Front Cover
 Title Page
 Part 1: Background information...
 Part 2: The concept of limiting...
 Part 3: Using models to predict...
 Part 4: Limiting environmental...
 Logarithmic scales
 Description of terms

Title: Beginner's guide to water management: nutrients
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066270/00001
 Material Information
Title: Beginner's guide to water management: nutrients
Physical Description: Book
Creator: Florida LAKEWATCH
Publisher: Gainesville, FL: University of Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences
Publication Date: 2000
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066270
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.

Table of Contents
    Front Cover
        Cover 1
        Cover 2
    Title Page
        Page i
        Page ii
        Page iii
    Part 1: Background information about algae
        Page 1
        Page 2
        Page 3
    Part 2: The concept of limiting nutrients
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
    Part 3: Using models to predict algal abundance
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Part 4: Limiting environmental factors other than nutrients
        Page 23
        Page 24
    Logarithmic scales
        Page 25
        Page 26
    Description of terms
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
Full Text

A Beginner's Guide to

Water Management Nutrients

Information Circular #102

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

August 2000

Institute of Food and Agricultural Sciences Ecsyst e ReearchAK

7922 NW71st Street
Gainesville, FL 32653-3071
Phone: (352) 392-9617 ext 228
Citizen message line: (800) 525-3928
Fax: (352) 392-3462
E-mail: lakewat@ufl.edu
Web address: http://www.ifas.ufl.edu/~lakewatch/index.htm

Copies are available for download from the LAKEWATCH web site at:

Copyright @ 2000
Limited reproduction of and/or quotation from this book is permitted, providing proper credit is given.

A Beginner's Guide to

Water Management Nutrients

Information Circular #102

Department of Fisheries and Aquatic Sciences
Institute of Food and Agricultural Sciences
University of Florida
Gainesville, Florida
August 2000

Institute of Food and Agricultural Sciences st. s1 WATCH


L~trgc I' ~~b13~~r nr r ,,,




'~--1 ~-I~C~3L~sC-* P~T lm
I ~







N nutrients are substances s. .1 by all organisms for :... :, and they are
found in all '...:, :: i. i:* Algae and aquatic plants need nutrients in order to
grow. To help you work more i. :1 I :1: waterbody managers, this circular
: 1' basic information about nutrients, their relationship to the : ... : of algae in
,. : .... ::,. and .... ::.. i : and mathematical tools you can use to achieve water
management goals relative to algal abundance.
You'll notice that our main ...:: is on algae -* 1: 1: ::. mention of rooted and/or
Si .:: leaved aquatic plants. While aquatic plants are a major factor influencing the
:::::.. of Florida lakes, for the : :: i of this circular, we've chosen to concentrate
on algae and the various .: :. *: that can limit or enhance algal abundance in waterbodies.
The dynamics of larger rooted and/or floating-leaved aquatic plants (called macrophytes)
i i be discussed in a .. :..:: :1- publication.
While reading this circular we'd like for you to .. i in mind that all water i:::
management: :: H..: : ,.1 : :. :.: 1 on nutrients or some other waterbody characteristic,
should be based on i defined management goals. And contrary to what most of us
might think, defining management goals often takes place in the public/political arena
instead of a scientific one. However a more scientific approach, including the information
provided in i:i:: circular, can be valuable in that it can ;:. : 1. a perspective for evaluating
various management options and their :.::.
When .' : :: : the : : ::.:::. water management arena, citizens as i i as water-
body managers should keep in mind that generalities, particularly statistically derived
ones, may not always apply to an individual waterbody. They should also be aware that
the management of a specific waterbody can be as much an art as it is a science at this
point in time.
It's also: .: ..: ,:. to remember that water management, and each of its related
disciplines (law, public : i: science, etc.), is ::. .:.::: evolving. The only commonal-
ity is that concerned and :.::, ,:. i people are involved throughout the process.
The: i. :: .... ..- described in this circular represent water management concepts
related to nutrients as Florida LAKEr'F.TCH professionals have come to know them:

I Background Information About Algae
T.i.. of Algae
Measuring Algae
The Role of Algae in ::. :. .. .
-.-: :: Are There Too Many Algae?
2 The Concept Of Limiting Nutrients
About Phosphorus
About Nitrogen
Determining The Limiting ": ::. ::; In A Waterbody
3 Using Models to Predict Algal Abundance
4 Limiting Environmental Factors Other Than Nutrients

Part 1





Algae are a wide variety of tiny and often microscopic plants, or plant-like
organisms, that live both in water and on land. If your management
goals include the manipulation of algae in your waterbody,
then the more you know about algae, the better.

Types of Algae The word algae is plural (pronounced AL-jee),
One common way to classify water-dwelling and alga is the singular form
algae is to categorize them based on where they (pronounced AL-gah).
live. Using this system, three types of algae are
commonly defined as follows:
The amount of and types of algae found in
+ phytoplankton float freely in the water; lakes (called phytoplankton community structure)
+ periphyton are attached to aquatic vegetation changes with increased nutrient concentrations.
or other structures; and In Florida, the phytoplankton of nutrient-
+ benthic algae grow on the bottom or bottom poor lakes are often dominated by green algae;
sediments. diatom abundance tends to be greatest in moder-
Algae may further be described as being ately nutrient-rich lakes; and blue-green algae
single-celled; colonial (grouped together in colo- (also called cyanobacteria) tend to be the pre-
nies) or filamentous (appearing as hair-like dominate phytoplankton in nutrient-rich lakes.
strands). The most common forms of algae are also
described by their colors: green, blue-green, red, Measuring Algae
and yellow. All these classifications may be used In addition to describing types of algae, it's
together. For example, to describe blue-green, hair- also useful to measure quantity. The amount of
like algae that are attached to an underwater rock, algae in a waterbody is called algal biomass.
you could refer to them as "blue-green filamentous Scientists commonly make estimates of algal
periphyton." biomass based on two types of measurements -
Free-floating algae, called phytoplankton, (1) chlorophyll concentrations and (2) counting
are further classified into three categories: green and measuring individual algae. These are
algae, diatoms, and blue-green algae. described as follows:

1 Chlorophyll Concentrations Algae can be categorized
Because almost all algae contain chlorophyll based on where they live.
(the green pigment found in plants), the concentration
of chlorophyll in a water sample is used to indicate Phytoplankton
the amount of algae or algal biomass present. float freely in the
Chlorophyll concentrations are expressed as units of water and can
be classified into
micrograms per liter (abbreviated gg/L) or in be classified into
milligrams per cubic meter (abbreviated mg/m3). the mn
These are equivalent units of measure and either diatoms .i
may be used to describe chlorophyll concentrations. + green algae
It should be remembered that collecting algae blue-green algae.
from water samples does not provide measurements
for all types of algae, only the phytoplankton.
It's a common practice for scientists to use the
phrases chlorophyll concentration or chlorophyll
a concentration when they are referring to the
amount of algae in a waterbody. Chlorophyll a is
one of several types of chlorophyll, as are chlorophyll g ag
b and chlorophyll c. The measurement of all three of
these types of chlorophyll in one water sample is Periphyton Benthic
referred to as a total chlorophyll concentration. are attached to aquatic algae grow
NOTE: In this document, all estimates of the vegetation or other on bottom or
amount of algal biomass in a waterbody will be underwater structures. bottom sediments.
based on total chlorophyll measurements.

ce See Appendix Bfor more information
on chlorophyll.

2 Counting Individual Algae
In certain cases, scientists prefer to count
individual algal organisms in a water sample. They
typically identify the individual algae by genus and Algae may
species, and then calculate cell volume by approxima- further be
tion to the nearest simple geometrical shape, such as a described as:
sphere or a cylinder. Using this information, the total
biovolume of algae in any sample can be estimated. + single-celled
+ colonial C
The Role of Algae in filamentous
Regardless of what humans might think, algae
are essential to aquatic systems. As a vital part of the
food web, algae provide the food and oxygen neces-
sary to support most aquatic animal life. Certain types
of algae, such as the larger benthic forms, also provide
habitat for aquatic organisms. On occasion, however,
algae can become troublesome. For instance:


* The concentration of phy- In Florida, when chlorophyll concentrations
toplankton (free-floating algae) .reach a level over 40 pg/L, some scientists will call it
in the water strongly influences C an algae bloom or algal bloom. The public, however,
water clarity. -U I usually has a less scientific definition often defining
Water clarity is commonly f algal blooms as events in which more algae can be
measured by using a Secchi seen in the water than they are accustomed to seeing. In
(pronounced SEH-key) disc. some cases, this may even be a relatively low amount.
A Secchi disc is a flat 8-inch
diameter disc that has a cordAlgae and Fish Kills
When algal biomass exceeds 100 Wg/L (measured
attached through the center. as chlorophyll concentrations), there is an increased
The disc is lowered into the water and the depth at probability of a fish kill. Fish kills, however, typically
which it vanishes from sight is measured, usually in only occur after three or four cloudy days. During this
feet or meters. This measurement of the transpar- time, algae consume oxygen rather than produce it
ency of the water is called the Secchi depth. In because they don't have sunlight available to help them
waters with low concentrations of phytoplankton photosynthesize more oxygen. This can lead to oxygen
(less than 10 pg/L), Secchi depths are generally depletion. Without oxygen, aquatic organisms, including
greater than 10 feet. In waters with high concentra- fish, die. Chlorophyll concentrations below 100 pg/L
tions of phytoplankton (greater than 40 pg/L), generally do not adversely affect fish and wildlife, but
Secchi depths are typically less than 3 feet. dead fish and wildlife can occasionally be found.
+ Benthic algal blooms, filamentous algal blooms, Health Concerns
and periphyton blooms can create accumulations Newspapers and magazines often present articles
along shorelines and have the potential to interfere describing toxic algae. However, most algae are not
with recreational activities such as boating and toxic and pose very little danger to humans. It should
fishing, as well as block lake access and navigation, be remembered that toxic algae can be found in all
* Algal blooms can block sunlight, shading aquatic environments. Known health problems associated
submersed aquatic plants which may be deemed with algal blooms in lakes and ponds have generally
desirable. been associated with high concentrations of three
species of blue-green algae: Anabaenaflos-aquae,
* An algal bloom can trigger a chain of events that Microcystis aeruginosa, and Aphanizomenonflos-
can result in a fish kill. This is most likely to occur aquae. With few exceptions, only fish and invertebrates
after several days of hot weather with overcast have died from the effects of these toxic algae.
skies and is related to oxygen depletion in the In Florida, it is extremely rare for algae to cause
water. It is not related to the toxicity of the algae. human illness or death. People are more likely to suffer
minor symptoms such as itching. However, several
When Are There Too Many species of algae produce gases that have annoying or
Algae? offensive odors, often a musty smell. These odiferous
Algalblooms may be caused by human gases may cause health problems for some individuals
Algal blooms may be caused by human
with breathing difficulties.
activities, or they may be naturally occurring. with breathing difficulties.
To be prudent, people should inform their doctor if
Sometimes, what seems to be an algal bloom is
me e, wt b n i they are experiencing any health problems and live near
merely the result of wind blowing the algae into a
a waterbody or use a waterbody often. This is critically
cove or onto a downwind shore, concentrating it in important in recent years because there is an alga called
a relatively small area. (This is known as wind-rowing.) Pfteseria that is known to cause severe health problems.
Looking at algae from the non-scientific point of Pfiesteria tends to be found primarily in tidal waters.
view, some people consider algae to be unsightly, While prudence must be the watchword when using any
particularly when it is abundant. For instance, a waterbody, it must also be recognized that people will
phytoplankton bloom can make water appear so face a greater risk during their drive home from the
murky that it's described as "pea soup." grocery store than from Pfiesteria or any other algae.


Part 2


Concept of



A limiting nutrient is a chemical necessary
for plant growth but is available in The chemical symbol for the
smaller quantities than needed for algae to element phosphorus is P.
increase their abundance. Once the limiting
nutrient in a waterbody is exhausted, the population
of algae stops expanding. If more of the limiting In waterbodies, phosphorus occurs in two
nutrient is added, larger algal populations will forms: dissolved and particulate.
result until their growth is again limited by nutrients Dissolved phosphorus is defined based on its
or by other limiting environmental factors. size, as that which is small enough to pass through
It's helpful to know if there is a limiting a 0.45 micron filter It includes phosphorus forms
nutrient (or some other limiting factor) in your like soluble reactive phosphorus and soluble
lake, as an increase of the limiting nutrient could organic compounds that contain phosphorus.
affect change in the lake. Its counterpart particulate phosphorus, is
There are many potentially limiting nutrients. too big to pass through a 0.45-micron filter. It is
For example, silica is sometimes known to limit formed when phosphorus becomes incorporated
the growth of diatoms. Although scientists may into particles of soil, algae, and small animals that
debate which nutrient is the limiting factor at any are suspended in the water. Both dissolved and
given time, phosphorus and nitrogen are most particulate phosphorus can change from one form
often the limiting nutrients in Florida waterbodies. to another very quickly (called cycling) in a water
body and there is ongoing scientific inquiry about
About Phosphorus when, where, and how often these specific forms of
phosphorus are found in waterbodies. This is
Phosphorus is an element that, in its different
Phosphorus is an element that, in its different important because algal cells and plants can only
forms, stimulates the growth of algae in waterbodies. phosphorus in certain forms.
Phosphorus compounds are also found naturally in Understanding the relationship between algae
many types of rocks and soils. In fact, phosphorus is and phosphorus is further complicated by the fact
mined in Florida and other parts of the world for a that an algal cell's ability to use specific forms of
variety of agricultural and industrial uses. In most phosphorus is strongly influenced by several
freshwater lakes in Florida, the limiting nutrient is factors including pH, water hardness (caused by the
believed to be phosphorus rather than nitrogen. gnesium), the
presence of calcium and/or magnesium), the

amount of dissolved oxygen in the water, and + Some areas of Florida and other parts of the
thermal stratification (layers of water having world have extensive phosphate deposits in the
different temperatures). soils. In these areas, rivers and water seeping or
This process of phosphorus cycling makes it flowing underground can become phosphorus
difficult to measure dissolved or particulate phosphorus enriched and may carry significant amounts of
in a waterbody at a given time. However, total phosphorus into waterbodies.
phosphorus concentrations (abbreviated TP), + Sometimes phosphorus is added intentionally
which include both dissolved and particulate forms, to waterbodies as a management strategy to
can be used to gain an estimate of the amount of increase fish production by fertilizing aquatic
phosphorus in a system. Florida LAKEWATCH plant and algal growth.
measures total phosphorus because it provides a
snapshot of the total phosphorus concentrations in a + Phosphorus can enter waterbodies inadvertently
lake at a given time. as a result of human activities like landscape
fertilization, crop fertilization, wastewater disposal,
There are many ways in which phosphorus and stormwater run-off from residential develop-
compounds find their way into waterbodies. m
ments, roads, and commercial areas.
Some of the more common pathways are
described as follows:

Waterbodies in the Florida LAKEWATCH database analyzed prior to January 1998,
had total phosphorus concentrations which ranged from less than 1 to over 1000 pg/L
(0.001 to 1 mg/L). Analysis of total phosphorus concentrations in Florida shows the
following relationships. These relationships should be of interest to anyone trying to
manage phosphorus concentrations in a Florida lake-and are important to consider
when attempting to evaluate the feasibility of goals you or others may set for
phosphorus levels in a waterbody.1

There seems to be a relationship between the location of a waterbody and its total
phosphorus concentration.
For example, lakes in the New Hope Ridge/Greenhead Slope Lake Regiono of northwestern
Florida (in Washington, Bay, Calhoun, and Jackson counties) tend to have extremely low total phos-
phorus values (below 5 ig/L). While lakes in the Lakeland/Bone Valley Upland Lake Region of central
Florida (in Polk and Hillsborough counties) tend to have very high values (above 120 pg/L).2

o Lake Regions are geographical areas in which lakes have similar geology, soils, chemistry, hydrology,
and biological features. In 1997, using Florida LAKEWATCH data and other information, the United
States Environmental Protection Agency designated 47 lake regions in Florida using these similarities as
their criteria. For more information, see Lake Regions in the Appendix A.

Using the Florida LAKEWATCH database, it can be shown that there is a seasonal pattern for
total phosphorus concentrations in Florida lakes.
Monthly total phosphorus concentrations tend to be lower during December and January, but
higher and more variable during the rest of the year. Typically, the maximum measured total phosphorus
concentration occurs most frequently in August and October or from February through May in some
lakes. Minimum measured total phosphorus concentrations occur most frequently from November
through February.
1 For more ,, ..... d- ,."., i on a 'pec ific LAKEWATCH waterbody, you can call the Florida LAKEWATCH office
(1-800-LAKEWATCH) and request a data ;, i., i r.. iri l., a r I.., 1 It's also recommended that you refer to the following
LAKEWATCH handouts Florida Lake Regions: A Classification System; Trophic State: A Waterbody'sAbility to Support
Plants, Fish, and Wdlidfe; and Florida LAKEWATCH Data What Does ItAll Mean?
2 Totalphosphorus concentrations in an individual waterbody may not be at these levels all the time; they can be
quite variable over time.


A Bum Rap
Because waterbodies with low concentrations
of total phosphorus (TP) will have relatively
clear water, the public may think their water
quality is better than waterbodies with higher TP.
It's a misconception however, that clearer water is
intrinsically better than water that is less clear.
Unfortunately, the association of clear water
with low phosphorus levels has given the public
the mistaken notion that phosphorus is a pollutant.

Total Phosphorus and
Total Phosphorus and For more information, see Part 4 Limiting
Biological Productivity Environmental Factors Other Than Nutrients
One major task that lake experts are faced onpage 23.
with in water quality management is assessing
the biological productivity of a waterbody Trophic State
and determining whether it's changing over time. While discussing a lake's biological produc-
However, overall biological productivity is tivity with aquatic scientists, you may hear the
difficult to measure in a waterbody because it term trophic state. Trophic state is just another
involves measuring many different parameters way of saying biological productivity. The
over a period of time. Such an approach would Trophic State Classification System is one
be prohibitively expensive and time consuming. method scientists use to quickly and easily
Because of this, many aquatic scientists use total describe the biological productivity of a water-
phosphorus measurements, often alone, as an body. It's one of the more commonly used
indirect way of assessing the biological productivity systems worldwide and is used by Florida
of a waterbody. LAKEWATCH.
Why? The Trophic State Classification System
Because phosphorus is one of the main classifies lakes and/or waterbodies into one of
nutrients that can limit the biological productivity four trophic states:
of a waterbody. However, this is not always the.
waterbodies with low productivity are called
most accurate way to assess the biological oligotrophic (oh-lig-oh-TROH-fic);
oligotrophic (oh-lig-oh-TROH-fic);
productivity of a waterbody. Other factors may
also limit biological productivity, such as availability those with moderate productivity are called
of light. mesotrophic (mes-oh-TROH-fic);
moderate-to-highly productive waters are called
c- See Color and Humic acids in Appendix B. eutrophic (you-TROH-fic);
eutrophic (you-TROH-fic);
+ and highly productive waters are called
Biological Productivity f hypereutrophic (HI-per-you-TROH-fic).
is the amount of algae,
aquatic plants, fish, and wildlife
that a waterbody can produce and c- For more information, see Trophic state
sustain. and Trophic State Index in Appendix B.


