United States
Environmental Protection
Agency
Environmental Monitoring
Laboratory
Las Vegas NV 89114 "'
Research and Development
EPA-600/S3-81-001 Feb. 1981
Project Summary
Modifications of Models
Predicting Trophic State of
Lakes: Adjustment of Models
to Account for the Biological
Manifestations of Nutrients
S. C. Hern, V. W. Lambou, L R. Williams, and W. D. Taylor
The strong relationship between
total phosphorus and phytoplankton
biomass in lakes has been clearly
confirmed by researchers. What is
now needed to predict algal biomass
for making better management
decisions for individual lakes is a
quantitative understanding of the
range in biomass (as measured by
chlorophyll a) per unit of phosphorus.
This range extends over several orders
of magnitude.
To determine the environmental
factors affecting the response of
phytoplankton chlorophyll a to total
phosphorus concentration, collected
data from 757 U.S. lakes were
analyzed showing that light
attenuation from interferences not
related to chlorophyll a can
dramatically affect the quantity of
phytoplankton biomass in many U.S.
lakes. The ratio of biologically
available phosphorus to nitrogen is, in
some cases, an important factor in
determining the amount of
chlorophyll a produced per unit of
phosphorus present.
This report presents methods to
modify nutrient ambient- and loading-
models that predict the trophic state
of lakes to:
1. change the trophic classifica-
tion based on an ambient total
phosphorus level to one based
on the biological manifesta-
tion of nutrients as measured by
chlorophyll a.
2. allow determination of the
critical levels of phosphorus
that will Jesuit in unacceptable
levels of chlorophyll a. and
3. account for the unique
characteristics of a lake that
affect the amount of chloro-
phyll a produced per unit of
phosphorus.
If chlorophyll a is used as the trophic
classification criterion rather than
total phosphorus, many U.S. lakes
would be classified lower, i.e., less
eutrophic.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory, Las
Vegas. NV, to announce key findings
of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Phosphorus supply is considered to
be the primary determinant of algal
community biomass and production in
most temperate zone lakes.
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Researchers clearly confirm the strong
correlation between total phosphorus
and phytoplankton biomass (as
measured by chlorophyll a). What is
presently needed to predict algal
biomass for management decisions on
individual lakes is a quantitative
understanding of the range in biomass
(chlorophyll a) per unit of phosphorus;
this range extends over orders of
magnitude.
Existing models predict ambient total
phosphorus lake concentration from
tributary phosphorus loading data.
These and similar models are widely
used to indicate the degree of eutrophy
of lakes and to evaluate the
environmental effects of ambient
phosphorus levels m lakes and,
accordingly, to make decisions about
lake management. For example, they
can be used to manipulate ambienttotal
phosphorus concentration to produce a
desired environmental effect. All these
models use levels of 10 and 20
micrograms per liter of ambient
phosphorus concentrations to divide
lakes into three standard trophic
classifications—oligotrophic, mesotro-
phic, and eutrophic—on the assumption
that the relationship of phytoplankton
biomass to phosphorus is the same for
all lakes.
However, the use and incorporation
of phosphorus into phytoplankton
biomass vanes significantly from lake to
lake. The efficiency of use of
phosphorus is largely dependent on the
availability of light, sufficient supply of
other nutrients, and biological
availability of the various phosphorus
species.
The use of these models to predict
phytoplankton biomass from actual or
potential phosphorus concentrations in
individual bodies of water could lead to
faulty management decisions if the
factors affecting the use of phosphorus
are not taken into consideration. This
study evaluates the factors affecting the
relationship of phytoplankton biomass
to phosphorus levels and shows how to
modify models predicting the trophic
state of lakes to take these factors into
account. The data base used to evaluate
the factors was derived from data
collected in the National Eutrophica-
tion Survey during the spring, summer
and fall of 1972 through 1975, and
involved 757 of the lakes surveyed
throughout the 48 conterminous
States.
