covered and are small, free-floating aquatic plants that are
is a measure of the percentage of a waterbody's suspended in the water column. They are
bottom area that has aquatic plants growing on sometimes called planktonicc algae" or just
or over it. Scientists use PAC to assess the "algae." Though small, phytoplankton perform
abundance and importance of aquatic plants in a important functions in waterbodies. For example,
waterbody. phytoplankton abundance often determines how
Waterbodies in the Florida LAKEWATCH biologically productive waterbodies can be -
database analyzed prior to January 2000, had how much fish and wildlife waterbodies can
PAC values that ranged from 0 to 100%. PAC support. Also, the public is concerned about the
values are linked with the biological productivity abundance of phytoplankton, because it
trophicc state) of waterbodies: significantly affects water clarity.
+ In the least productive (oligotrophic) water- Aquatic scientists assess phytoplankton
bodies, PAC values are usually low. In rare cases relative abundance by estimating its biomass.
where PAC values are high (occasionally reach- Two common methods are used: (1) viewing
ing 100%), it is usually due to a thin layer of phytoplankton through a microscope and counting
small plants growing along the bottom. them, and (2) measuring the chlorophyll concentra-
tions in water samples. Florida LAKEWATCH
* In moderately productive (mesotrophic) and uses the chlorophyll method because it's faster
highly productive (eutrophic) waterbodies, PAC and less costly.
values are generally greater than those measured
in oligotrophic waterbodies, and the average Planktonic Algae
plant biomass is also greater. See Phytoplankton.
+ In extremely productive (hypereutrophic) PVI
waterbodies that are dominated by algae, PAC
values are often less than 25%. In Florida how- is a measure of the percentage of a waterbody's
ever, many hypereutrophic waterbodies contain volume that contains aquatic plants. Historically,
mostly aquatic plants, not algae. In these cases, PVI represented the percent volume infested with
PAC values often tend to be greater than 75%. aquatic plants. Recently, it has become an
abbreviation for the more neutral phrase percent
Particulates volume inhabited. Regardless of the terminology,
are any substances in the form of small particles PVI is used to assess the abundance of aquatic
that are found in waterbodies, often suspended in plants in a waterbody.
the water column. Substances in water are either In Florida:
in particulate form or in dissolved form. Passing Numerous plant surveys performed on
water through a filter will separate these two Florida LAKEWATCH lakes have shown that
forms. The filter will trap most of the particulates, prior to January 2000, PVI values ranged from 0
allowing the dissolved substances to pass through. to 100%. In Florida, PVI values are strongly
linked with the biological productivity trophicc Total Nitrogen
state) of waterbodies as described below:
state) of waterbodies as described below: is a measure of all the various forms of nitrogen
+ In the least biologically productive waterbodies, that are found in a water sample. Nitrogen is a
(oligotrophic) PVI values are generally low. necessary nutrient for the growth of aquatic
+ In moderately biologically productive macrophytes and algae. Not all forms of nitrogen
waterbodies (mesotrophic) and highly productive can be readily used by aquatic macrophytes and
waterbodies (eutrophic) dominated by aquatic algae, especially nitrogen that is bound with
plants, PVI values are higher than those measured dissolved or particulate organic matter. The
in oligotrophic waterbodies. chemical symbol for the element nitrogen is N,
and the symbol for total nitrogen is TN.
+ The most highly biologically productive Total nitrogen consists of inorganic and
(hypereutrophic) waterbodies that are dominated organic forms. Inorganic forms include nitrate
by algae usually have low PVI values. However, (NO), nitrite (NO2), unionized ammonia (NH3),
hypereutrophic waterbodies dominated by inied ni (N ), and nioen gas (N)
aquatic plants usually have high PVI values. io aid an p i are naturally-occurring
Amino acids and proteins are naturally-occurring
Secchi depth organic forms of nitrogen. All forms of nitrogen
is a measurement that indicates water clarity, are harmless to aquatic organisms except union-
is a measurement that indicates water clarity.