Phosphorus As A
Limiting Nutrient
Because phosphorus is frequently the limiting
nutrient in the growth of free-floating algae in
lakes, it is strongly believed in the scientific
community that waterbodies with higher phosphorus
levels will have higher levels of algae and water-
bodies with low phosphorus concentrations will
have lower levels of algae. This belief is based in
part on surveys of lakes, both in Florida and
throughout the world, and on results of whole-
lake experiments.
A picture of this relationship emerges when i
average yearly chlorophyll concentrations, from a
group of LAKEWATCH lakes, are plotted on a
graph versus the total phosphorus concentrations.
(See Figure 1 on page 8.) The graph shows that
increasing phosphorus values are generally
accompanied by increasing chlorophyll levels. See Determining the Limiting Nutrient
Consequently, aquatic scientists almost always In A Waterbody (page 11); Part 3 Using
recommend the manipulation of phosphorus, called Models to Predict AlgalAbundance (page
Models to Predict Algal Abundance (page
phosphorus control, as a primary management 17), and Limiting Environmental Factors
strategy for controlling algal biomass. Other Than Nutrients (page 23).
The high priority placed on phosphorus
control by regulatory and professional manage-
control by regulatory and professional manage- 3 This relationship is indicated by the observation that the
ment agencies in Florida is evidenced by its use points in the graph that are further to the right are also
in the multi-million dollar lake management generally higher up. The correlation is true in spite of the
programs at Lake Apopka and Lake Okeechobee. fact that chlorophyll concentrations can be highly variable
However, phosphorus is not always the for any specific totalphosphorus concentrations.
limiting nutrient and phosphorus removal may not 4 This distribution oftrophic state is based solely on total
be the best management approach to controlling phosphorus values without utilizing information on nitrogen
algal biomass. concentrations, chlorophyll concentrations, Secchi depth, or
aquatic plant abundance.

Total Phosphorus and Trophic State
Using ONLY average concentrations of total phosphorus (TP) from the Florida
LAKEWATCH database, Florida lakes were found to be distributed into the four trophic states as
described below.4
+ Approximately 42% of the lakes (those with TP values less than 15 pg/L) would be classified as olig-
otrophic. Oligotrophic lakes have very low levels ofbiological productivity.
+ About 20% of the lakes (those with TP values between 15 and 25 pg/L) would be classified as me-
sotrophic. Mesotrophic lakes have moderate levels of biological productivity.
30% of the lakes (those with TP values between 25 and 100 pg/L) would be classified as eutrophic.
Eutrophic lakes have moderately high levels ofbiological productivity.
+ Nearly 8% of the lakes (those with TP values greater than 100 pg/L) would be classified as
hypereutrophic. Hypereutrophic lakes have very high levels ofbiological productivity.




0.1 -------i------------i------i
F 100igue


01 1 10 10 1000

The relationship between total phosphorus concentrations and total chlorophyll
concentrations for Florida lakes.

The graph shown here in Figure 1 is a scatter plot graph. Scatter plot graphs are good for
plotting more than one type of measurement on the same graph. Notice how this scatter plot graph
represents both total chlorophyll and total phosphorus concentrations at the same time. You'll see
more of these types of graphs in this circular as well as water management publications, meetings,
and seminars.
While studying this graph, you may also notice that the numbers on each axis are represented
in multiples of 10. It's arranged this way because this particular scatter plot graph is formatted using a
common logarithmic scale.5 Rather than plotting the phosphorus and chlorophyll concentrations directly,
we plotted logarithms of the concentrations. By using this type of logarithmic scale, we were able to
stretch out the scale at the lower end of the graph so that more of the individual points could be seen.

For more on logarithmic scales see Appendix A.

5 Remember that common !., i,,,' are the exponents ofthe number 10. For example in the equation 102 = 100,
we can see that the !. ,' ,, ,i'ii of 100 is 2. And using the equation 103=1000, we can see that the !. ',i ri,'i
of 1000 is 3. Similarly, the equation 10'=10 tells us that the !. ii, ,i,, of 10 is 1.


About Nitrogen The chemical s mbol for the
Nitrogen is also a necessary nutrient for the element iitrOell is N.
growth of algae and aquatic plants. Various forms
of nitrogen can be found in water including organic
and inorganic forms. inorganic forms of nitrogen such as nitrates,
Organic forms of nitrogen are derived from nitrites, and ammonia.
living organisms and include amino acids and Nitrogen finds its way into aquatic environments
proteins, from both natural and man-made sources
Inorganic forms are composed of materials including:
other then plants or animals (i.e., mineral based) + the air some algae can "fix" nitrogen, or pull
and include nitrate (NO,-), nitrite (NO,- ), unionized nitrogen out of the air in its gaseous form and
ammonia (NH4), ionized ammonia (NH,'), and convert it to a form they can use;
nitrogen gas (N,). + stormwater run-off- nitrogen can even come
Total nitrogen (abbreviated TN) is a measure from "natural" run-off from areas where there is no
of all the various forms of nitrogen found in a water human impact because it is a naturally-occurring
sample, except nitrogen gas. Not all forms of nutrient found in soils and organic matter;
nitrogen can be readily used by algae especially 4 fertilizers; and
nitrogen bound with particulate organic matter. In + animal and human wastes (sewage, dairies,
general, algae and aquatic plants directly utilize feedlots, etc.).

Waterbodies in the Florida LAKEWATCH database analyzed prior to January 1998,
had total nitrogen concentrations which ranged from 50 to over 6000 pg/L (0.05 to 6
mg/L). Analysis of total nitrogen concentrations in Florida shows the following
relationships that should be of interest to anyone trying to examine nitrogen
concentrations in their lakes. As with phosphorus, these relationships provide a
useful background against which a waterbody manager can evaluate the feasibility
of specific management goals.

The location of a waterbody has an important effect on its total nitrogen concentration.
For example, lakes in the New Hope Ridge/Greenhead Slope Lake Regiono in northwestern
Florida (Washington, Bay, Calhoun, and Jackson counties) tend to have very low total nitrogen
values (below 220 .ig/L). While lakes in the Lakeland/Bone Valley Upland Lake Region in central
Florida (Polk and Hillsborough counties) tend to have high values (above 1700 pg/L).
o Lake Regions are geographical areas in which lakes have similar geology, soils, chemistry, hydrology,
and biological features. In 1997, using Florida LAKEWATCH data and other information, the United
States Environmental Protection Agency designated 47 lake regions in Florida using these similarities as
their criteria.

c For more information, see Lake Regions in Appendix B.

Total nitrogen concentrations, like phosphorus concentrations, can vary seasonally in
individual lakes.
The variability in monthly total nitrogen concentrations is relatively low however, when
compared to the amount of variation observed in algal levels and in total phosphorus concentrations in
Florida lakes throughout a year. If there is a period when total nitrogen concentrations can be expected to
be low, it generally occurs during the months of January and February. Maximum total nitrogen concentra-
tions generally occur most frequently during the months of April, May, and October.


Nitrogen As A Limiting
Like phosphorus, nitrogen is an essential
nutrient for all aquatic plants. In some cases, an
inadequate supply of TN in waterbodies has been
found to limit the growth of free-floating algae

divided by the TP concentration is less than 10 s-olEr
(TN/TP < 10).

l Fit r morn, n ocrmtos nsee etermmonl n the
Limiting Nutrient In A Waterbody on pages nte
11-16 a nitrogeLimitingotal Environmen tale
tFact ors Other Than Nutrients on page 23.a-

Like many people throughout the world, Howver nittesare a lyolem
Floidians are concerned about water quality. Water Sunsetshingfor bass on Lake Elbert in Winter Haven
quality is sometimes defined in terms of human
health effects and toxicity to aquatic organisms. nitrates are readily taken up by plants and used as
With regard to nitrogen, total nitrogen in surface nutrients. Most forms of nitrogen are harmless to fish
waters does not reach high enough levels to pose a and aquatic organisms except unionized ammonia
direct threat to human health. The maximum and nitrite, which can be toxic.
allowable level of nitrate, a component of the total However, nitrites are usually not a problem in
nitrogen measurement, is 10 mg/L in drinking waterbodies; if there is enough oxygen available in
water Generally, concentrations of nitrates in the water, nitrte s oxidize and are readily la converted
surface waterbodies do not reach thi level because to nd traties. orgai cpt uio

Total Nitrogen and Trophic State
When ONLY the average concentrations of total nitrogen (TN) from the Florida LAKE-
WATCH database are used, Florida lakes were found to be distributed into the four trophic states as
described below.6
+ Approximately 14% of the lakes (those with TN values less than 400 gg/L) would be classified
as oligotrophic. Oligotrophic lakes have very low levels of biologicalproductivity.
+ About 25% of the lakes (those with TN values between 401 and 600 gg/L) would be classified
as mesotrophic. Mesotrophic lakes have moderate levels of biological productivity.
+ 50% of the lakes (those with TN values between 601 and 1500 gg/L) would be classified as
eutrophic. Eutrophic lakes have moderately high levels of biological productivity.
+ Nearly 11% of the lakes (those with TN values greater than 1500 gg/L) would be classified as
hypereutrophic. Hypereutrophic lakes have very high levels of biological productivity.
6 This distribution oftrophic state is based solely on total nitrogen values without utilizing information on
total phosphorus concentrations, chlorophyll concentrations, Secchi depth, or aquatic plant abundance.


