Front Cover
 Title Page
 A listing of Florida Lakewatch...
 Table of Contents
 List of Figures
 Part 1: Two ways to define color...
 Part 2: More about suspended and...
 Part 3: Light and color in...
 Part 4: Color and its influence...
 Part 5: Water clarity and...
 How to use an empirical model
 Back Cover

Title: Beginner's guide to water management: color
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00055859/00001
 Material Information
Title: Beginner's guide to water management: color
Series Title: Florida LAKEWATCH Information Circular 108
Alternate Title: Color
Physical Description: Book
Language: English
Creator: Florida LAKEWATCH.
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences
Publication Date: 2004
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00055859
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: notis - ocm5486

Table of Contents
    Front Cover
        Cover 1
        Cover 2
    Title Page
        Title 1
        Title 2
    A listing of Florida Lakewatch information circulars
        Page i
    Table of Contents
        Page ii
    List of Figures
        Page iii
        Page iv
    Part 1: Two ways to define color in a waterbody
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
    Part 2: More about suspended and dissolved substances
        Page 7
        Page 8
        Page 9
        Page 10
    Part 3: Light and color in water
        Page 11
        Page 12
        Page 13
        Page 14
    Part 4: Color and its influence on algae and aquatic plants
        Page 15
        Page 16
        Page 17
        Page 18
    Part 5: Water clarity and algae
        Page 19
        Page 20
    How to use an empirical model
        Page 21
        Page 22
        Page 23
    Back Cover
        Page 24
Full Text

A Beginner's Guide to

Water Management Color

Information Circular 108

Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida

N[VETOF January 2004
F.LORI 1st Edition
I ....F-i ,.i .. T0

This publication was produced by:

Florida LAKEWATCH 2004
University of Florida / Institute of Food and Agricultural Sciences
Department of Fisheries and Aquatic Sciences
7922 NW 71st Street
Gainesville, FL 32653-3071
Phone: (352) 392-4817
Toll-Free Citizen Hotline: 1-800-LAKEWATch (1-800-525-3928)

E-mail: lakewat@ufl.edu
Web Address: http://lakewatch.ifas.ufl.edu/

Copies of this document and other information circulars are available for
download from the Florida LAKEWATCH website:


As always, we welcome your questions and comments.

A Beginner's Guide to

Water Management Color

Information Circular 108

Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida
January 2004
1st Edition

r..UN I[V ESTY OF # L.
I.... ,, F. n I np...m ....i .A.. I.

This publication was produced by:

Florida LAKEWATCH 2004
University of Florida / Institute of Food and Agricultural Sciences
Department of Fisheries and Aquatic Sciences
7922 NW 71st Street
Gainesville, FL 32653-3071
Phone: (352) 392-4817
Toll-Free Citizen Hotline: 1-800-LAKEWATch (1-800-525-3928)

E-mail: lakewat@ufl.edu
Web Address: http://lakewatch.ifas.ufl.edu/

Copies of this document and other information circulars are available for
download from the Florida LAKEWATCH website:


As always, we welcome your questions and comments.

Note: For more information related to color in lakes,
we recommend that you read Circulars 101,102 and 103.

Beginner's Guide to Water Management The ABCs (Circular 101)
This 44-page publication, in a user-friendly glossary format, provides a basic introduction to the
terminology and concepts used in today's water management arena.

A Beginner's Guide to Water Management Nutrients (Circular 102)
A basic introduction to the presence of phosphorus and nitrogen in lakes two nutrients commonly
associated with algal growth and other forms of biological productivity in lakes. Limiting nutrients are
discussed, along with conceptual and mathematical tools that can be used to achieve a variety of water
management goals. The booklet is 36 pages in length.

A Beginner's Guide to Water Management Water Clarity (Circular 103)
Anyone interested in the subject of water clarity can benefit from reading this 36-page circular. Topics
include the many factors that can affect water clarity in Florida lakes, techniques for measuring it, as
well as discussion of the methods used for managing this important lake characteristic.

A Beginner's Guide to Water Management Lake Morphometry (Circular 104)
Knowledge of the size and shape of a lake basin (i.e., lake morphometry) can tell us a great deal about
how a lake system functions. It can also help us appreciate lakes for what they are and manage them
with more realistic expectations. This 36-page booklet is recommended for anyone interested in learning
more about the terminology and techniques currently being used to study lake morphometry in Florida.
A Beginner's Guide to Water Management Symbols, Abbreviations and Conversion Factors (Circular 105)
This 44-page booklet provides the symbols, abbreviations and conversion factors necessary to commu-
nicate with water management professionals in the U.S. and internationally. Explanations for expressing,
interpreting and/or translating chemical compounds and various units of measure are included.
A Beginner's Guide to Water Management Bacteria (Circular 106)
This 38-page booklet provides a brief tutorial on the presence of bacteria in Florida lakes and the
aquatic environment in general, followed by a discussion of the possible sources of bacterial contami-
nation and how one might test for it. Also included: a comparison of wastewater treatment plants
versus septic tank systems; indicators used for detecting bacterial contamination; and laboratory
methods commonly used for detection of bacteria. Lastly, an easy 4-step process is provided for track-
ing down bacterial contamination in a waterbody.
A Beginner's Guide to Water Management Fish Kills (Circular 107)
In an effort to alleviate concerns voiced by the general public regarding fish kills, this 16-page booklet
discusses five of the most common natural causes of fish kills: low dissolved oxygen; spawning fatalities;
mortality due to cold temperatures; diseases and parasites; and toxic algae blooms. Human-induced
events are also covered, along with a section on fish stress a component of virtually every fish kill
situation. The last section of the circular provides steps one can take to help biologists determine the
cause of the event including a listing of fish health diagnostic laboratories and instructions on how to
collect fish and/or water samples for analysis.