ized ammonia and nitrite, which can be toxic to
Traditionally, the transparency or water clarity of a .
fish. Nitrite is usually not a problem in
waterbody has been measured using an 8-inch .
aterd a en as s a waterbodies because nitrite is readily converted to
diameter disc called a Secchi disc, that was
named in honor of its inventor. A Secchi disc is
usually painted in alternating quadrants of black The Role of Nitrogen
and white, although it can be solid white. There in Waterbodies:
is a line (a rope or chain) attached through the Like phosphorus, nitrogen is an essential
Secchi disc's center that is marked off in nutrient for all plants, including aquatic macrophytes
intervals, usually in feet or meters. and algae. In some cases, the inadequate supply
To use the Secchi disc to measure water clarity, of TN in waterbodies has been found to limit the
it's lowered into the water to find the depth at growth of free-floating algae (i.e., phytoplankton).
which it first vanishes from the observer's sight. This is called "nitrogen limitation," and occurs
Note that if the disc can still be seen as it rests most commonly when the ratio of total nitrogen
on the lake bottom or if it disappears into plant to total phosphorus is less than 10 (in other
growth, the depth at which this happens is not a words, the TN concentration divided by the TP
measurement of the waterbody's Secchi depth. concentration is less than 10: TN/TP < 10). TN
in waterbodies comes from both natural and
Surface Water man-made sources, including:
is water found on the earth's surface. It is + the air (some algae can "fix" nitrogen; that is,
distinguished from "groundwater" which is found the algae can pull it out of the air in its gaseous
underground. Surface waters include many types form and convert it to a form they can use);
of waterbodies such as estuaries, lakes, marshes, stormwater run-off (even "natural" run-off
ponds, reservoirs, rivers, streams and swamps. from areas where there is no human impact,
because nitrogen is a naturally-occurring nutrient
Total Chlorophyll found in soils and organic matter);
is a measure of all types of chlorophyll. The
+ fertilizers; and
Florida LAKEWATCH abbreviation for total
chlorophyll is CHL. + animal and human wastes (sewage, dairies,
In Florida: Total Phosphorus
Waterbodies in the Florida LAKEWATCH
is a measure of all the various forms of phosphorus
database analyzed prior to January 2000, had
Sr that are found in a water sample. Phosphorus is an
total nitrogen concentrations which ranged from
Element that, in its different forms, stimulates the
less than 50 to over 6000 pg/L. Using these
growth of aquatic macrophytes and algae in
average concentrations of total nitrogen from
waterbodies. The chemical symbol for the element
this same database, Florida lakes were found to
phosphorus is "P" and the symbol for total
be distributed into four trophic states as follows. ph s is "e r
phosphorus is "TP." Some phosphorus compounds
+ approximately 14% of the lakes would be are necessary nutrients for the growth of aquatic
classified as oligotrophic (those with TN values macrophytes and algae. Phosphorus compounds are
less than 400 pg/L); found naturally in many types of rocks. Mines in
+ about 25% of the lakes would be classified as Florida and throughout the world provide phosphorus
mesotrophic (those with TN values between 401 for many agricultural and industrial uses.
and 600 pg/L); The Role of Phosphorus in Waterbodies:
+ 50% of the lakes would be classified as Like nitrogen, phosphorus is an essential
eutrophic (those with TN values between 601 nutrient for the growth of all plants, including
and 1500 pg/L); and aquatic macrophytes and algae. Phosphorus in
waterbodies takes several forms, and the way it
+ nearly 11% of the lakes would be classified as ateroes taes seea ad te wy
hypereutrophic (those with TN values greater changes from one form to another, also called
hypereutrophic (those with TN values greater c p
cycling, is complex. Because phosphorus
S1 changes form so rapidly, many aquatic scientists
The location of a waterbody has a strong generally assess its availability by measuring the
influence on its total nitrogen concentration. For concentration of total phosphorus rather than the
example, lakes in the New Hope Ridge/Greenhead concentration of any single form. In some water-
Slope lake region in northwestern Florida (in bodies, phosphorus may be at low levels that
Washington, Bay, Calhoun, and Jackson counties) limit further growth of aquatic macrphytes and/
tend to have total nitrogen values below 220 pg/L. or algae. In this case, scientists say phosphorus is
While lakes in the Lakeland/Bone Valley Upland the "limiting nutrient."