Determining The Limiting Nutrient

In A Waterbody

Aquatic scientists routinely recommends mre o h T r
cr See pages 13-16for more on how TN/TPratios
nutrient (phosphorus and nitrogen) control to andPhosphorus Threshold Values can be used to
manipulate algae populations in a waterbody. determine limiting nutrients in waterbody.
Controlling nutrients, as a way of manipulating
algae, is one strategy for managing fisheries, Some Nutrient Other Than
water clarity, and wildlife populations. This Phosphorus Or Nitrogen Is the
strategy, however, only works if phosphorus and/ Limiting Nutrient
or nitrogen are the environmental factors limiting As mentioned earlier in this circular, nutrients
algal abundance. like silica can be limiting in some Florida water-
If nutrients are the environmental factors limit- bodies. In addition, micronutrients8 that are also
ing algal abundance, you necessary for the growth
may be able to achieve over- of plants and algae (such
all management goals If nutrients, as molybdenum and zinc),
through nutrient control.7 rather than some other may be in limited supply
However, there are many environmental factor, in some circumstances.
methods for managing the are limiting the growth of Tests to evaluate these
growth of algae in water- algae in a waterbody, there substances as potential
bodies and the appropriate are a few possibilities that limiting nutrients are
method of nutrient control deserve consideration. sometimes recommended.
is often debated at length. The tests are relatively
This debate can often be expensive, so they should
sidetracked by discussions over which nutrient, only be considered if phosphorus and nitrogen
phosphorus or nitrogen, is limiting, are eliminated as possibilities.
If nutrients, rather than some other environmental In addition, nutrients are not always the
factor, are limiting the growth of algae in a water- limiting factor. Other environmental factors such
body, there are a few possibilities that deserve as highly colored water can also influence the
consideration: abundance of algae in a waterbody.

+ Phosphorus and/or Nitrogen Is cr For more on limiting environmental factors
The Limiting Nutrient see Part 4 Limiting Environmental Factors
There are two approaches that can be used to Other Than Nutrients on pages 23 and 24.
help you and/or a water manager decide whether:

* phosphorus is the limiting nutrient, 7 Ifyou have not developed a managementplan yet, you
s te l g n t, or may want to read the booklet How To Create a Lake
Snitrogen is the limiting nutrient, or
Management Plan by Jess VanDyke, Northwest Florida
* both phosphorus and nitrogen are limiting Regional Biologist, Department ofEnvironmental
nutrients in a waterbody. Protection/Bureau ofAquatic and Invasive Plant
Management. Free copies are available from Florida
One involves the use of a TN/TP Ratio (total LAKEWATCH.
nitrogen/total phosphorus ratio) and the other 8 The term micronutrient indicates that plants and algae
involves the use of Phosphorus Threshold Value. need only tiny amounts of this nutrient. Contrary to its name,
a micronutrient is of no smaller importance than a nutrient.





qa "a


Calculating a relatively simple ratio can sometimes provide a useful
clue as to the relative importance of nitrogen or phosphorus toward
the abundance of algae in a waterbody. Studies of Florida lakes
have shown that the ratio of total nitrogen to total phosphorus (TN/TP)
may indicate which nutrient plays the most significant limiting role.

By calculating TN/TP ratios for 534 Florida lakes...
and plotting them on a scatter plot graph, a useful relationship emerges.
The scatter plot graph shown here (see Figure 2), illustrates that based on the relation-
ship between total chlorophyll, total phosphorus values, and TN/TP ratios, Florida
waterbodies can be loosely divided into three groups:
Lakes with a TN/TP ratio less than 10 (represented by a A in the graph)
Lakes with a TN/TP ratio between 10 and 17 (represented with a O in the graph)
Lakes with a TN/TP ratio greater than 17 (represented with a 0 in the graph)

TN/TP i10
o TN/TP 10-17

TN/TP 217


0.1 ---------------------------
0.1 1 10 100 1000

Total Phosphorus (jig/L)

Figure 2. The relationship between total phosphorus concentrations and chlorophyll
concentrations for Florida lakes.


Lakes with a TN/TP ratio less than 10 (represented with a A in the graph)
Notice that lakes in this group tend to be grouped in the upper right-hand comer of the graph, beyond
where the 50 pg/L mark would be on the total phosphorus axis. Also notice that none of these lakes appear
on the graph anywhere below the 50 pg/L mark on the total phosphorus axis. This can be interpreted to
mean that phosphorus may not be the only factor affecting the growth of algae in these lakes.

Lakes with a TN/TP ratio between 10 and 17 (represented with a O in the graph)
Notice that, similar to the lowest TN/TP ratio group, this group of lakes also tends to be grouped in
the upper right-hand corer of the scatter plot graph with a few lakes scattered down toward the
bottom left hand comer. Also, notice how many of lakes with higher TN/TP ratios (greater than 17 and
represented by @), have higher chlorophyll levels than lakes with the same amount of phosphorus (lakes
represented by the A and the 0).
This can be interpreted to mean that a specific amount of phosphorus in the A or O lakes will not
produce as much algae as that same amount of phosphorus in lakes with a TN/TP ratio greater than 17
(@ lakes). Something is limiting the growth of algae (chlorophyll) in these lakes. However, it's unclear as
to whether it's nitrogen or phosphorus.

Lakes with a TN/TP ratio greater than 17 (represented with a 0 in the graph)
The broad range of the black dots on graph can be interpreted to mean that lakes with the highest
TN/TP ratio (greater than 17) generally have more chlorophyll per unit of phosphorus than lakes with
lower TN/TP ratios. In other words, there seems to be a stronger correlation between phosphorus and
chlorophyll in these lakes.

In light of these observations, some scientists think that
something other than phosphorus must be limiting the algal growth
in the lower two TNITP ratio groups (the A and 0 lakes) possibly nitrogen.
Therefore, these scientists hypothesize:

when the TN/TP ratio is less than 10, a lake is nitrogen-limited;
when the TN/TP ratio is between 10 and 17, there appears to be a gray area
(nitrogen or phosphorus could be limiting);
when the TN/TP ratio is greater than 17, a lake is phosphorus-limited.

Aquatic scientists have differing opinions as to whether 10 and 17 are the exact boundary
values and whether this relationship applies to all waterbodies. Perhaps the TN/TP ratio can be useful
in helping you decide whether nitrogen or phosphorus is the limiting nutrient in your waterbody.

To calculate a TN/TP ratio...

Take the TN (total nitrogen) value and divide it by the TP (total phosphorus) value.
For example: If your lake's TN value is 300 and the TP value is 30, you'll need to divide
300 by 30 ... giving you a TN/TP ratio of 10.

300 + 30 = 10

NOTE: The TN and TP values you use to calculate this ratio can be from one day water sample, or they
can be attained by averaging a year's worth of monthly sample concentrations -called an annual mean.


There appears to be two phosphorus thresholds in Florida lakes.

I n lakes with TP concentrations above 100 up/L, there is the potential for
nitrogen to be the limiting nutrient, rather than phosphorus. Why?
It seems reasonable to assume that when the phosphorus concentration is high
(e.g., above some threshold level), the probability that phosphorus will be the
limiting nutrient decreases simply because of its abundance. Several observations
support the idea that a phosphorus threshold of 100 pg/L separates Florida lakes
that are phosphorus limited from those that are not. For example:
+ Many lakes with total phosphorus concentrations exceeding 100 pg/L have TN/TP
ratios that suggest they are not phosphorus limited (their TN/TP is generally less than
17, as shown in Figure 3 on page 16).
+ Evidence from surveys of lakes suggests that concentrations of total phosphorus above
a threshold value of 100 pg/L do not correspond to higher concentrations of chlorophyll.

2 A second threshold of 50 u/iL is evident from Figure 3 on page 16.
Lakes with a TN/TP ratio less than 10 (which suggests a nitrogen limitation) generally do
not occur when total phosphorus concentrations are less than 50 pg/L. This observation
suggests that waterbodies with TP less than 50 pg/L are indeed most likely to be
phosphorus limited.
0 There is one documented exception.

Check your Florida LAKEWATCH data to see if your waterbody falls into one of
these categories. Ifso, phosphorus may NOT be a limiting nutrient

Phosphorus control pitfall...

In nitrogen limited waterbodies, it may well be that both phosphorus and nitrogen need to be
controlled simultaneously in order to manipulate chlorophyll concentrations. The TN/TP ratio offers a
clue to understanding why. There is a positive relationship between phosphorus and chlorophyll in
lakes with a TN/TP ratio less than 10, as shown in the upper graph in Figure 3 (page 16). This
relationship suggests that by lowering the phosphorus concentration in one of these lakes, the
chlorophyll concentration can be made to decrease.
This may be true if the lake maintains a TN/TP less than 10. However, as TP is lowered,
the TN/TP ratio will increase because its denominator is becoming smaller. If the TN/TP ratio
becomes greater than 10, the chlorophyll concentration could actually stay the same, even though
phosphorus has been reduced. Additional research is needed before phosphorus control techniques
alone can be relied upon to reduce chlorophyll concentrations in lakes with high phosphorus
concentrations (TP greater than 50 ig/L).



250 chlorophyll = (0.302 x TP) 1.533


'1 500A A A
0 -
0 50 100 150 200 250 300 350 400 450 500

Chlorophyll = (0.628 x TP) 2.402

o 200- o

150- -p
& 0 o
0L 0
o0 Lt 0o

Lo 0 50 100 150 0 2 250 300 350 400 450 500

S* chlorophyll = (1.248 xTP) 9.904
250- *

Al 200
CL b 0* **
S150- *


0 50 100 150 200 250 300 350 400 450 500

Total Phosphorus (pg/L)

Figure 3. The relationship between total phosphorus concentrations and chlorophyll
concentrations for Florida lakes with different total nitrogen to total phosphorus ratios.