Copies of these publications can be obtained by contacting the Florida LAKEWATCH office at
1-800-LAKEWATch (1-800-525-3928). They can also be downloaded forfree from the Florida LAKEWATCH web site at:
http://lakewatch.ifas.ufl.edu/LWcirc.html or from the UF/IFAS Electronic Document Information System (EDIS):

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Aside from water clarity, the color of water less or "clear" water is traditionally considered the
in a lake is one of the main attributes that ultimate water quality standard in many states.
captures people's attention particularly While this may be true when it comes to drinking
if the color begins to change. Such events often water, it doesn't always apply for many of
take us by surprise as many of us carry a mental Florida's unique aquatic systems.
image of a lake or waterbody as we first saw it So, how do we know if colored water is
and generally don't expect changes to occur. In natural?
reality, however, many lakes and waterways in Collecting long-term color data is a good
Florida can display a wide variety of hues over way to start especially if it's combined with
time, ranging from a clear blue to vivid green, to long-term water chemistry measurements for
orange or almost black. algae (total chlorophyll), nutrients (total nitrogen
Water color can be influenced by any number and total phosphorus) and water clarity. With
of factors: some colors occur naturally; some may such information, we can discern much about
be human-induced or result from a combination what is happening in a lake or waterbody.
of circumstances. For example, heavy rain events Thanks to Florida LAKEWATCH (FLW) volunteers
are known to wash organic substances into the and a dedicated staff, we now have access to data
water where they dissolve and act as a dye; for hundreds of lakes throughout the state. In fact,
seasonal algae blooms can result in such high much of the material provided in this circular was
concentrations of algae that the water becomes made possible by our volunteers and the samples
tinted with the coloration of the algal cells; or wind they've collected. (In addition to the usual monthly
events may stir up fine particles off the bottom, sampling regimen, our water chemistry technicians
re-suspending them into the water column. Color have been able to use the same water samples to
may also be the result of inorganic materials (e.g., conduct color analysis on FLW lakes two to four
clay particles, etc.) from storm-water runoff or times a year.)
shoreline erosion. These efforts have allowed us to compile
It's no wonder that many visitors and/or and analyze data from thousands of samples
residents are often bewildered when they see the and, as a result, identify some rather strong
spectrum of colored lakes and waterbodies patterns between a lake's water color and its
throughout Florida. Sometimes, these differences biological productivity (i.e., the amount of algae,
are misinterpreted as an indication of pollution. aquatic plants, fish and other wildlife). We've
In rare instances, they may be correct, but most of also learned just how important color can be to
the time lake color is a result of naturally occurring lake management even though it is often over-
processes that have more to do with the geology shadowed by concerns about nutrients, algae or
of the soils under the lake bed or runoff from aquatic plants. This is unfortunate as color may
areas within the surrounding watershed. well be influencing many of these same lake
Admittedly, accepting colored lakes as characteristics.
"normal" can be difficult, especially since color- We hope you find this publication useful in


your quest to learn more about the aquatic information into agreeable portions for everyone.
environment and, as always, we welcome your Think of this circular as an educational buffet; feel
questions and comments. Because this material is free to take what you want and leave the rest for
intended for a varied audience including citizens, those with larger appetites.
students and scientists, we've tried to organize the Bon appetite!

Included in this circular:

Introduction i
List of Figures iii

Part 1 Two Ways to Define Color in a Waterbody 1
Apparent Color 1
Measuring Apparent Color 1
Sidebar: Suspended and Dissolved Substances (definitions) 1
Important Points to Remember About Apparent Color 2
Sidebar: Illustrating the Difference Between Suspended and Dissolved Substances 2
True Color 2
Measuring True Color 2
Important Points to Remember About True Color 2
Sidebar: How Does Florida LAKEWATCH Measure True Color? 3
Sidebar: An Anecdote About True Color 4

Part 2 More About Suspended and Dissolved Substances 7
Suspended Substances 7
Algal Matter 7
Non-algal Matter 7
Sidebar: A Mystery Color? 7
Dissolved Substances 8
Organic Matter 8
Sidebar: Can Nutrients Such as Nitrogen and Phosphorus Add Color to a Lake? 8
Inorganic Matter 9
Sidebar: Long-term Color Changes 9

Part 3 Light and Color in Water 11
Visible Light 11
So Why Do We Need to Know About Light Absorption in Lakes? 12
Measuring Light in a Lake 12
Sidebar: What Is a Secchi Disk? 13
Sidebar: Mathematical Formula Used to Calculate Light Attenuation 13

Part 4 Color and Its Influence on Algae and Aquatic Plants 15
Algae, Color and Light 15
Aquatic Plants, Color and Light 15
Sidebar: What About Emergent or Floating-leaved Plants? 16
Sidebar: Aquatic Plants and Color (Example: Tsala Apopka Chain-of-Lakes) 17

Part 5 Color, Water Clarity and Algae 19

Appendix 1: How to Use an Empirical Model 21
Selected Scientific References 23
Florida LAKEWATCH 24


Figure 1 5
Color Frequency in Florida Lakes

... L Figure 2 9
SColor Measurements from Lake Santa Fe
Over Time (i.e., from 1989 2000)

Figure 3 11
Earth's Electromagnetic (EM) Radiation
SSpectrum: An Illustration of How the Various
: Forms of EM Radiation are Categorized
... ..... ..
:= Figure 4 12
.. Transmission of Light by Distilled Water
.... ........ at Six W avelengths
Figure 16

Color and Its Influence on PAC and PVI
in Florida Lakes

Figure 6a 17
Color Measurements for the Tsala Apopka

Figure 6b 17
Plant Abundance in the Tsala Apopka

Fom fE aito r aeoie

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There are two basic terms commonly used
when referring to the color of the water in Suspended substances can include algal matter
a lake or waterbody. One is known as (i.e., floating in the water column or stirred up
apparent color and the other as true color. When bottom sediments due to wind mixing or boating
communicating with lake residents, lake manag- activity) or non-algal matter such as finely ground
ers and others, it's helpful to know the difference calcium carbonate particulates from limestone.
between the two. In Part 1, we'll begin our Depending on the source, these substances may
discussion by defining these two types of lake impart any number of colors to the water, including
color and in Part 2, we will go into greater detail a blue-green tint, various shades of brown, gray,
about what influences both types. In Part 3, we'll green or even orange.
delve even deeper to see how color can influence
the biology of a lake (i.e., the amount of algae, Dissolved substances often include metallic
aquatic plants and wildlife), ions of iron and manganese from natural sources
(e.g., rocks and soils) as well as humic acids and
Apparent Color tannins derived from organic matter (e.g., dead
Apparent color is the color of the water as leaves and plants, etc.). These substances
seen by the human eye. For example, when a usually impart a reddish or brown stain to the
person looks into a lake, the water may appear to water.
be colorless (i.e., clear), blue, green, yellow, red,
brown, black or somewhere in between.
Most of the time, apparent colors are the Forel-Ule color scale, a system developed by
result of substances that are either suspended or European lake scientists. The scale classifies lake
dissolved in the water column. However, there color into 22 categories ranging from blue, green-
are other factors that can affect the apparent color ish blue, bluish green, green, greenish yellow,
of a lake including the color of the lake bottom yellow and brown. Using the scale, one can
(i.e., is it light or dark?); the depth of the lake; determine the apparent color of a lake by visually
reflections from the sky, trees or structures sur- matching (i.e.,, with the naked eye) the color of
rounding the waterbody; and the presence or the water with the Forel-Ule color spectrum.
absence of aquatic plants. Some of these factors In Europe, this system has been used as a
can be difficult to measure, which is why lake way of defining the various levels of a lake's
management professionals prefer to use true color productivity: lakes with water that appears to be
measurements when assessing a lake. blue are considered to be less productive. Green,
See page 2 for information on true color, yellow and brown lakes are considered to be more
productive. In the United States, the Forel-Ule
Measuring Apparent Color system is seldom used for measuring the apparent
Some water management professionals color of water because it is cumbersome and
assess the apparent color of lake water using the difficult to use.