lake region in central Florida (in Polk and For example, waterbodies having TP con-
Hillsborough counties) tend to have values above centrations less than 10 pg/L will be nutrient
1700 pg/L. poor and will not support large quantities of
Health Concerns: algae and aquatic macrophytes. There are many
The concentration of total nitrogen in water ways in which phosphorus compounds enter
is not a known direct threat to human health. It is water. The more common ones are described below:
the individual forms of nitrogen that contribute + Some areas of Florida and other parts of the
to the total nitrogen measurement and the use of world have extensive phosphate deposits. In these
the water that need to be considered. areas, rivers and water seeping or flowing under-
For example, nitrate in drinking water is a ground can become phosphorus enriched and may
concern. Drinking water with nitrate concentra- carry significant amounts of phosphorus into
tions above 45 mg/L has been implicated in waterbodies.
causing blue-baby syndrome in infants. The
Sb Sometimes phosphorus is added intentionally
maximum allowable level of nitrate, a component to waterbodies to increase fish production by
to waterbodies to increase fish production by
of the total nitrogen measurement, is 10 mg/L in
Sfertilizing aquatic macrophytes and algal growth.
drinking water. Concentrations of nitrate greater
than 10 mg/L generally do not occur in waterbodies, Phosphorus can enter waterbodies inadvert-
because nitrate is readily taken up by plants and ently as a result of human activities like landscape
used as a nutrient, fertilization, crop fertilization, wastewater
disposal, and stormwater run-off from residential four groups according to their level of biological
developments, roads, and commercial areas. productivity. The adjectives denoting each of
In Florida: these trophic states, from the lowest productivity
Waterbodies in the Florida LAKEWATCH level to the highest, are oligotrophic, mesotrophic,
database analyzed prior to January 2000, had eutrophic, and hypereutrophic. Aquatic scientists
assess trophic state by using measurements of
total phosphorus concentrations which ranged assess state us measurements of
one or more of the following:
from less than 1 to over 1000 pg/L. Using these one or more of the following:
average concentrations of total phosphorus from + total phosphorus concentrations in the water;
this same database, Florida lakes were distributed + total nitrogen concentrations in the water;
into the four trophic states as follows: + total chlorophyll concentrations a measure
Approximately 42% of the lakes would be of free-floating algae (phytoplankton), in the
* approximately 42% of the lakes would be
classified as oligotrophic (those with TP values water column;
less than 15 pg/L) water clarity, measured using a Secchi disc;
+ aquatic macrophyte abundance.
* about 20% of the lakes would be classified as
The Florida LAKEWATCH professionals
mesotrophic (those with TP values between 15r
and 25 pg/L base trophic state classifications primarily on the
amount of chlorophyll in water samples. Chlorophyll
* 30% of the lakes would be classified as concentrations have been selected by LAKE-
eutrophic (those with TP values between 25 and WATCH as the most direct indicators of biological
100 pg/L); and productivity, since the amount of algae actually
+ nearly 8% of the lakes would be classified as being produced in a waterbody is reflected in the
hypereutrophic (those with TP values greater than amount of chlorophyll present. In addition,
100 pg/L). Florida LAKEWATCH professionals may modify
their chlorophyll-based classifications by taking
The location of a waterbody has a strong
the aquatic plant abundance into account.
influence on its total phosphorus concentration.