Part 3

Using Models

to Predict



I t is possible to estimate how much algae can Once you have a chlorophyll prediction, you
be expected in your lake based on hypothetical can decide whether a specific management strategy
changes in the amount of nutrients entering is realistic. You can then evaluate whether it is
the waterbody. These hypothetical situations can be worth the cost of implementing that strategy. For
graphed as nutrient-chlorophyll relationships example, is it worth a large expenditure of dollars
similar to Figure 1 on page 8, or converted into to decrease algal levels so that water clarity
mathematical formulas referred to as empirical increases from 0.5 foot of visibility to 1.0 foot?
models. If not, you may wish to focus on other management
Unlike experimental models, where phosphorus problems such as aquatic weed control.
would actually be added to a waterbody and then Probably the most important lesson learned
observed for changes, an empirical model is a when using empirical models is that small changes
mathematical equation that is derived purely from in nutrient concentrations will not always produce
statistical data analysis of available data from a noticeable changes in algal levels and water clarity,
chosen group of lakes. except perhaps in oligotrophic waterbodies. In other
In this circular, we've provided three empirical words, if you want to decrease chlorophyll concen-
models that were developed using data from 534 trations (meaning algal levels) to the point where
waterbodies within the Florida LAKEWATCH people actually see a change in water clarity, you may
database (see pages 19-20). Using these models, or have to decrease nutrient concentrations dramatically.
formulas, you can predict what algae levels (chloro-
phyll concentrations) could be expected in your lake
F or step-by-step instructions on how to do
based on a hypothetical nutrient concentration (e.g., e l m g c
empirical modeling calculations see
total phosphorus, total nitrogen, or both). How to Use Empirical Models on page 18.
In other words, if you're concerned about how
a possible increase in phosphorus might affect your c See page 19 for anAn Empirical Model That
Predicts Algae Levels from Phosphorus.
lake, you can plug a hypothetical total phosphorus PredictsAlgae Levelsfrom Phosphorus.
concentration (perhaps a higher TP concentration W See page 19 forAn Empirical Model That
than your lake is currently experiencing) into the PredictsAlgae Levels from Nitrogen.
formula at the top of page 19. The answer you get W See page 20 forAn Empirical Model
from calculating the equation will represent an That PredictsAlgae Levels from Both
estimated or predicted chlorophyll (algae) concen- Phosphorus and Nitrogen.
tration for your lake.

How To Use An Empirical Model

Consider that a h1 pothetical lake called I Lake has an a\ eraue total phosphorus ( TP)
concentration of 10 jig'L Lets suppose that. for \\ hate\ er reason. \ oIo suspect that the total
phosphorus concentration in l. Lake ma\ inciease to as much Ias 20 jig'L i sing the tfollo\ mu
phosphorus-chlorop)IhIll empirical equation. \ou I can pedlict \\ hat the chloroph~h1 II \ value in the
lake miuht be if total phosphorus ( TP) \ alue reaches 20 p~'L

Log (('liloroliyll) = -0.369 + 1.053 Log (TP)

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 TP concentration of 20 pg/L
into the equation. (Replace "TP" with the number 20.)
Now find the Log of 20 on your calculator.
/., I l. I /l,. / ". / *I i illtill!".i l l. ,I ,. i0/,.111I 0 0 l *,l\ H0 I-0 I,. i101 ll ll !,0", i 0 l I/I,.
/ ,''. \I Li. 1 1 0i 1 1 / 11 l i l\, 1 l .,. Ilt "/ 10 1 I / 1, 0 II 0 I l.I 0 I.0 I0., l !", l l 0I II' / 1 '. P l- l l 1 1- 0 M ,'o "I ,
., 'li \,. \,,\, l.,*l \lh.,l./.'. l ,l\,.10 10 0 l l../lllll \ 11 I4, i. /. "I.

Example: Log (chlorophyll) = 0.369 + 1.053 x Log (20)

Log (chlorophyll) = -0.369 + 1.053 x 1.3010

Step 2 Multiply that number (1.3010) by 1.053 (from the equation).

Log (chlorophyll) = 0.369 + 1.053 x 1.3010

Log (chlorophyll) = 0.369 + 1.3700
Step 3 Now add 0.369 (from the equation).
Example: Log (chlorophyll ) = 0.369 + 1.3700

Log (chlorophyll) = 1.0010
Step 4 Find the antilog of your result.
_/,, h l ,l III,. HIl ,,'" k, l ,. Ih, ,. I II. "01,,11 l l,. h,. I ,. / ll, *I / -lh ll\ I Illll,. 1,- I h .. .. 1 hll W -0,*
j. ,h 11. I hI. / ,. ,. II,. ,, ',.llll l lllll ll ii li ,. I ,,ll l Ihllll',;. i, r1 ll lll l h I h,. ,.- I h ,r ,l I,',. I
S I II,. I l i \ lh \ h l, ll\ ,.'/ ,. 0,. HI,. l ,'"\ III,. \ \ i1ll!', 'l |" i l \ /l ,. 0 l ,. h i t. ,* IJ -,10 \, I 0 l I/ l hll / A, I
,./l,. ,.l, 1 .. 11 ,1 l 4 ,0, oi l- h hi ,l", ,I *; Y l,/,l,. I 1

You should get an answer of 10.0231 which can be rounded down to 10 to give
you a predicted average chlorophyll concentration of 10 pg/L.

Lo2 iS a11n aLbbilc\ IN1to fo t 1 Ill 111cat llhlc tical tcn1111 IO ll'illll .i A1 lo1 h111 is t "i \polk t tll 11t(h iililcatn
thIl' ipo r\ to \\ ll l Illlll.c'l Ia. 1.i 1i '1 d to Ioill'cc a 11i Illlllmi'l I c' oi (thc Illllllc'l'I 1II lthc
lo_'jalnith of 1 is 2|
A\nlilo2 is anl abblc\ iltion II t 1' I1math l'imatical t'111 :111iloo :1'ril h An anitilol 'a 1tliiii is "th l llll c'l'
corr.'l)po lin' to a l\L 'I i lo aiinthli I For tIh l' %q tio Il = liI. 1' tl. anltlo' of 2 is lI II


An Empirical Model That Predicts Algae Levels
from Phosphorus
For Florida lakes, the following empirical phosphorus-chlorophyll model was
developed using data from 534 waterbodies within the Florida LAKEWATCH database.
Using this model, you can predict algae levels (chlorophyll) by plugging in a hypothetical total
phosphorus concentration for a lake. [See the sidebar How To Use Empirical Models 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,
Chlorophyll is the annual mean chlorophyll concentration in Pg/L, and
TP is the annual mean total phosphorus concentration in [tg/L.
Data analysis shows this model has a 95% confidence limit that ranges from 30% to 325%. For more on
confidence limits, see How Much Confidence Can You Have In An Empirical Model? on page 21.

An Empirical Model That Predicts Algae Levels
From Nitrogen
Empirical nitrogen-chlorophyll models can be derived in a manner similar to that described
for the phosphorus-chlorophyll model above. Some aquatic scientists believe that both the
nitrogen-chlorophyll models and the phosphorus-chlorophyll models should be used simulta-
neously to provide a more realistic prediction of how chlorophyll levels will be affected
by specific changes in nutrient levels.
For Florida lakes, the following empirical nitrogen-chlorophyll model was developed using
data from 534 waterbodies within the Florida LAKEWATCH database. Using this model, you
can predict chlorophyll levels (algae levels) by plugging in a hypothetical total nitrogen concentra-
tion for a lake. Using a hypothetical total nitrogen concentration for your lake, see if you can
predict what your chlorophyll levels would be using the equation below. [See the sidebar example
entitled How to Use an Empirical Model for step-by-step instructions. Apply the same steps to
the equation below.]

Log (Chlorophyll) = 2.42 + 1.206 Log (TN)

Where: Log is the common logarithm,
Chlorophyll is the annual mean chlorophyll concentration in Pg/L, and
TN is the annual mean total nitrogen concentration in [tg/L.
Data analysis shows this model has a 95% confidence limit 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 21.


An Empirical Model That Predicts Algae Levels
From Both Phosphorus and Nitrogen
The most reliable model (with the smallest confidence interval) is an empirical nutrient-
chlorophyll model that factors in both nitrogen and phosphorus concentrations to predict
chlorophyll levels. Using this model, you can predict algae levels (CHL concentrations) by
plugging in hypothetical total phosphorus and total nitrogen concentrations for a lake.
For Florida lakes, the following empirical nutrient-chlorophyll model was developed
using data from 534 waterbodies within the Florida LAKEWATCH database. [See the sidebar
example entitled How to Use an Empirical Model for step-by-step instructions. 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,
Chlorophyll is the annual mean chlorophyll concentration in [tg/L,
TP is the annual mean total phosphorus concentration in Pg/L, and
TN is the annual mean total nitrogen concentration in pg/L.

Data analysis shows that this model is the best available model for Florida lakes. It has a 95%
confidence limit ranging from 33% to 312% for predicted chlorophyll concentrations. This is the
smallest confidence range for any published empirical nutrient-chlorophyll model that has been
testedfor Florida lakes. The confidence limit is also smaller than those establishedfor 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 21.

Keep practicing

Using the phosphorus-chlorophyll
empirical model again (see top of page
21), plug in the following hypothetical
total phosphorus concentration to see if
you can predict an average chlorophyll
concentration. Be sure to check your answer

With a total phosphorus concentration
of 178 jg/L the formula should yield an
average chlorophyll concentration of ?
See answer below.