Criteria used to determine apparent color is
considered to be somewhat subjective whereas
true color is based on actual water analysis

The following activities provide a handy visual True Color
aid for understanding the difference between True color is defined as the color of water
suspended and dissolved substances: resulting from dissolved substances only; all
suspended substances have been removed and
Suspended substances are therefore not allowed to "conceal" or influence
Fill a clear glass jar with water. Next, find several the color of the water. In the United States,
pieces of chalk and break them up or grind them into a when lake management professionals talk about
coarse powder. Add this mixture to the water. If you "color," they are generally referring to true
were to stir the mixture vigorously with a spoon, the color.
chalk would become suspended into the water,
changing the overall color. Depending on how much
agitation is used, the chalk may stay in the water Measuring True Color
column, sink to the bottom or float to the top. Tre color is determined by first filtering a
water sample to remove all suspended substances.
Dissolved Substances After the samples have been filtered, they are
Now fill a clear jar with water and place three to four compared to a specific color scale. This comparison
tea bags into it and place it out in the sun for about 30 is generally done in a laboratory with a spectro-
minutes. Tannins from the tea leaves will begin to stain photometer.
or color the water. This is an example of dissolved In the United States, the most commonly
substances. The same process can be found in lakes used color scale is the platinum-cobalt color scale.
that are colored by tannins from decaying leaf material, This system is comprised of 1,000 color units or
plant stems or roots found within the waterbody and/or platinum-cobalt units (PCU or Pt-Co units). If
surrounding watershed. one were to use the platinum-cobalt color scale to
measure lake water that is especially clear (i.e.,
colorless), the color readings would probably be
Important Points to Remember About less than 10 PCU, whereas lakes that have a little
Apparent Color color will have a true color measurement ranging
from 20 to 50 PCU. On the far end of the spec-
* Most of the recognizable apparent color in a
trum, lake water that is extremely dark in color
lake is the direct result of suspended substances ae a r r r
will have a color reading of 500 PCU or higher.
in the water (i.e., both algal and non-algal matter). I lia color generally ranges
In Florida lakes, true color generally ranges
from 5 PCU to 600 PCU. Of course, there are
* Apparent color can change significantly once t i ar
always exceptions: Lake Charles in Marion
suspended substances are filtered out of a water Cl in ar
County has shown true color readings approach-
sample. That is why scientists rely less on apparent ing 7 P To te e ee c appears
r n s ing 700 PCU! To the naked eye, such a lake appears
color when studying lakes and instead, usually to be almost black in color.
insist on taking true color measurements before
coming to any conclusions about lake color. Important Points to Remember About True Color

* Nutrients can influence the apparent color of True color measurements are especially helpful
water indirectly by increasing algal populations. to lake scientists because they provide a standard-
(e.g., Once algal populations increase, the algae ized way of assessing the color of a waterbody.
themselves are known to release organic substances
True color is a component of apparent color.
that can tint the water various shades of green or
even brown.) True color does not stay constant; it can increase


dramatically, especially after prolonged rain suspended particles, true color measurements will
events or it can decrease under severe drought not necessarily coincide with apparent color. For
conditions. example, algae or clay particles can make the water
appear to be a certain color, but once the particles
SWaterbodies with limited suspended particles are filtered out, its appearance may change signifi-
will generally have true color measurements that cantly That's one reason why lake management
coincide with the visual appearance of the water. professionals prefer to use true color measurements
However, in waterbodies with an abundance of instead of apparent color.

Florida LAKEWATCH measures the true color of lake water using the platinum-cobalt color scale.
These measurements are processed two to four times a year from water samples that volunteers provide
(i.e., the same samples that are being processed for total nitrogen and total phosphorus analysis each
month). Usually, an equal amount of water is taken from each of the bottles collected for a lake, for a given
month. The water is then combined to form an 'average' color sample for that lake. In some instances,
water samples from individual stations are analyzed separately, particularly if there are obvious visible color
differences between the sampling stations.

Water is filtered to remove any particulate material.
Afterwards, the sample is spun in a centrifuge,*
measured on a spectrophotometer,** and compared
to a series of platinum-cobalt standards
(i.e., standards that simulate the color of lake water).

At the end of the year, color measurements are averaged
for each lake and that number is included as part of the
annual periodic water chemistry data.

Color data from Florida LAKEWATCH lakes can be
obtained by:

accessing the annual data report on the FLW
website: http://lakewatch.ifas.ufl.edu/

calling 1-800-LAKEWATch (1-800-525-3928) to
obtain a copy of the printed page from the annual

requesting a printout of the annual data packet,
which now includes color data in the form of
tables and graphs.

A centrifuge is a machine that uses centrifugal force
(i.e., intense spinning) for separating substances of
different densities.
Note: Florida LAKEWATCH evaluated frozen water
** A spectrophotometer is an instrument that is used for samples among a wide range of true color values
measuring the relative intensities of light found in different
parts of a light spectrum, and they did not change significantly over time.
parts of a light spectrum.



Grasshopper Lake in Lake County.

An Anecdote About True Color

True color has been found to be strongly linked to the amount of seasonal rainfall a
watershed receives and the amount of runoff that seeps into a waterbody. This phenom-
enon has been documented with LAKEWATCH data numerous times. For example, in
1993 -1994, Grasshopper Lake, in Lake
County, had Secchi depth values greater
than 12 feet. This happened to be during a
time of extremely dry weather. Following
heavy rains in 1995 1996, the same lake
had Secchi depth measurements of less
than three feet. The difference in water
clarity was associated with a change in z
true color from 0 PCU in the dry years to Bladderwort (Utricularia spp.), a submersed
plant, grows abundantly in Grasshopper Lake
more than 50 PCU after the heavy rains, wen water clarity is sufficient.

when water clarity is sufficient.


Figure 1 Color Frequency in Florida Lakes

The graph below is an illustration of the frequency of occurrence that color values
occurred in lakes throughout Florida. To be more specific, 3,223 true color measure-
ments have been collected and plotted from 670 waterbodies, located within 48 Florida
counties. Color measurements ranged from 0 PCU to 930 PCU, with a median of 18 PCU.
Because there are very few lakes that had color measurements exceeding 500, the
y-axis in this graph (i.e., Frequency of Occurrence) stops at 500. Notice the left portion
of the graph clearly shows that the lion's share of lakes in this data set (i.e., about 79
percent) had color measurements that were less than 50 PCU.




