United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-84-027 Apr. 1984
SEPA Project Summary
Impact of Algal-Available
Phosphorus on Lake Erie Water
Quality: Mathematical Modeling
Douglas K. Salisbury, Joseph V. DePinto, and Thomas C. Young
Accurate estimates of the forms and
bioavailability of phosphorus loadings
are necessary for loading trend analysis
and water quality model development.
Total phosphorus loading data for Lake
Erie from 1970 to 1980 were catego-
rized into three forms, based on phos-
phorus bioavailability studies. The three
forms were soluble reactive phos-
phorus (SRP) (immediately available for
algal uptake), external ultimately-avail-
able phosphorus (EUP) (not immedia-
tely available but converted to an avail-
able form at a specific rate), and ex-
ternal refractory phosphorus (ERP)
(never available for algal uptake).
From 1970 to 1980, 23.4% of the
total external unavailable phosphorus
load to Lake Erie was bioavailable. The
11 -year total phosphorus load to Lake
Erie contained 34.4% SRP, 15.3% EUP,
and a significant 50.3% ERP. Trend
analysis demonstrated that from 1970
to 1980 bioavailable phosphorus load-
ing decreased at a slower rate than
point source phosphorus loading de-
creased.
The significance of the phosphorus
proportioning technique was ascer-
tained using a multi-nutrient phyto-
plankton model2 with the 1970 and
1975 phosphorus loading data. The
model was modified to represent the
distinction between allochthonous and
autochthonous unavailable phosphorus.
Comparisons between the original and
modified models showed that the modi-
fied phosphorus dynamics proved to be
a viable alternative to the concept of
settling soluble phosphorus from the
water column.
Sensitivity analyses were performed
and demonstrated the need for additional
research to examine the in-lake dynam-
ics of allochthonous ultimately-avail-
able phosphorus from Lake Erie tribu-
taries. Continued research on the ex-
tent and rate of SRP release, and the
settling velocity of external ultimately-
available phosphorus from Lake Erie tri-
butaries is recommended.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Duluth. MN. 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
Regulation of phosphorus loading is
considered to be the primary method of
eutrophication control for Lake Erie. Non-
point phosphorus control or additional
point source control will be necessary, in
addition to current point source controls,
to meet the annual phosphorus load of
11,000 metric tons recommended by the
1978 Great Lakes Water Quality Agree-
ment. Knowledge of the forms and
bioavailability of all phosphorus sources
is essential since implementation of the
most cost-effective load control measures
depend upon identification of sources
carrying high bioavailable phosphorus
loads.
Research has been undertaken at
Clarkson College of Technology3'4'5 to im-
prove the accuracy of estimates of the
form and reactivity of phosphorus loadings
to Lake Erie. Since mathematical models
of the eutrophication process are used to
form management strategies, it is neces-
sary to develop them to represent
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biochemical and physical processes as
accurately as current scientific knowledge
permits. It was the purpose of this study
to incorporate the phosphorus bioavail-
ability findings at Clarkson College into
an existing (calibrated and verified) Lake
Erie phytoplankton model.2
Recent deterministic phytoplankton
models of the Great Lakes considered
phosphorus loads and water column
phosphorus from internal and external
sources to exist in two forms, available
and ultimately-available. Available phos-
phorus was considered to be soluble
reactive P (SRP) and immediately available
for phytoplankton uptake Ultimately-
available phosphorus, on the other hand,
was considered to be paniculate P (PP) or
dissolved P which required a biochemical
conversion before becoming available for
algal uptake No distinction was made
between the reactivities of ultimately-
available P produced within the water
column by primary productivity (internal
or autochthonous) and ultimately-available
P supplied by tributaries, shore erosion,
and municipal effluent discharges (exter-
nal or allochthonous) The empirical rate
coefficient for the conversion of ultimate-
ly-available P to available P, utilized in
water quality models, was based primari-
ly on the conversion of autochthonous
ultimately-available P. In the past, this
discrepancy was unavoidable due to the
lack of empirical evidence to demonstrate
the difference between the release of
available P from allochthonous ultimate-
ly-available PP and the conversion to
available P from autochthonous ultimate-
ly-available P. However, researchers 3'4'5
have demonstrated subtle differences in
these processes.
The scope of the modeling effort
reported here was to examine the
usefulness of differentiating between
allochthonous and autochthonous ulti-
mately-available P forms with respect to
predictions on m-lake phosphorus and
algal dynamics The refinement of the
Lake Erie phytoplankton model2 involved
the addition of two newforms of phospho-
rus, total external unavailable P (TEUP)
and external refractory P (ERP), to the
model framework. TEUP was the sum of
ERP and the external ultimately-available
P (ŁUP). Both EUP and ERP were sedi-
ment-bound particulate phosphorus
which entered the lake via direct or in-
direct (tributary) sources EUP, not im-
mediately available for algal uptake, was
converted to bioavailable P ERP, in con-
trast, did not contribute to the available P
pool while the sediments were in the
water column.