[1M 001 :ijOsuV]


How Much Confidence Can
You Have In An Empirical
Scientists often choose to answer this question
by calculating confidence limits for their predictions.
By doing a mathematical analysis from the very
same database used to create the empirical models,
scientists can calculate these confidence limits.
A 95% confidence limit gives the range that
an empirical model prediction can be expected to
fall into 95% of the time. Confidence limits can be
calculated for 90% confidence ranges, 85% confi-
dence ranges, etc. (Water managers usually prefer. .
to use higher confidence limits.)
Let's use the hypothetical lake My Lake
again as an example:
After analyzing the same 534 waterbodies
from the LAKEWATCH database, our staff found
that the 95% confidence limit for the phosphorus-
chlorophyll empirical model example used for My
Lake (pg. 18) ranges from 30% to 325%.
Lake (pg. 18) ranges from 30% to 325%. Before water chemistry data can be used in empirical
In other words, there is a 95% confidence models or for lake management plans, water samples must
that the predicted chlorophyll (of 10 vg/L) will first be collected and analyzed. Florida LAKEWATCH
fall somewhere between 30% and 325%. We can volunteer Phyllis Brumfeld collects a "grab sample" of
translate this percentage range into real numbers lake water that will be analyzed in a UFIFAS water
So chemistry laboratory at the Department o i, ,l ...... and
and check to see if this is true by doing a couple chemistry laboratory at the Department and
of calculations. Aquatic Sciences in Gainesville.
of calculations.
Empirical Models and
Calculate this yourself Their Limitations
Using the phosphorus-chlorophyll empirical
While the confidence limit for the phosphorus-
model example from page 18, we know that a
chlorophyll concentration of 10 pg/L was predicted. chlorophyll model may seem large (30% to
We can use this predicted chlorophyll concentration 325% is a rather expansive range), it's not
of 10 pg/L along with the 95% confidence limit unusual. The confidence limit of even the most
range of 30%to 325%, to do the following reliable empirical model can yield a broad range
calculations: of chlorophyll values particularly in Florida.
This broad range of confidence limits, based
30% of 10 CLg/L is 3 ptg/L
on Florida LAKEWATCH lakes, truly reflects the
pand variability of chlorophyll concentrations found in
325% of 10 Ltg/L is approximately 33 lg/L. waterbodies in this state. As you can see in Figure
1 (page 8), chlorophyll concentrations found in
Florida lakes range from as little as 2 1g/L all the
In other words, the actual chlorophyll value way up to 500 ig/L.
for this sample lake (My Lake) should be Such variability makes predictions from all
somewhere between 3 &g/L and 33 g/L for empirical nutrient-chlorophyll models somewhat
950% of the time. uncertain, particularly when only small changes
occur in nutrient concentrations.


Use of a 95% confidence limit also reflects
the desire of professionals to have their predictions
correct 95% of the time. Confidence limits,
however, can be smaller when the degree of
certainty does not need to be as stringent
(e.g., 80% confidence limit or 90% confidence
Also, keep in mind that in 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
dealing with real-world scenarios and all the
possible factors that can come into play.
In the case of water management, empirical
models are used frequently because they have a
proven record of providing inexpensive, reasonable
results in a short time. Some scientists argue that
more complicated experimental models are
better than empirical models. However, experi-
mental models are often very time consuming
and expensive.
Because other environmental factors, such
as local climate, can influence algal biomass
(chlorophyll), managers may make their predictions Florida LAKEWATCH volunteer Susan Wright (above)
more accurate by using empirical models developed prepares tofilter lake waterfrom Clear Lake. As the water is
for waterbodies in their local geographic region. pumped ;i,. ,,ghi the special glass filter (see photo below),
When developing these empirical models, a basic algae are trapped on the surface. The j r...I nI then be
understanding of how waterbodies function in processed and analyzed in a water chemistry laboratory for
that area should be combined with all the best (algae)
available data. concentrations.
Of course there are instances when an These chlorophyll
individual lake may fall outside the predictions data can then be
used as baseline
found while using any empirical model. When used as baseline
this happens, it's important for that lake to be Clear Lake, as
studied independently of others in its region to well as an
determine what is "driving" the basic productivity c'0 t'e riV tool for
of the lake. developing
While there are several empirical models management
currently being used throughout the state, we
strongly suggest that lake managers and/or
citizens consider using the models provided in Lastly, it's important to remember that
this circular. These models are based on a large empirical models merely provide a framework for
number of Florida lakes and offer a good starting evaluating the potential effects on algal biomass
point for determining the most appropriate of changing nutrient concentrations in a water-
management options for a waterbody. body. They provide a guide, not absolute answers.


Part 4




Other Than


L imiting environmental factors are factors The hydraulic flushing rate is the rate at which
whose presence or absence causes the water flows out of a waterbody. The flushing rate
growth of aquatic plants and/or algae to be can influence algal growth significantly. Waterbodies
restricted. Often the management of a waterbody is with high flushing rates (such as many of Florida's
focused solely on the manipulation of nutrients as a springs, reservoirs, and lakes that are actually just
strategy for controlling growth of algae and/or wide spots in rivers) have low algal levels even
plants, and the potentially limiting environmental though they may have high nutrient concentrations.
factors are overlooked. A skilled manager will This seemingly paradoxical condition exists
evaluate all the potentially limiting environmental because algae are flushed out of the system before
factors along with the limiting nutrients and consider they have the time to grow to their maximum potential.
all the possibilities. Several important limiting Florida's famous Silver Springs provide a good
environmental factors are described below: example. Water samples taken from the springs are
initially crystal clear and yet when analyzed in a water
* Suspended solids (tiny particles stirred up from chemistry laboratory, they contain tremendously high
the bottom sediments or washed in from the levels of phosphorus concentrations. However, that
watershed) can reach concentrations high enough same crystal clear water turns green with algae when
that the growth of algae is limited because sunlight left sitting in ajar, for a period of time.
is blocked out. This is a common situation in
shallow lakes, especially those with heavy wave Aquatic macrophytes should also be considered
action such as Lake Okeechobee in south Florida. as a limiting environmental factor, because their
presence may limit the growth of free-floating
* The color of dissolved substances, though algae in Florida waterbodies indirectly. If macro-
translucent, can block sunlight and retard the phyte coverage (PAC or "percent area covered") is
growth of aquatic plants and sometimes limit algal less than 30% of a waterbody, the presence of
abundance. Many of Florida's lakes are tea-colored macrophytes does not appear to influence open-
(reddish or reddish-brown) because of dissolved water algal levels.
organic substances in the water. Even when tea- However, lakes with aquatic macrophytes
colored lakes are rich in nutrients, the growth of covering over 50% of their bottom area typically
algae and submersed aquatic plants can be limited, have reduced algal levels and clearer water.


Suspended solids can reach concentrations great The color of dissolved substances, such as tannins released
enough that the growth ofalgae is limited because from cypress trees, can block sunlight and retard the growth
sunlight is blocked out. ofaquatic plants and sometimes limit algal abundance.

Waterbodies with high flu.\hing rates, such as many of Lakes with aquatic macrophytes covering over 50% of
Florida springs have low algal levels even ;h. ,'ii, their bottom area typically have reduced algal levels
they may have high nutrient concentrations. and clearer water

One explanation is that either aquatic + Macrophytes also provide calm water conditions
macrophytes, or perhaps the algae attached to within their beds. This lack of water movement
them, use the available phosphorus in the water, keeps algal cells from being suspended in the water
competing with the free-floating algae for this column.
necessary nutrient. Another explanation is that
the macrophytes anchor the nutrient-rich bottom ce See Appendix B for more information on
sediments in place, buffering the action of wind, Aquatic Macrophytes.
waves, and human effects, and thereby deprive
the free-floating algae of nutrients contained in
the sediments that would otherwise be stirred up.


Scientists often use graphs to illustrate relationships
between two different measures. In this example, graphs
are used to compare phosphorus and chlorophyll in lakes.

Figure 1
Such a plot is shown in Figure 1 (page 26) with a "best fit" line that can be used to
estimate the amount of chlorophyll when the total phosphorus measurement is known.
A problem with this type of graph is that we have trouble distinguishing the points for
very low levels of phosphorus and chlorophyll. In Figure 1 the phosphorus concentrations
range from a low of about 3 to a high of 350 and there are 51 points with concentrations of 10
or less. However, the distance on the scale from 0 to 10 takes up only 1/40th of the distance
for the phosphorus axis; so the points are squeezed together.

Figure 2
In Figure 2 we have expanded the scale and cut it off at 100 instead of 400, in order
to spread out the points; but they are still packed together, and we can no longer see the
points for lakes with higher values of phosphorus and chlorophyll.

Figure 3
A common solution to this problem is shown in Figure 3. Rather than plotting the actual
phosphorus concentrations, we can plot the logarithms of the concentrations.o This allows us
to see more individual points that otherwise would be crowded in the lower corer of the
For example, by using logarithms we are able to stretch out the scale at the lower end so
that the distance between 2 and 7 is the same at the distance between 20 and 70 higher up on
the scale. This is evident by comparing Figure 2 with Figure 3.
There are two other benefits. For many measurements the variability or sampling error
increases as the value of the measurement increases. Notice in Figure 1 how the points are
very close to the "best fit" line at low values of phosphorus and chlorophyll and show a much
greater scatter as the values increase.

o Recall that common I, g t, il,,o are the exponents of the number 10. For example:
102 =100, so the !I,,gi, iti ofl00 is 2. And using the equation 103=1000, we can see that
the !',, ,, iin ofl000 is 3. Similarly the, !,,,'g ,liifm of l would be 1. [Note: Tables or computer
programs can be used to find 1, g1 ,,,li:ni of other numbers that are not exact multiples of 10.]


500 100

400 80 3- a
30 0 60
2 0 a 0 a
0 200 40 0 a 0c0

100 20 a

0 00
0 100 200 300 400 0 20 40 60 80 100
Figure Total phosphorus Figure 2. Total phosphorus
Figure 1. Figure 2.