Z3 The highest true color value sampled within
the LAKEWATCH database was Charles Lake
LL in Marion County, which had a true color
measurement of 930 PCU! 1111

Color in Platinum-Cobalt Units (PCU}


.. ... .... ...:.

... .. .. .. ..

... .. .. -. .

Suspended algal matter can easily be seen in this glass beaker. The sample was pulled from the
adjacent pond at the UF/IFAS Department of Fisheries and Aquatic Sciences in Gainesville.


Now that we've learned about the two
basic ways that lake scientists define A Mystery Color?
color, we will discuss suspended sub- During the months of March, April or May, many
stances and dissolved substances in greater detail, Florida lakes have been known to take on a bright
as they are particularly important to understanding yellow hue. As a result, FLW has heard from many
the color of a waterbody. people who are under the impression that an algal
bloom is occurring in their lake when, in fact, what
Suspended Substances they are seeing is pine and oak pollen floating on
the surface or suspended in the water column.
There are any number of naturally occurring A clue for determining whether or not it's algae:
suspended substances that can be found in Florida If a yellow powdery substance has collected on
lakes or waterbodies. In lake management circles, cars, windows and other outdoor objects in the
they are also referred to as suspended matter or area, there's a good chance it's pollen.
particulates and they are usually classified into
two basic groups: algal matter, which consist of Yen c s ae fy
Yellowish-brown colors are frequently noticed
algae cells suspended in the water column and
Sc s i t w c in waterbodies where diatoms dominate the algal
non-algal matter which includes fine soil particles population.
or non-living plant material. Both are described in
greater detail below. Botryococcus (pronounced Ba TREE o cockus)
is a type of algae that gives many Florida lakes a
Algal Matter rusty or orange-brown color. It is often most
In many cases, apparent color in Florida visible during afternoon hours when it tends to
lakes is due to large concentrations of algae float to the surface. At times, Botryococcus produces
suspended in the water. In other words, if there an oily sheen on the water, fooling people into
are enough algae in the water column, lake water thinking there's been a gasoline or oil spill.
will appear to be the same color of the actual algal Many turbid lakes display a green hue due to
Many turbid lakes display a green hue due to
cells. Sometimes, this results in a short-term event
an algae bloom for example or sometimes green chlorophyll pigments within the algae.
- an algae bloom for example or sometimes
Sa e b f e r s However, at times, some waterbodies have been
lakes maintain a particular color for many months However at te oe ater e e bee
known to develop a blood-red color. The cause of
or years, due to the presence of a dominant algal ths red coloraon is he ala ulea hch
s. g n e s e a t this red coloration is the alga Euglena, which
species. Depending on the species and the
species. Dep ng on the species ad the produces a red pigment during intense periods of
amount of algal cells in the water, such blooms
am nt of algl cells n the water, sh b s sunlight to protect its green chlorophyll pigment.
can impart a variety of colors to the water:
* Blue-green algae are dominant in the more Non-algal Matter
eutrophic lakes and impart a dull-green appearance Suspended particulate matter that is not of
to the water. When large amounts of blue-green algal origin can also influence the apparent color
algae float to the surface, it may look like someone of water. This includes both organic matter (e.g.,
dumped a bucket of blue-green paint into the water. tiny particulates from dead aquatic and terrestrial


plants) or inorganic particles (e.g., clay, sand, soils).
These materials are usually introduced to a lake Can nutrients, such as nitrogen and
from storm-water runoff or erosion of the shoreline, phosphorus, add color to water in a lake?
In Florida, these lakes, which are often described Phosphorus and nitrogen are nutrients found in
as "muddy," are in the minority. However, they do virtually every lake or waterbody. They are also
exist. In the northern part of the state, some lakes naturally occurring in plants and soils. In fact,
receive large amounts of red clay resulting in a Florida's phosphorus-rich soils are what motivated
distinctive reddish "Georgia clay" appearance. many farmers to move to Florida in the early
Lake Talquin and Lake Seminole are good examples. 1900s. Phosphate mines are also prevalent in
Other lakes receive inputs of grayish-white colored various regions of the state, for the same reason.
clay, giving them a milky white appearance. As far as color is concerned, when nitrogen
In flatter parts of the state, erosion or runoff- and phosphorus are dissolved in water, the
inorganic compounds are generally colorless so
related color is rare. However, the central and inorganic compounds are generally colorless so
they don't really add to the apparent and/or true
southern portion of Florida does have its share of color of a waterbody directly. However nutrients
lakes that are influenced by non-algal suspended can affect color indirectly by influencing the growth
sediments from within the lake itself. Lake Okee- of algae.
chobee and Lake Apopka are prime examples. In Example 1: In lakes where algae are abundant,
Okeechobee, water depth at numerous mid-lake the apparent color of the lake is affected because
locations is typically 2.7 meters (8.8 feet) and in you are seeing the color of algal cells in high
Lake Apopka, the average mid-lake depth is 1.7 densities.
meters (5.6 feet). Because of the shallow depth Example 2: Should algae concentrations begin
and large amount of fetch in both lakes, wind is to increase in lakes that previously had low algal
able to constantly re-suspend sediments from the abundance, one of the first things people notice is
bottom and mix them throughout the water a shift in from a bluish color to various shades of
column causing changes in apparent color.' This is green. This change is largely due to the release of
organic matter from within the algal cells, which
known as turbidity-related color. Needless to say, organic matter from within the algal cells, which
will be evident in a true color measurement.
water clarity at these same mid-lake locations isn A n
For more on nutrients and algae, see A Beginner's
quite low (i.e., as measured by a Secchi disk), Guide to Water Management Nutrients (Circular 102).
typically less than 0.33 meters (one foot).
There are also instances in which colloidal
types of terrestrial and aquatic plants. There are
substances (i.e., particles tiny enough to pass literally hundreds of lakes in Florida that are col-
through a filter) remain in a sample and affect
S l ored due to the presence of these substances. (See
water color. This is particularly true in limerock A m n 1
Figure 1 on page 5.) As mentioned in Part 1, Lake
pits where inorganic materials such as calcium and C i t O Ni r
Charles, in the Ocala National Forest, is a good
magnesium carbonates will give water a green or
magnesium carbonates will give water a green or example of this type of lake; the clear brown tea-
emerald hue.
emera ue. colored water is the result of humic acids entering
the waterbody from the surrounding watershed.
Dissolved Substances Lakes with small amounts of these substances
Dissolved substances can affect both true and will generally appear green in color. (In this
apparent color. These substances enter lakes via a instance, the color is not related to algae.) As the
variety of pathways including surface water runoff waterbody receives more dissolved organic matter,
from the surrounding watershed, following rain the color will begin to shift from green to yellow-
events, and the leaching of organic compounds from green, to a yellow-brown and then a "clear" brown.
decomposing plant material within the lake itself. In addition to the compounds described above,
Organic algae can be another source of dissolved organic
matter in water. The substances are released
The dominant dissolved substances found in
water are typically organic compounds including 1 Fetch is the distance that wind can travel over water
humic acids and tannins that originate from many before intersecting a land mass.