Model Refinement
In order to address the hypothesis that
distinguishing between allochthonous
and autochthonous phosphorus is a
reasonable alternative to SRP settling
for describing phosphorus/phytoplankton
dynamics in Lake Erie, comparisons were
made among simulations of three Lake
Erie phytoplankton models. The only
differences among the three models lie in
the mathematical representation of
phosphorus dynamics. These differences
are illustrated conceptually in Figure 1.
Two of the models, LEM1 and LEM2, had
two P forms, SRP and unavailable P In
LEM1 and LEM2 the total external load of
particulate P and the ultimately-available
P internally recycled from organisms
were both treated as having the same
characteristics The phosphorus dynamics
ofbothLEM! and LEM2 are shown on the
left side of Figure 1 The only difference
between the two models is that LEM1
contains a sink term for SRP settling
(dashed arrow), whereas LEM2 omits this
term altogether LEM1 is identical to the
Lake Erie model 2
In contrast, LEM3 made a distinction
between allochthonous and autochthon-
ous unavailable phosphorus As a result
(right side of Figure 1), LEM3 contained
four phosphorus forms in addition to
biologically-bound P. Note that SRP
does not settle from the water column in
LEM3
In LEM3 a basin-specific fraction (FA)
of the total external unavailable P
(TEUP) was considered to be external
ultimately-available P (EUP). The re-
maining fraction was considered to be
unavailable to algae while in the water
column (ERP) EUP was converted to SRP
at a temperature dependent, first-order
rate (0.154/day at 20°C) The conversion
from EUP to SRP was mathematically re-
presented as.
S = K 0'
([TEUP] - [ERP])
where1
S =smk term for EUP and source term
for SRP (mg P/L-day)
K = rate coefficient for the conversion
of EUP to SRP (/day)
0=temperature coefficient for K
T=water column temperature (C)
[TEUP]=concentration of total external
unavailable P (mg P/L)
[ERP]=concentration of external refrac-
tory P (mg P/L)
I Allochthonous \
I Phosphorus i
I
Autochthonous
Phosphorus
Ultimately
A vailable
settling
v — 02m/d
I settling
v = 04m/d
Figure 1. Comparison of LEM1, LEM2 (left) and LEM3 (right) phosphorus kinetics, loading,
and settling
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Phosphorus Loading
Compilation
Total phosphorus (TP) and soluble
reactive phosphorus (SRP) loads to Lake
Erie for the period of 1970 - 1980 were
provided by the U.S. Army Corps of
Engineers, Buffalo District 1 Loading data
were provided on a monthly basis and
were divided into direct point, indirect
point, non-point and atmospheric sources.
For use in LEM3, these phosphorus loads
were categorized into three forms:
soluble reactive phosphorus (SRP),
external ultimately-available phosphorus
(EUP), and external refractory phosphorus
(ERP). The fractionation of total external
unavailable phosphorus (TEUP =TP-SRP)
into EUP and ERP was based on phospho-
rus bioavailability studies conducted
between 1979 and 1982 at Clarkson
College.34'5 This fractionation is given
in Table 1.
Table 1. Bioavailable fractions of Total
External Unavailable Phosphorus
Utilized in LEM3
30
Input Source
Municipal wastewater
Tributaries
Detroit River
Western Basin
Central Basin
Eastern Basin
(EUP/TEUPI x 100
63
20
25
28
8
Using the above fractionation scheme,
the eleven year average total phosphorus
load to Lake Erie contained 34 4% SRP,
153% EUP and a substantial 503%
external refractory phosphorus (ERP)
Annual TP loading to Lake Erie from 1 970
- 1 980, divided into these three forms, is
shown m Figure 2. While the bioavailable
P load (SRP + EUP) demonstrated a
reasonably linear decrease after 1972,
the ERP load, which is strongly dependent
on annual runoff, fluctuated irregularly
Note that most of the TP load reduction to
the lake in recent years appears to be
derived from the ERP (refractory) poo!
Model Simulations
In order to evaluate the effect of the
model modification on in-lake phospho-
rus dynamics, simulations of LEM1,
LEM2 and LEM3 were compared All
three models used the same loading data
discussed above. These P loading data
were not identical to those used by other
investigators2 to calibrate their model.