3 1000

>r 2- 100

0 1 Ca 10 a


-1 .1 I
0 0- 1 P

0 1 2 3 1 10 100 1000
Log total phosphorus Total phosphorus

Figure 3 Figure 4

In the logarithmic plot in Figure 3 the scatter of the points along the line is more even from
the low to the high end of the scale. It can be shown that in this case the per cent error is more or
less constant. Lastly, we can often use a logarithmic plot to fit a straight line to data that form a
curve on a direct plot. Note in Figure 2 that there is an upward curve to the points. This is
straightened out in the logarithmic plot in Figure 3. This property makes it easier to find a
mathematical relationship between the two measurements.

Figure 4
To make our plots easier to understand, we often use a logarithmic scale rather than the
actual logarithms, which was done in Figure 3. Note how much easier it is to find the values for a
point in Figure 4 when a logarithmic scale is used.


Aquatic Macrophytes Aquatic macrophytes are a natural part of
are aquatic plants that are large enough to be waterbodies, although in some circumstances
apparent to the naked eye. In other words, they are they can be troublesome. The same plant may be
larger than microscopic aquatic plants. The general a desirable aquatic plant in one location and a
term "aquatic plants" usually refers to aquatic nuisance weed in another. When exotic aquatic
macrophytes, but some scientists use it to mean plants have no natural enemies in their adopted
aquatic macrophytes and algae. area, they can grow unchecked and may become
Aquatic macrophytes characteristically overly abundant.
grow in water or in wet areas and are quite a In Florida for example, millions of dollars
diverse group. For example, some are rooted in are spent each year to control two particularly
the bottom sediments, while others float on the aggressive and fast-growing aquatic macrophytes:
water's surface and are not rooted to the bottom. water hyacinth, an exotic aquatic plant that is
Aquatic plants may be native to an area, or they thought to be from Central and South America,
may have been imported (referred to as exotic). and hydrilla, an exotic aquatic plant that is
Some aquatic macrophytes are vascular thought to be from Africa.
plants, meaning they contain a system of fluid- The term "weed" is not reserved solely for
conducting tubes, much like human blood exotic aquatic plants. In some circumstances, our
vessels. Cattails, waterlilies, and hydrilla are native aquatic plants can cause serious problems,
examples. Large algae such as Cladophora, too. When assessing the abundance of aquatic
Lyngbya, and Chara are examples of non-vascular plants in a waterbody, scientists may choose to
plants that are also included in the category of measure or calculate one or more of the following:
aquatic macrophytes. + PVI (Percent Volume Infested or Percent
Even though they are quite diverse, aquatic Volume Inhabited) is a measure of the percentage
macrophytes have been grouped into three general of a waterbody's volume that contains aquatic
categories: macrophytes;
+ emergent aquatic plants are rooted in the + PAC (Percent Area Covered) is a measure of the
bottom sediments and protrude up above the percentage of a waterbody's bottom area that has
water's surface; aquatic plants growing on or over it;
+ submersed aquatic plants primarily grow + frequency of occurrence is an estimate of the
completely below the water's surface; and abundance of specific aquatic plants; and
+ floating-leaved aquatic plants can be rooted to + average plant biomass is the average weight
the waterbody's bottom sediments and also have of several samples of fresh, live aquatic plants
leaves that float on the water's surface. growing in one square meter of a lake's area.


The Role of Aquatic Macrophytes in Chlorophyll can be abbreviated CHL and total
W Aquatic macrachlorophyll can be abbreviated TCHL.
Aquatic macrophytes perform several
functions in waterbodies, often quite complex The Role of Chlorophyll in Waterbodies:
ones. A few are briefly described below. Measurements of the chlorophyll concentra-
tions in water samples are very useful to scientists.
* Aquatic macrophytes provide habitat for fish,
S aohe pi h For example, they are often used to estimate algal
wildlife, and other aquatic animals. .
biomass in a waterbody and to assess a
* Aquatic macrophytes provide habitat and food waterbody's biological productivity.
for organisms that fish and wildlife feed on.
In Florida:
+ Aquatic macrophytes along a shoreline can protect Waterbodies in the Florida LAKEWATCH
the land from erosion caused by waves and wind. database analyzed prior to January 1998 had
+ Aquatic macrophytes can stabilize bottom average chlorophyll concentrations which ranged
sediments by dampening the wave action. from less than 1 to over 400 [g/L.
Using these average chlorophyll concentra-
+ The mixing of air into the water that takes place ts these average cloroyll c enra
tions from this same database, Florida lakes were
at the water's surface can be obstructed by the found to be distributed into the four trophic
found to be distributed into the four trophic
presence of floating plants and floating-leaved states as follows:
plants. In this way, they can cause lower oxygen
levels in the water. + 12% of the lakes (those with chlorophyll values less
* Floating plants and floating-leaved plants create than 3 g/L) would be classified as oligotrophic;
shaded areas that can cause the growth of sub- + about 31% of the lakes (those with chlorophyll
mersed plants beneath them to be slowed, values between 4 and 7 pg/L) would be classified
* When submersed aquatic plants become more as mesotrophic;
abundant, these plants can cause water to become + 41% of the lakes (those with chlorophyll values
clearer. Conversely, the removal of large amounts between 8 and 40 [g/L) would be classified as
of submersed aquatic plants can cause water to eutrophic; and
become less clear. + nearly 16% of the lakes (those with chlorophyll
+ When aquatic macrophytes die, the underwater values greater than 40 [tg/L) would be classified as
decay process uses oxygen from the water, which hypereutrophic.
can become severely oxygen-poor if massive In Florida, characteristics of a lake's
In Flonda, characteristics of a lake's
amounts of plants die simultaneously. .
amounts of plants die simultaneouslygeographic region can provide insight into how
+ Decayed plant debris (dead leaves, etc.) contrib- much chlorophyll may be expected for lakes in
utes to the buildup of sediments on the bottom. that area. For example, water entering the water-

Chlorophyll bodies by stream flow or underground flowage
Through fertile soils can pick up nutrients that
is the green pigment found in plants and found
can then fertilize the growth of algae and aquatic
abundantly in nearly all algae. Chlorophyll plants. In this way, the g gy physiogra-
w pplants. In this way, the geology and physiogra-
allows plants and algae to use sunlight in the watershed can influence a waterbody's
phy of a watershed can influence a waterbody's
process of photosynthesis for growth. Thanks to ioloical roductiit
biological productivity significantly.
chlorophyll, plants are able to provide food and
oxygen for the majority of animal life on earth. Health Concerns:
Scientists may refer to chlorophyll a, Chlorophyll (algae) poses no direct threat to
which is one type of chlorophyll. Chlorophyll b human health. There are some rare cases where
and chlorophyll c are two other types. algae can become high enough in abundance to
A measurement of all three of these types cause concern. However, algae are generally not
combined is known as total chlorophyll. a health threat.


Color Health Concerns:
There is no known direct health hazard of
in waterbodies has two components:
in waterbies has two component: color. Consequently, an acceptable level of color
(1) apparent color is the color of a water sample depends on personal preference. Water transpar-
t h n p f o depends on personal preference. Water transpar-
that has not had particulates filtered out;
1ency, however, may be reduced in highly colored
(2) true color is the color of a water sample that has ency, however, may be reduced in highly colored
Sa p f o o t waters (greater than 50 PCU) to the point where
had all particulates filtered out of the water
underwater hazards may be concealed, creating a
The measurement of true color is the one most nderater s may be once cre g
potentially dangerous situation for swimmers,
commonly used by scientists. To measure true color, enia aners s
skiers, and boaters.
the color of the filtered water sample is matched to
one from a spectrum of standard colors. Each of Humic acids
the standard colors has been assigned a number on a are produced when organic matter such as dead
scale of platinum-cobalt units (abbreviated as either leaves decay. Humic acids can color water so that it
PCU or Pt-Co units). On the PCU scale, a higher appears reddish or reddish-brown, like tea. In some
value of true color represents water that is darker in cases, the water can appear almost black.
Lake Region
The Role of Color in Waterbodies:
Dissolved organic materials humicc acids from is a geographic area in which lakes have similar
decaying leaves), and dissolved minerals can give geology, soils, chemistry, hydrology, and biological
water a reddish brown "tea" color. features. In 1997, using Florida LAKEWATCH
The presence of color can reduce both the data and other information, the United States
quantity and quality of light penetrating into the Environmental Protection Agency divided Florida
Into 47 lake regions using these similarities as their
water column. As a result, high color concentra-
tions (greater than 50 PCU) may limit both the crtera.
quantity and types of algae growing in a waterbody. Lakes in an individual ke region ehibit
Changing the quantity and quality of light reach- remarkable similarities. However, lakes in one
ing the bottom of a waterbody can also influence lake region may differ significantly from those in
the depth of colonization and the types of aquatic a different lake region For example, most lakes
in the New Hope Ridge/Greenhead Slope lake
plants that can grow there. In some waterbodies, in the New Ho Ridge/Greenhead Slope lake
color is the limiting environmental factor. region in northwestern Florida (in Washington,
In Florida: Bay, Calhoun, and Jackson Counties) tend to
Waterbodies in the Florida LAKEWATCH have lower total nitrogen, lower total phospho-
database analyzed prior to January 1998 had rus, lower chlorophyll concentrations and higher
average color values ranging from 0 to over 700 Secchi depths when compared to other Florida
PCU. Over 75% of these waterbodies had color lakes. While lakes in the Lakeland/Bone Valley
values less than 70 PCU. Upland lake region in central Florida (in Polk
Waterbodies that adjoin poorly drained areas and Hillsborough Counties) tend to have higher
(such as swamps) often have darker water, especially total nitrogen, higher total phosphorus, higher
after a rainfall. Consequently, the location of a chlorophyll concentrations and lower Secchi
waterbody has a strong influence on its color, depths when similarly compared.
For example, lakes in the well-drained New Hope Using descriptions of Lake Regions, water-
Ridge/Greenhead Slope lake region in northwestern body managers can establish reasonable, attain-
Florida (in Washington, Bay, Calhoun, and Jackson able water management goals for individual
Counties) tend to have color values below 10 PCU. lakes. Lake Region characteristics can also be
While lakes in the poorly-drained Okefenokee used to help choose management strategies that
Plains lake region in north Florida (in Baker, are likely to be effective in achieving manage-
Columbia, and Hamilton Counties) tend to have ment goals. In addition, lakes with water chemistry
values above 100 PCU. that differs markedly from that of other lakes in the