directly into the water from the algal cells. This type Inorganic
of organic matter can change the true color of a Dissolved inorganic substances can also
lake by affecting light absorption. (See Part 3 for influence color in lakes. For example, in
more on this.) However, when algae are very waterbodies that receive inputs that are high in
abundant, they can also affect apparent color as dissolved iron compounds, apparent color
described on page 7. (See section on Algal Matter.) might be described as rusty or orange-brown.

anmiple in: 12. 2i and again in r;r1i.". i sidenis IIC.ni Lake 1"' e p:ia i i n l. : Fic'nda ii a:o lak ."::ud .:e. "ic'
iic.. values e.ceing PC See Fge Figure 2 Color Measurements Over Time(1989-2000)
I ". .. ... ....
... ... ...

Due iC a lnhaked la ek Cdiiia re ile Li Ias less i I a.l .

CO ppoinni a FIc la Ias and oICi a lng pdissolved st siuane again ieied i Pa 1 e sae sea
l ell as al seep li e lake s sees ll ge C le pe Lake Lake C a

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.......... 1... r 1 and again II..n. ..;" : il-i.ugl m-.id-l.. 30 -
Hi h eis pLi o.h a~ e diHc iay events Choange cve a as Sec depl easuemens wn r 12 leer
pec Cdl FIyeais based cas aegck nal i hainale paens Fei less an 20 lee 'uise c eve g s is easy_
eavp ale in 1 alunde sagan es lai esdes i Lake c e plan n F da iI a lake s d happen
gana Fe -lle ding vlaii ild wee lacked ange a gee a w e s a g
ic see iIeding s'i p TIe s wae csied l iC e a lea gese ow.i :ed all pa e.ns

19 values e eedng 1990 P Fgu Figure 2 Color Measurements Over Time (1989- 2000)

dissolved su ea s l. Ie e lS a esu Lake S-

heavy aie. .hunde. sc, we. s ian ware. quickly .
LiiiLdig swamp TIus esLilled In hle release l ... ....... ... .


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ow that we know a little more about the eyes to detect (i.e., between
various substances that can cause lake 400 and 700 nanometers on
water to change and/or retain color, we the EM spectrum).2 Most of
will begin to explore the effects that it may have the time, this light appears
on a lake's biological productivity (i.e., the amount to be white in color. How-
of plant and animal life supported by the lake). ever, if one were to pass a
But before we do, we need to take a short detour beam of light through a
into the physical sciences to learn about visible prism, the primary colors of
light. Granted, it may seem a bit odd to go off on the rainbow would emerge: I
this tangent, but rest assured, it is quite relevant; red orange, yellow, green,
without visible light, we would not be able to see blue, indigo and violet. This -. -
any of the colors discussed in this circular or occurs because the prism
much else, for that matter. Ultimately, our little separates the wavelengthls.
detour will lead us back to one of the main allowing us to see
interests that lake scientists have regarding lakes: them individually. .. I .........
the growth of algae and aquatic plants. When light shines
down into a lake, it
Visible Light behaves in much the same
Whether emanating from a firefly, a lightbulb way. While passing through S
or the sun, visible light is a form of electromag- water, light becomes re- .
netic (EM) radiation. Like other types of EM fracted and is absorbed at
radiation (e.g., radiowaves, microwaves, infrared different rates and depths.
light, X-rays, etc.), visible light is a form of energy Colors such as red orange
that travels outward from its source and widens and yellow are the first to be
as it goes. See Figure 3 for an illustration of how absorbed as they have a
the various forms of EM radiation are categorized lower radiant energy than Figure 3
within the earth's electromagnetic spectrum. blue wavelengths.3 Once
Notice that the low energy forms of radiation, red and yellow wavelengths are absorbed by
such as radio waves have the longest wave- water, the higher energy blue wavelengths (indigo,
lengths (i.e., around 3,000 nm) and high energy blue and violet) are the only ones still visible to
radiation, such as x-rays and gamma rays have the eye. This explains why deep lakes, with clear
shortest wavelengths (100 nm). water, often appear to be blue in color.
As its name suggests, visible light is the only
type of EM radiation that is visible to the human 2 Within the EM system, radiation wavelengths range from 100 to
eye. That's because, when we see light, we are 3,000 metric units known as nanometers (nm).
seeing a combination of electromagnetic waves 3 Red has the lowest radiant energy of the visible light spectrum
that are just the right size and intensity for our with an electron voltage of1.8 and violet has the highest radiant
energy with an electron voltage of3.1


Of course, light behaves differently in lakes particularly interested in PAR, as it is a measure of
with suspended or dissolved matter, as these the amount of light that is available at various
substances can alter the way light refracts and is depths and therefore can help us predict the abun-
absorbed as it passes through the water column. dance and type of plants (including algae) that
This is crucial to the biology of a lake. might be expected to grow under those conditions.

So, Why Do We Need To Know Measuring Light in a Lake
About Light Absorption In Lakes? Generally, a light meter is used to measure
Humans aren't the only ones who make use the intensity of light just below the surface and
of light and its radiant energy. It is also critical to at different depths. Information from the meter
the survival of nearly all animals and plants, both can then be used to calculate light attenuation (i.e.,
terrestrial and aquatic. When radiant energy falls the depth at which light is no longer able to pen-
on a lake it is reflected off the surface; utilized by etrate into the water column). For more on how
plants; or converted to heat, contributing to the this is done, see the mathematical formula provided
thermal stratification of the water. This in turn on page 13. For the average citizen, the only
affects the rates of various biological and chemical drawback to this process is that light meters are
processes within the lake even the behavior of expensive and require special training to operate.
aquatic organisms.4 Fortunately, there is another way to assess
Photosynthesis is one of the most important the depth of light penetration in water: A simple
of these processes as it enables plants and algae to and inexpensive device known as a Secchi disk
utilize sunlight for creating food, and then releases can be used. This device generally consists of a
oxygen into the water as a by-product. In fact, 20-cm (8-inch) disk that is white in color or has
during this process, plants use the same portion black and white quadrants painted on it. A string
of the visible light spectrum that we do (i.e., wave- or rope is attached through the center. The rope is
lengths between 400 and 700 nm). In the lake
science arena, this portion of the spectrum is known 4 Further discussion ofchemical processes and/or the behavior of
as Photosynthetic Active Radiation or PAR. aquatic animals, as it relates to light, is outside the scope of this
Scientists who study light in waterbodies are publication. For more on the subject, see G.E. Hutchinson (1957)
Scientists who study light in waterbodies are
and R.G. Wetzel (1975).