The Corps of Engineers revised their
estimates of total and orthophosphate
loadings to Lake Erie subsequent to the
modeling effort of DiToro and Connolly 2
20
o
I
10 —
ERP
EUP
SRP
0
1970
Figure 2.
1972
1974
1976
1978
1980
Yearly soluble reactive phosphorus (SRP), external ultimately-available phosphorus
(EUP), external refractory phosphorus (ERP), and resultant total phosphorus loadings
to Lake Erie, 1970-1980
LEM1 and LEM2 were not recalibrated
to the P loading data utilized Model
coefficients utilized by researchers2were
used for LEM1 and LEM2 However,
LEM2 did not employ SRP settling from
the water column. LEM3 was not calibrated
for this research Rather, LEM3 utilized
model coefficients determined by these
same investigators,2 for those state
variables which were common to the
models For the state variables unique to
LEM3 (TEUP and ERP) unadjusted empiri-
cal coefficients obtained from laboratory
experimentation were utilized. Therefore,
the differences in P dynamics in the three
models were readily compared.
Model comparisons were made for
1970 and 1975 An example of the
comparisons, showing total Chlorophyll a
simulations for three segments of Lake
Erie during 1975, is presented in Figure
3 In both 1970 and 1975 spring and early
summer simulations of all three models
were nearly identical This is because the
models only differed in their phosphorus
representation, and the phytoplankton
were not P-limited atthistime. During the
late summer-fall P-limited bloom, how-
ever, the three simulations were different.
LEM2 drastically overpredicted chloro-
phyll biomass because SRP available in
the water column was overpredicted at
this time. LEM1 and LEM3 predicted
similar biomass peaks, although the
LEM3 simulations were slightly lower
due to its calculation of more severe
phosphorus limitation at this time This
difference appears to be the result of
differences in the mechanism for conver-
sion of external unavailable phosphorus to
available P
Conclusion
The fractionation of Lake Erie phospho-
rus loading data into immediately avail-
able, ultimately available and refractory
forms was quite illuminating and demon-
strated the utility of deterministic models
which differentiate between available
and unavailable phosphorus forms
Although both the Lake Erie model
(LEM1) and the model containing the
proposed phosphorus dynamics refi/ie-
ments (LEM3) compared quite favorably
m the phosphorus and phytoplankton
simulations, it is felt that the LEM3
phosphorus submodel would better
reflect m-lake r.esponses to various
phosphorus management strategies
This is because it offers a mechanistic
explanation of the m-lake phosphorus
observations which is consistent with
experimental evidence on external phos-
phorus bioavailability and is a plausible
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' Central Basin Epilimnion
Central Basin Hypolimnion '
LEM1
-LEM2
-LEM3
7975
Figure 3. LEM1, LEM2, and LEM3 simu-
lations for total chlorophyll-a
in the Western Basin, Central
Basin ep/limnion, and Centra/
Basin hypolimnion of Lake Erie,
1975
alternative to the concept of SRP settling
from the water column.
References
1 Yaksich, S M., D.A Melfi, D.A. Baker,
and J A. Kramer, "Lake Erie Nutrient
Loads, 1970-1980," Lake Erie Waste-
water Management Study technical
report, U S. Army Corps of Engineers,
Buffalo District (1980).
2 DiToro, D.M. and J.P. Connolly,
"Mathematical Modeling of Wa-
ter Quality in Large Lakes, Part 2:
Lake Erie," EPA-600/3-80-065, U.S.
Environmental Protection Agency,
Duluth, MN (1980).
3. DePmto, J.V., T.C. Young, and S.C
Martin, "Algal-available Phospho-
rus in Suspended Sediments from
Lower Great Lakes Tributaries,"
Jour. Great Lakes Res.. 7(3), 311-
325 (1981).
4. Martin, S C., "Bioavailability of Sedi-
ment Phosphorus Inputs to the Low-
er Great Lakes," Ph.D Dissertation,
Clarkson College of Technology,
Potsdam, NY (1983)
5. Young, T.C., J.V DePinto, S.E. Flint,
et a/., "Algal Availability of Phos-
phorus in Municipal Wastewaters,"
Jour. Water Pollut. Control Fed.,
54(11), 1505-1516(1982).
Douglas K. Salisbury, Joseph V. DePinto, and Thomas C. Young are with Clarkson
College of Technology, Potsdam, NY 13676.
W. L. Richardson is the EPA Project Officer (see below).
The complete report, entitled "Impact of Alga I-A vaitable Phosphorus on Lake Erie
Water Quality: Mathematical Modeling," (Order No. PB 84-154 269; Cost'
$11 SO, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Large Lakes Research Station
Environmental Research Laboratory—Duluth
U S Environmental Protection Agency
Grosse He, Ml 48138
•fo U S GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7648
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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