For example, lakes in the New Hope Ridge/ Water Clarity
Greenhead Slope lake region in northwestern W
is the transparency or clearness of water. While
Florida (in Washington, Bay, Calhoun, and
many people tend to equate water clarity with
Jackson Counties) tend to have total phosphorus water alty, ts a m oeton to o o.
water quality, it's a misconception to do so.
values below 5 pg/L. While lakes in the Lake-
SContrary to popular perceptions, crystal clear
land/Bone Valley Upland lake region in central w m c
water may contain pathogens or bacteria that
Florida (in Polk and Hillsborough Counties) tend w m i h
would make it harmful to drink or to swim in,
to have values above 120 ug/L.
to have values above 120 g/L. while pea-soup green water may be harmless.
Health Concerns: Water clarity in a waterbody is commonly
There is no known level of total phosphorus measured by using an 8-inch diameter Secchi
in water that poses a direct threat to human health. disc, attached to a string/rope. The disc is lowered
Transparency into the water, and the depth at which it vanishes
from sight is measured. Measured in this way,
See Water Clarity.
Water clarity is primarily affected by three
Trophic State components in the water:
is defined as "the degree of biological productivity free-floating algae called phytoplankton,
of a waterbody." Scientists debate exactly what is + dissolved organic compounds that color the
meant by biological productivity, but it generally water reddish, brown, or black, and
relates to the amount of algae, aquatic macrophytes, fish
sediments suspended in the water, either stirred
and wildlife a waterbody can produce and sustain.
Waterbodies are traditionally classified into up from the bottom or washed in from the shore.
Waterbodies are traditionally classified into
Water clarity is important to individuals buffering the action of wind, waves, and
who want the water in their swimming areas to human effects depriving the free-floating algae
be clear enough so that they can see where they of nutrients contained in the bottom sediments
are going. In Canada, the government recommends that would otherwise be stirred up.
that water should be sufficiently clear so that a
Secchi disc is visible at a minimum depth of 1.2 In Florida:
meters (about 4 feet). This recommendation is one Waterbodies in the Florida LAKEWATCH
reason that many eutrophic and hypereutrophic database analyzed prior to January 2000, had
lakes that have abundant growths of free-floating Secchi depths ranging from less than 0.2 to over
algae do not meet Canadian standards for swimming 11.6 meters (from about 0.7 and 38 feet).
and are deemed "undesirable." It should be noted The trophic state of a waterbody can be
that these lakes are not necessarily "undesirable" for strongly related to the water clarity. Using these
fishing nor are they necessarily polluted in the average Secchi depth readings, Florida lakes
sense of being contaminated by toxic substances. were found to be distributed into four trophic
states as follows:
The Role of Water Clarity in Waterbodies:
Water clarity will have a direct influence on approximately 7% of the lakes would be
the amount of biological production in a water- classified as oligotrophic (those with Secchi
body. When water is not clear, sunlight cannot depths greater than 3.9 meters- about 13 feet);
penetrate far and the growth of aquatic plants about 22% of the lakes would be classified as
will be limited. Consequently aquatic scientists mesotrophic (those with Secchi depths between
often use Secchi depth measurements (along 2.4 and 3.9 meters between about 8 and 13
with total phosphorus, total nitrogen, and total feet);
chlorophyll concentrations) to determine a + 45% of the lakes would be classified as
waterbody's trophic state. eutrophic (those with Secchi depths between 0.9
Because plants must have sunlight in order and 2.4 meters between about 3 and 8 feet); and
to grow, water clarity is also directly related to
how deep underwater aquatic macrophytes will 26% of the lakes (those with Secchi depths
be able to live. This can be estimated using less than 0.9 meters -about 3 feet) would be
Secchi depth readings. A rule of thumb is that classified as hypereutrophic.
aquatic macrophytes can grow to a depth of The location of a waterbody has a strong
about 1.5 times the Secchi depth measurement. influence on its water clarity. For example, lakes
For example, for a Secchi depth measurement of in the New Hope Ridge/Greenhead Slope lake
3 feet, the depth at which aquatic macrophytes region (in Washington, Bay, Calhoun, and
can grow is limited to about 4.5 feet. Jackson counties) tend to have Secchi depths
Water clarity affects plant growth but greater than 9 feet (3 meters). While lakes in the
conversely, the abundance of aquatic plants can Lakeland/Bone Valley Upland lake region (in
affect water clarity. Polk and Hillsborough counties) tend to have
Generally, increasing the abundance of Secchi depths less than 3 feet (0.9 meters).
submersed aquatic macrophytes to cover 50% or Health Concerns:
more of a waterbody's bottom may have the effect Water clarity is not known to be directly
of increasing the water clarity. related to human health.