Conclusions
Previous workers have established a
strong relationship between CHLA (a
measure of phytoplankton biomass) and
total phosphorus in lakes. They report
an extremely high log-log product
moment correlation coefficient ranging
from 0.85 to 0.98 between chlorophyll a
and total phosphorus in lakes. The
implication of these findings is that
phosphorus is the element that controls
algal biomass. However, we believe that
the relationship between chlorophyll a
and total phosphorus described by these
workers represents the situation under
nearly ideal conditions, i.e., without
major interferences.
In this study of 757 U.S. lakes, a log-
log product moment correlation
coefficient of only 0.60 was found
between chlorophyll a and total
phosphorus, and the response ratio (i.e.,
the amount of chlorophyll a produced
per unit of total phosphorus) was found
to vary greatly. It therefore appeared
that many U.S. lakes do not reach
maximum production of chlorophyll a
because of interference factors.
Interference factors that may prevent
phytoplankton chlorophyll a from
achieving maximum theoretical
concentrations based upon ambient
total phosphorus levels in a lake include
availability of light, limitation of growth
factors other than total phosphorus
components, domination of the aquatic
flora by vascular plants rather than
phytoplankton, short hydraulic reten-
tion time, and the presence of toxic
substances. We found that light
attenuation from non-chlorophyll a
related interferences can dramatically
affect the quantity of phytoplankton
biomass present in lakes In some cases
the ratio of biologically available
phosphorus to nitrogen is an important
factor in determining the amount of
chlorophyll a produced per unit of total
phosphorus, while temperature is not
an important factor.
Most of the trophic classification
schemes for lakes use nutrient levels
rather than the biological manifestation
of nutrients as measured by chlorophyll a
as the basis of classification. When
classified on the basis of chlorophyll a
rather than total phosphorus, 25 per-
cent of the 757 lakes used in the study
were classified lower, i.e., less eutro-
phic. If, in fact, the large population of
lakes used in this study is representa-
tive of conditions throughout the U.S.,
and if the manifestations of nutrients
rather than their absolute concentra-
tions are the primary criteria for
beneficial water use, many communi-
ties could be spared the burden of costly
nutrient-removal programs suggested
by phosphorus-based trophic classifica-
tions.
This study developed methods to
modify loading and ambient modelsthat
predict the trophic state of lakes to (1)
change the trophic classification based
on an ambient total phosphorus level to
one based on the biological
manifestation of nutrients measured by
chlorophyll a; (2) determine the critical
levels of total phosphorus which will
result in an unacceptable level of
chlorophyll a so that the level of total
phosphorus can be manipulated to
achieve the desired use of a given
waterbody; and (3) account for the
unique characteristics of a lake that
affect the amount of chlorophyll a
produced per unit of total phosphorus.
Recommendations
The commonly used ambient and
loading models predict the trophic state
of a lake from total phosphorus levels
and assume that all lakes will respond in
the same manner to a given ambient
total phosphorus concentration.
Because non-chlorophyll a light
interferences and other interferences in
many U.S. lakes significantly decrease
the amount of chlorphyll a produced per
unit of total phosphorus, and since
excessive algal growth or other
manifestations of nutrients are more
important from a water quality
standpoint than a trophic classification
based on an arbitrary ambient total
phosphorus level, we recommend that
ambient and loading models predicting
trophic state be adjusted to account for
the amount of chlorophyll a produced
per unit of total phosphorus m a lake.
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The EPA authors S. C. Hem, V. W. Lambou, L. R. Williams, and W. D. Taylor
arere with the Environmental Monitoring Systems Laboratory, Las Vegas,
NV89114.
V. W. Lambou is the EPA Project Officer (see below).
The complete report, entitled "Modifications of Models Predicting Trophic State
of Lakes: Adjustment of Models to Account for the Biological Manifestations of
Nutrients." (Order No. PB 81-144 362; Cost: $6.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
P.O. Box 15027
Las Vegas, NV89114
• U* GOVERNMENT PMNTTNO OfnCE; f»1 -757-064/OZ58
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