S1 10 100 m


S 60o

10 100
Percentage of Incident Light

Figure 4
An illustration of the transmission of light by distilled water at
six wavelengths (i.e., the percentage of incident light that would
remain visible after passing through various water depths).
0" This diagram is based on the common (base 10) logarithmic
Scale, as indicated by the x-axis along the bottom.
Red (R) = 720 nm 0 ,,., (0) = 620 nm L i.i,, (Y) = 560 nm Green (G) = 510 nm Blue (B) = 460 nm Violet (V) = 390 nm


What is a Secchi Disk?
A simple and inexpensive device known as a Secchi
disk can be used to measure light attenuation in
water. This device generally consists of a 20-cm
(8-inch) disk that is white in color or has black and
white quadrants painted on it. (LAKEWATCH uses
white disks.)
A string or rope is attached through the center.
The rope is marked off in increments of meters or
feet and a small weight is attached underneath the
disk itself so that it will sink quickly when lowered
into the water. As the disk is being lowered, the
individual holding the device can use the markings
on the rope to determine the depth at which the
disk disappears. This measurement is commonly
referred to as a Secchi depth or water clarity
The Secchi disk was invented around 1860 <
by an Italian named Pietro Angelo Secchi.

marked off in increments of meters or feet and a
small weight is attached underneath the disk itself Mathematical Formula Used
so that it will sink quickly when lowered into the to Calculate Light Attenuation
water. As the disk is being lowered, the person
holding the device can use the markings on the e tet ht
The intensity of light diminishes with water
rope to determine the depth at which the disk depth in an exponential way. Professionals
depth in an exponential way. Professionals
disappears. This measurement is commonly
disapeas. This measuement is cy measure the intensity of light just below the
referred to as a Secchi depth or water clarity
measurement. Once it is obtained, the measure-
ment can be used to estimate light attenuation. meters. They mathematically describe the
A general rule of thumb is to multiply the Secchi decrease in light, with depth, by the follow-
depth by two. For example, if the average depth ing equation:
of a lake is eight feet and the Secchi reading is five
feet, then we can multiply the Secchi depth by I e kz
two to get our estimate: z

2 x 5-foot Secchi depth = 10feet of light penetration.
In this particular example, our calculation
tells us that light is reaching the bottom of the Iz is the intensity of light at depth Z.
lake. However, in the same lake, if the Secchi
reading is only three feet, then it would tell us I is the intensity of light immediately below the
that light does not penetrate to the bottom: surface of the water.
e is the natural logarithm
2 x 3-foot Secchi depth = 6feet of light penetration. k is the vertical attenuation (reduction) coefficient for

If this were the case, submersed aquatic the downward penetration of light (also known as
plants would not be able to grow on the bottom, irradiancee").
due to the lack of sufficient sunlight.



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because light is so critical to the growth of In other words, it can limit the amount of algae
aquatic plants and algae, lake scientists growing in the water column. However, it must
have spent a great deal of time observing also be said that, most of the time, phosphorus
and documenting just how much light is required and nitrogen have a much greater effect on algal
for optimal growth. From their research, they abundance.
have developed special terminology that helps For more on the relationship between algae, color,
describe their observations. For example, the nutrients and water clarity, see Part 5.
terms euphotic zone and euphotic depth are
frequently used to describe just how far light is Aquatic Plants, Color and Light
able to penetrate into the water column: As mentioned earlier, when light levels fall
*Euphotic zone is the portion of the water column below 10 PAR, there is insufficient light for most
where light is still present (i.e., greater than one submersed aquatic plants to grow or remain
percent of the surface light level), established in a lake.
How do we know this?
*Euphotic depth is considered to be the depth at LAKEWATCH data and analysis by
which light levels fall below one percent of PAR. Bachmann et al. (2002) shows that Florida lakes
Both the euphotic zone and euphotic depth with low true color measurements (i.e., values less
of a lake are directly influenced by the amount of than 50 PCU) can have as much as 100 percent of
color in the water (i.e., the amount of suspended the lake bottom covered in plants.6 (In scientific
and/or dissolved substances). As the amount of circles, this is expressed as 100 Percent Area
color increases in a lake, light penetration decreases Covered or 100 PAC.) However, once the true
and results in a limited amount of algal and/or color exceeds 50 PCU, the percentage of the
submersed aquatic plant growth. To be more bottom that is covered seldom exceeds 40 PAC
specific, algae have a difficult time growing when In addition, researchers have used the same
light levels fall below one percent surface PAR data to calculate how light affects submersed
(1 PAR) and aquatic plants are affected when light aquatic plants that grow up through the water
falls below 10 percent of the surface PAR (10 PAR). column. This measurement is known as the
Of course, there is some variation to this rule as percent volume inhabited or PVI. The results are
different species of algae and aquatic plants
different species of algae and aquatic plants 5 Claude D. Brown, Mark V Hoyer, Roger W. Bachmann and
require differing amounts of light. Daniel E. Canfield, Jr 2000. Nutrient-chlorophyll relationships:
an evaluation of empirical nutrient-chlorophyll models using
Algae, Color and Light Florida and north-temperate lake data. Canadian Journal of
Fisheries and Aquatic Sciences. 57(8): 1574-1583.
Thanks to Florida LAKEWATCH data and
6 Roger W Bachmann, Christine A. Horsburgh, Mark V Hoyer,
an analysis made by Brown et. al. (2000), we know Laura K. Mataraza and Daniel E. Canfield, Jr 2002. Relations
that a high true color value (i.e., 50-100 PCU) can between trophic state indicators andplant biomass in Florida
have a negative effect on a lake's algal biomass.5 lakes. Hydrobiologia. 470: 219-234.


similar.7 Lakes with color measurements
that fall below 50 PCU have supported
plant growth that takes up as much as
80 percent of the lake's volume (80 PVI).
However, once true color values exceed
50 PCU, things change dramatically as
these lakes generally have plant growth
totaling 10 PVI or less. (See Figure 5 below.)