One explanation is that either the sub-
mersed macrophytes, or perhaps the algae Water Depth
attached to the aquatic macrophytes, use the is the measurement of the depth of a waterbody
available nutrients in the water, depriving the free- from the surface to the bottom sediments. Water
floating algae of them. Submersed macrophytes also depth can vary substantially within a waterbody
anchor the nutrient-rich bottom sediments in place based on its morphology (shape).
Florida LAKEWATCH volunteers measure Water quality guidelines developed by the
water depth using a weighted Secchi disk attached Florida Department of Environmental Protection
to a string or cord that is marked in one-foot (FDEP) provide standards for the amounts of
increments. The weighted Secchi disk is dropped certain substances that can be discharged into
down until it hits bottom and then the distance is Florida waterbodies (Florida Administrative
determined by measuring the length of rope Code 62.302.530). The FDEP guidelines provide
between the bottom and the surface of the water. different standards for waterbodies in each of
These measurements are then recorded for future five classes that are defined by their assigned
reference. designated use as follows:
Water depth can also be measured using a + Class I waters are for POTABLE WATER
device called a fathometer by bouncing sonic SUPPLIES;
pulses off the bottom and electronically calculating + Class II waters are for SHELLFISH
the depth. Several fathometer readings taken PROPAGATION OR HARVESTING;
continuously along a number of transects + Class III waters are for RECREATION,
(shore-to-shore trips across the waterbody) are PROPAGATION AND MAINTENANCE OF A
used to calculate an average lake depth. This HEALTHY, WELL-BALANCED POPULATION OF
technique can be used instead of the traditional FISH AND WILDLIFE;
method of dividing the lake's volume by its + Class IV waters are for AGRICULTURAL WATER
surface area to obtain a "mean depth." SUPPLIES; and
+ Class V waters are for NAVIGATION, UTILITY
Water Quality AND INDUSTRIAL USE.
is a subjective, judgmental term used to describe All Florida waterbodies are designated as
the condition of a waterbody in relation to Class III unless they have been specifically
human needs or values. The terms "good water classified otherwise; refer to Chapter 62.302.400,
quality" or "poor water quality" are often related Florida Administrative Code for a list of
to whether the waterbody is meeting waterbodies that are not Class III.
expectations about how it can be used and what
the attitudes of the waterbody users are. Watershed
Water quality is not an absolute. One person
is the area from which water flows into a
may judge a waterbody as having good water
quality, while someone with a different set of waterbody. Drawing a line that connects the
values may judge the same waterbody as having highest points around a waterbody is one way to
poor water quality. For example, a lake with an delineate a watershed's boundary. A more
abundance of aquatic macrophytes and algae in accurate delineation would also include areas
the water may not be inviting for swimmers but that are drained into a waterbody through
may look like a good fishing spot to anglers. underground pathways.
Water quality guidelines for freshwaters In Florida, these might include drainage
have been developed by various regulatory and pipes or other man-made systems, seepage from
governmental agencies. For example, the Cana- high water tables, and flow from springs. Activi-
dian Council of Resource and Environmental ties in a watershed, regardless of whether they
Ministers (CCREM) provides basic scientific are natural or man-made, can affect the charac-
information about the effects of water quality teristics of a waterbody.
parameters in several categories, including raw
water for drinking water supply, recreational
water quality and aesthetics, support of freshwater
aquatic life, agricultural uses, and industrial