same lake region can be identified and investigated Trophic State Index (TSI)
to determine the cause of their being atypical. is a scale of numbers from 1 to 100 that can be
The lake regions are mapped and described used to indicate the relative trophic state of a
in a report entitled Lake Regions of Florida waterbody. Low TSI values indicate lower levels of
(EPA/R=97/127). The Florida LAKEWATCH biological productivity, and higher TSI values
Program can provide a free pamphlet describing: indicate higher levels. The use of TSI is an attempt
(1) how and why the Lake Regions project was to make evaluations of biological productivity easier
developed; to understand.
(2) how to compare your lake with others in its Using mathematical formulas, TSI values
Lake Region; and can be calculated using four parameters: total
nitrogen concentrations, total phosphorus con-
(3) how the Lake Region Classification System can centrations, total chlorophyll concentrations, and
be useful to you. Secchi depth. Sometimes a single TSI value for

Trophic State a waterbody is calculated by combining selected
individual TSI values.
is defined as the degree of biological productivity The State of Florida has classified its
of a waterbody. Scientists debate exactly what is waterbodies according to the designated uses the
meant by "biological productivity," but it generally state has assigned to each. (See Water Quality
relates to the amount of algae, aquatic plants, fish in this Appendix, for a more detailed description.)
and wildlife a waterbody can produce and sustain. The Florida Department of Environmental
Waterbodies are traditionally classified into Protection (FDEP) assesses water quality in
four groups according to their level of biological Florida by evaluating whether each waterbody
productivity. The adjectives denoting each of was able "to support its designated use."
these trophic states, from the lowest productivity [The Florida Water Quality Assessment 305(b)
level to the highest, are oligotrophic, mesotrophic, Report 1996]. Their assessment is based solely
eutrophic, and hypereutrophic. Aquatic scientists on TSI values as follows:
assess trophic state by using measurements of
one or more of the following: + waterbodies with TSI values from 0 to 59 are
rated as "good and fully support use";
+ total phosphorus concentrations in the water;
+ those waterbodies with TSI values between 60-
+ total nitrogen concentrations in the water; 69 are rated as "fair and partially support use"; and
+ total chlorophyll concentrations (a measure of + waterbodies with TSI values from 70 to 100 are
free-floating algae in the water column); and rated as "poor and do not support use."
+ water clarity (measured using a Secchi disc); and
Individual TSI values may be further
+ aquatic plant abundance. combined in a special type of averaging to
Florida LAKEWATCH professionals base produce an Average Trophic State Index
trophic state classifications primarily on the amount (abbreviated TSIave). Government and regulatory
of chlorophyll in water samples. Chlorophyll agencies responsible for water management
concentrations have been selected by LAKE- often use the average value, overlooking the fact
WATCH as the most direct indicators of biological that the designing author, Dr. Robert Carlson of
productivity, since the amount of algae actually Kent State University in Ohio, never intended
being produced in a waterbody is reflected in the TSI values to be reduced to a single number. TSI
amount of chlorophyll present. In addition, Florida values for the individual parameters could differ
LAKEWATCH professionals may modify their markedly within any specific waterbody and this
chlorophyll-based classifications by taking the significant variation will be obscured when the
aquatic plant abundance into account. TSIave is calculated.

Dr. Carlson has noted that TSI values The Role of Water Clarity in Waterbodies:
should not be averaged, as consideration of the Water clarity will have a direct influence on
differences in individual TSI values in a waterbody the amount of biological production in a water-
can provide insight and a better understanding of body. When water is not clear, sunlight cannot
its biological productivity, penetrate far and the growth of aquatic plants will
The Florida LAKEWATCH Program does be limited. Consequently aquatic scientists often
not use the TSI system (the TSIae or individual use Secchi depth (along with total phosphorus, total
TSI values). Instead LAKEWATCH finds it more nitrogen, and total chlorophyll concentrations) to
informative to use the individual values of the determine a waterbody's trophic state.
four measured parameters without transforming Water clarity affects plant growth, but
them into TSI values, conversely, the abundance of aquatic plants can
affect water clarity. Generally, increasing the
Water Clarity abundance of submersed aquatic plants to cover
is the transparency or clearness of water. While 50% or more of a waterbody's bottom area may
many people tend to equate water clarity with have the effect of increasing the water clarity.
water quality, it's a misconception to do so. One explanation is that either the submersed
Contrary to popular perceptions, crystal clear water plants, or perhaps the algae attached to aquatic
may contain pathogens or bacteria that would make plants, use the available nutrients in the water,
it harmful to drink or to swim in, while pea-soup depriving the free-floating algae of them. Another
green water may be harmless. explanation is that the submersed plants anchor the
Water clarity in a waterbody is commonly nutrient-rich bottom sediments in place buffering
measured by using an 8-inch diameter Secchi the action of wind, waves, and human effects -
disc attached to a cord. The disc is lowered into depriving the free-floating algae of nutrients
the water, and the depth at which it vanishes contained in the bottom sediments that would
from sight is measured. Measured in this way, otherwise be stirred up.
water clarity is primarily affected by three Because plants must have sunlight in order to
components in the water: grow, water clarity is also directly related to how
deep underwater aquatic plants will be able to live.
+ free-floating algae called phytoplankton, This depth can be estimated using Secchi depth
+ dissolved organic compounds that color the readings.
water reddish or brown, and In Florida:
+ sediments suspended in the water (either stirred Waterbodies in the Florida LAKEWATCH
up from the bottom or washed in from the shore). database analyzed prior to January 1998, had
Secchi depths ranging from less than 0.2 to over
Water clarity is important to individuals who 11.6 meters (from about 0.7 to 38 feet).
want the water in their swimming areas to be clear The trophic state of a waterbody is strongly
enough so that they can see where they are going. related to the water clarity. Using these average
In Canada, the government recommends that water Secchi depth readings from the Florida LAKE-
should be sufficiently clear so that a Secchi disc is WATCH database analyzed prior to January 1998,
visible at a minimum depth of 1.2 meters (about 4 Florida lakes were found to be distributed into the
feet). This recommendation is one reason that four trophic states as follows:
many eutrophic and hypereutrophic lakes that have
abundant growths of free-floating algae do not approximately 7% of these lakes would be
meet Canadian standards for swimming and are classified as oligotrophic (lakes with Secchi depths
deemed undesirable. It should be noted that these greater than 3.9 meters [about 13 feet]);
lakes are not necessarily undesirable for fishing nor o t l w b c
About 22% of these lakes would be classified as
are they necessarily polluted in the sense of being mc ( w i d
Sy i s mesotrophic (lakes with Secchi depths between
contaminated by toxic substances.

2.4 and 3.9 meters [between about 8 and 13 feet]); Water quality guidelines developed by the
S4% o t l w b c a Florida Department of Environmental Protection
* 45% of these lakes would be classified as
c ( s wh S i d s b n 0.9 (FDEP) provide standards for the amounts of some
eutrophic (lakes with Secchi depths between 0.9
substances that can be discharged into Florida
and 2.4 meters [between about 3 and 8 feet]); and .es i
waterbodies (Florida Administrative Code
* 26% of these lakes would be classified as 62.302.530). These FDEP guidelines provide
hypereutrophic (lakes with Secchi depths less different standards for waterbodies in each of five
than 0.9 meters [about 3 feet]). classes. They are defined by their assigned designated
The location of a waterbody has a strong use as follows:
influence on its water clarity. For example, lakes
in the New Hope Ridge/Greenhead Slope lake Class I waters are for POTABLE WATER
region (in Washington, Bay, Calhoun, and SUPPLIES;
Jackson Counties) tend to have Secchi depths + Class II waters are for SHELLFISH
greater than 3.0 meters. While lakes in the Lake- PROPAGATION OR HARVESTING;
land/Bone Valley Upland lake region (in Hillsborough + Class III waters are for RECREATION,
and Polk Counties) tend to have Secchi depths less HEALTHY, WELL-BALANCED POPULATION
than 0.9 meters. OF FISH AND WILDLIFE;
Health Concerns: + Class IV waters are for AGRICULTURAL
Water clarity is not known to be directly WATER SUPPLIES; and
related to human health. + Class V waters are for NAVIGATION,

Water Quality All Florida waterbodies are designated as
is a subjective, judgmental term used to describe Class III unless they have been specifically
the condition of a waterbody in relation to human classified otherwise (refer to Chapter 62-302.400,
needs or values. The phrases "good water quality" Florida Administrative Code for a list of waterbodies
or "poor water quality" are often related to whether that are not Class III).
the waterbody is meeting expectations about how it
can be used and what the attitudes of the waterbody
users are. Water quality is not an absolute.
One person may judge a waterbody as being
high quality, while someone with a different set
of values may judge the same waterbody as
being poor quality. For example, a lake with an
abundance of aquatic plants in the water may not
be inviting for swimmers but may look like a
good fishing spot to anglers.
Water quality guidelines for freshwaters
have been developed by various regulatory and
governmental agencies. For example, the Canadian
Council of Resource and Environmental Ministers
(CCREM) provides basic scientific information
about the effects of water quality 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 water supply.


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