So what does this mean to the average leave a
lake user or lakefront resident? What About Emergent or Fating-leaved Plants?
It means that if a lake has significant While color can have a negative influence on submersed
color in the water, there is the potential for plants, things are a bit different when it comes to
the lake to maintain less plant growth. emergent or floating-leaved plants. In some instances,
This may be good news for individuals highly colored lakes (i.e., lakes with values higher than
who aren't fond of aquatic plants in their 50 PCU) have been known to support an abundance
lake (e.g., swimmers, waterskiers, etc.). of emergent plants such as maidencane or various
However, others who like to bird watch or floating-leaved plants such as water hyacinth, water
fish near aquatic plants might be less lettuce or spatterdock (a.k.a. cow lilies). While some of
happy. these plants may be submersed under water, growth is
still possible as long as some of the leaves are above
7 PVI is also used as an acronym for "Percent water or able to receive light.
Volume Infested. "

Figure 5 Color and Its Influence on PAC and PVI in Florida Lakes

The two graphs pictured here reflect some -..
interesting patterns regarding aquatic plant "
abundance and color in Florida lakes:
The top graph illustrates the relationship between
color and the area of a lake that is covered by plants 40 -
(i.e., Percent Area Covered or PAC).
The bottom graph illustrates the relationship between
color and the volume of a lake that is inhabited by I
plants (i.e., Percent Volume Inhabited or PVI). Color
Notice that when lake waters are highly colored, we
do not find high percentages of the lake area inhabited
by plants, nor do we find high percentages of the lake
volume inhabited by plants. In fact, the highest
percentages of plant abundance are found only in
lakes with low color values (i.e., below 100 PCU).
For lakes with color measurements below about 75 1
PCU, there is a good spread of PAC values (0 to
100%). However, notice that in the PVI graph, the 2
majority of these lakes have PVI values below 10%.
Only a few of them have higher values (i.e., ranging lo s
up to about 77%). coor


Aquatic Plants and Color

The Tsala-Apopka Chain-of-Lakes, located
in Citrus County, in the west central portion of
Florida, provides an excellent example of the
influence that lake color can have on aquatic
plant growth.
Dozens of lakes belong to this aquatic
system a fascinating network of islands, ':
marshes, canals and lakes that seem to be condition .
loosely grouped into three pools: the Floral years (2000 2003)
City, Inverness and Hernando Pools. The "
lakes within each pool are linked to each
other by natural and artificial means, but .
many of the interconnecting waterways are The lake shown in the distance is part of the Floral City Pool,
intermittent, depending on rainfall and result- within the greater Tsala-Apopka Chain-of-Lakes. The low water
water levels. level is the result of severe drought conditions that i, ,.1 this
ing particular lake system for several years (2000 2003).
Figure 6a, shown below, depicts the
amount of color measured in the Chain-of-Lakes in the late 1990s. Figure 6b provides an indication
of the abundance of aquatic plants found in the Chain-of-Lakes within the same time frame. Compare
Figure 6a with 6b. Notice that as color decreases (Figure 6a), the amount of submersed plants
increases considerably (6b).

Note: The horizontal lines below each bar graph indicate which lakes are located in the three pools.
Also, the lakes are listed in geographical order as they exist within the prevailing water flow, which
runs from south (S) to north (N) in this system.

Figure 6a Figure 6b
Color Measurements for the Tsala Apopka Chain-of-Lakes Plant Abundance in the Tsala Apopka Chain-of-Lakes

150 0

I |. | 1 g 0 |

Floral City Inverness Hemamno Floral City Inverness Hemando

s N S N


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Lake Wauberg in Gainesville, Florida.
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Without a doubt, water clarity is a major model is generally used to predict water clarity.
concern for most citizens who live on Usually these equations only include Secchi depth
or use lakes. In Florida, water clarity is and chlorophyll measurements as variables.
affected by three main factors: However in Florida, color can also affect water
St a o clarity so it must be factored into the equation.
* the amount of algae in the water;
St a o in The following Secchi-chlorophyll model was
* the amount of true color in the water;
developed using FLW data from hundreds of
* the amount of non-algal solids in the water deeloed usg F ata
lakes throughout the state:
(e.g., inorganic sediments such as sand and clay;
and organic solids such as dead plant material). Log (Secchi) = 0.86 0.36 Log (CHL) 0.27 Log (color)

For the most part, algae and true color are Where:
the major players.8 We know this thanks to many Log is the common (base 10) logarithm.
scientific studies and long-term water chemistry Secchi is the mean Secchi depth in meters.
CHL is the mean chlorophyll concentration in
data that have been collected by FLW volunteers micrograms per liter (Rg/L).
around the state since 1986. However, it's not Color is the mean true color concentration
always easy to tell which of the two is having the in Platinum Cobalt Units (PCU).
greatest influence on water clarity. That's why Note: The model above is .. from the Secchi-
scientists rely on mathematical equations, also chlorophyll model introduced in A 7 -'s Guide to Water
known as empirical models, to try and predict Management- Water Clarity (Circular 103). When it was
such things and as a way of checking each factor published, LAKEWATCH did not have the data needed to include
color in the equation. Now that we do, we can a new version
against the other. The models are derived from for those who are interested in exploring the that color
statistical analysis of a specific set of data. In this might be having on a specific lake.
case, they are derived from many years' worth of See Appendix 1, on page 21, for more on how
monthly water chemistry data, courtesy of Florida to use an empirical model.
Using these equations, one can plug in known
information from a data set (e.g., chlorophyll con-
centrations, Secchi depth or color measurements)"
and then do the necessary calculations. If the h
numbers don't match up, it may be an indication ..
that some other factor is limiting water clarity,
such as non-algal suspended solids.
For lakes that are considered to be "algae
dominated," an empirical Secchi-chlorophyll / ;
.. ... ....,
8 For more about water clarity and empirical models, see ..
SFor more aout water la an emp a e ee Students from the Narcoossee Community School in Osceola County,
A Beginner Guide to Water Management Water Florida hold up water samples collected as part of a LAKEWATCH
Clarity (Circular 103). training session.


If a lake should have high true color and nitrogen have the strongest influence.
measurements, an equation known as a multiple But that's not all.
nutrient-color regression model can be used to The model also helps us predict what chloro-
determine whether nutrients or color are having a phyll concentrations should be for a specific lake.
greater influence on chlorophyll concentrations For example, if we were to work through the
(i.e., algae) in a lake. The equation looks like this: equation by plugging in actual TP, TN and color
measurements from a lake, we could compare our
Log (CHL) = -1.92 + 0.70 Log (TP) + 0.75 Log(TN) 0.15 Log(Color) answer with chlorophyll measurements (aka CHL)
from the same lake. If the chlorophyll value is
Where: similar to our calculated answer (i.e., from the
Log is the common (base 10) logarithm, equation), we can conclude that nutrients and
CHL is the average chlorophyll concentration in color are most likely having the greatest influence
micrograms per liter (Rg/L). on algal abundance in the lake.
TP is the average total phosphorus concentration in gg/L. However, if the actual chlorophyll value
TN is the average total nitrogen concentration in gg/L. from the lake is less than the value we've calculated,
Color is the mean true color concentration in Platinum we can conclude that some other factor is influenc-
Cobalt Units (PCU). ing algal abundance and generally it's the
presence of suspended solids.
This exercise is important because it gives us
This equation was developed in much the a way of determining whether a lake's algal
la way of determining whether a lake's algal
same way as the Secchi-chlorophyll model abundance is being affected by nutrients, true
described on page 19. Both are the result of hours color or some other factor. It also tells us that
of mathematical analysis, using data from hundreds further examination will be necessary to pinpoint
of Florida lakes, exactly which factor is having the most influence.
Using this model, FLW researchers have
been able to demonstrate that true color does have See Appendix 1 on page 21, for a step-by-step
an influence on algal abundance, but phosphorus example of how to use an empirical model.


Appendix 1

How to Use an Empirical Model

To illustrate how to use an empirical model, we will work with data from a hypothetical lake named
My Lake. Let's say this waterbody has average chlorophyll concentrations of 10 micrograms per
liter (10 pg/L) and a true color measurement of 50 Platinum Cobalt Units (50 PCU). Using the
empirical model below, we can plug in these numbers and solve the equation for Secchi depth.
Once we have an answer, we can compare it with the actual Secchi depth of the lake to see if they
are similar. If the actual Secchi depth of My Lake is different from our calculated answer, there may
be something else affecting the water clarity (i.e., other than true color), such as suspended sediments.

Log (Secchi) = 0.86 0.36 Log (CHL) 0.27 Log (Color)

Log is the common (base 10) logarithm.
Secchi is the mean Secchi depth in meters.
CHL is the mean chlorophyll concentration in micrograms per liter (igg/L).
Color is the mean true color concentration in Platinum Cobalt Units (PCU).

You will need a calculator with a logarithm (LOG) button and an antilogarithm (anti-LOG)
button to make the following calculations:

Step 1 Start by finding the LOG of the chlorophyll concentration for My Lake.
To find the LOG of a number on your calculator, type in the number on the keypad (in this instance,
type in the number 10) and then push the button marked LOG. For this exercise, you should get an
answer of 1. Now that we know the LOG of our chlorophyll concentration, plug that into the equation.
In other words, replace the letters Log (CHL) with the number 1.

Log (Secchi) = 0.86 0.36 Log (CHL) 0.27 Log (Color)

Log (Secchi) = 0.86 0.36 (1) 0.27 Log (Color)

Step 2 Multiply the chlorophyll LOG of 1 by 0.36 as provided in the equation.
Log (Secchi) = 0.86 0.36 (1) 0.27 Log (Color)

Log (Secchi) = 0.86 0.36 0.27 Log (Color)


Step 3 Find the LOG of the color measurement for My Lake.
To find the LOG of a number on your calculator, type in the number on the keypad (in this
instance, type in the number 50) and then push the button marked LOG. For this exercise, you should
get an answer of 1.699. Now that we know the LOG of the color measurement for My Lake, plug that
into the equation (i.e., replace the words "Log (Color)" with the number 1.669) and calculate as
shown below:

Log (Secchi) = 0.86 0.36 0.27 Log (Color)

Log (Secchi) = 0.86 0.36 0.27 (1.699)

Log (Secchi) = 0.86 0.36 -0.45873

Step 4 Now do the remaining calculations (subtractions).

Log (Secchi) = 0.86 0.36 0.45873

Log (Secchi) = 0.04127

Step 5 Now find the antilogarithm of your result.
To do this, enter the logarithm into your calculator (i.e., the number from the right side of the equation).
You should type in the number 0.04127. While that number is on your calculator screen, push the
antilogarithm key on the keypad, which is usually represented by the symbol 10x.

Note: If your calculator doesn't have an antilog key, check the instruction booklet. Also, some
calculators rely on a,;/ lli'r m ii i1 offinding the antilog of a number. To do this, one would
need to use the y button on the calculator where y = 10 and x = 0.04127 (from the equation above).

Step 6 Check your answer.
You should get an answer of 1.0998, which can be rounded to a hypothetical Secchi depth of approximately
1.1 meters, based on chlorophyll concentrations and true color values for My Lake. If the actual Secchi
depth for My Lake happens to be 1 meter, we could say that the two numbers "agree" and the lake's
water clarity (Secchi depth) is most likely affected mostly by chlorophyll (algae) and/or true color.
However, If the actual Secchi depth of My Lake was substantially less than 1.1 meters (by ~ 0.5 meters
or more), then the model would suggest that non-algal suspended solids may be impacting water clarity.


Selected Scientific References

Books and Circulars

Hutchinson, G. E. 1957. A Treatise on Limnology. Volume I. Geography, Physics and Chemistry.
John Wiley & Sons, Inc., New York, New York.

Florida LAKEWATCH. 1999. A Beginner's Guide to Water Management The ABCs / Descriptions of Commonly
Used Terms. Information Circular #101. Florida LAKEWATCH, Department of Fisheries and Aquatic Sciences,
University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS), Gainesville, Florida.

Florida LAKEWATCH. 2000. A Beginner's Guide to Water Management Nutrients. Information Circular #102.
Florida LAKEWATCH, Department of Fisheries and Aquatic Sciences, University of Florida/Institute of Food
and Agricultural Sciences (UF/IFAS), Gainesville, Florida.

Florida LAKEWATCH. 2000. A Beginner's Guide to Water Management Water Clarity. Information Circular
#103. Florida LAKEWATCH, Department of Fisheries and Aquatic Sciences, University of Florida/Institute of
Food and Agricultural Sciences (UF/IFAS), Gainesville, Florida.

Wetzel, R. G. 1975. Limnology. W. B. Saunders Company. Philadelphia, Pennsylvania.


Roger W. Bachmann, Christine A. Horsburgh, Mark V. Hoyer, Laura K. Mataraza, and Daniel E. Canfield, Jr.
2002. Relations between trophic state indicators and plant biomass in Florida lakes. Hydrobiologia 470: 219-234.

Brezonik, P.I. 1978. Effect of organic color and turbidity on Secchi disc transparency. Journal of the Fisheries
Research Board of Canada 35:1410-1416.

Brown, Claude D., Mark V. Hoyer, Roger W. Bachmann, and Daniel E. Canfield, Jr. 2000. Nutrient-chlorophyll
relationships: an evaluation of empirical nutrient-chlorophyll models using Florida and north-temperate lake
data. Canadian Journal of Fisheries and Aquatic Sciences. 57(8): 1574-1583.

Canfield, D. E., Jr. and L. M. Hodgson. 1983. Prediction of Secchi disc depths in Florida lakes: impact of algal
biomass and organic color. Hydrobiologia 99: 51-60.

Canfield, D. E., Jr., S. B. Linda and L. M. Hodgson. 1984. Relations between color and some limnological charac-
teristics of Florida lakes. Water Resources Bulletin 20: 323-329.

Web Resources

For information about the earth's electromagnetic spectrum:

For information about plant management in Florida waters:



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