U.S. ENVIRONMENTAL PROTECTION AGENCY
NATIONAL EUTROPHICATION SURVEY
WORKING PAPER SERIES
RELATIONSHIPS OF PRODUCTIVITY AND PROBLEM
CONDITIONS TO AMBIENT NUTRIENTS:
National Eutrophication Survey Findings
for 418 Eastern Lakes
WORKING PAPER NO. 725
CORVALLIS ENVIRONMENTAL RESEARCH LABORATORY - CORVALLIS, OREGON
and
ENVIRONMENTAL MONITORING & SUPPORT LABORATORY - LAS VEGAS, NEVADA
~ G.P.O. 699-440
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RELATIONSHIPS OF PRODUCTIVITY AND PROBLEM
CONDITIONS TO AMBIENT NUTRIENTS:
National Eutrophication Survey Findings
for 418 Eastern Lakes
WORKING PAPER NO. 725
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RELATIONSHIPS OF PRODUCTIVITY AND PROBLEM CONDITIONS
TO AMBIENT NUTRIENTS:
National Eutrophication Survey Findings
for 418 Eastern Lakes
by
Llewellyn R. Williams, Victor W. Lambou,
Stephen C. Hern and Robert W. Thomas
Water and Land Quality Branch
Monitoring Operations Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
Working Paper No. 725
National Eutrophication Survey
Office of Research and Development
U.S. Environmental Protection Agency
November 1977
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FOREWORD
The National Eutrophication Survey was initiated in 1972 in
response to an Administration commitment to investigate the nationwide
threat of accelerated eutrophication to freshwater lakes and reservoirs.
The Survey was designed to develop, in conjunction with State environmental
agencies, information on nutrient sources, concentrations, and
impact on selected freshwater lakes as a basis for formulating
comprehensive and coordinated national, regional, and State management
practices relating to point source discharge reduction and nonpoint
source pollution abatement in lake watersheds.
The Survey collected physical, chemical, and biological data
from 815 lakes and reservoirs throughout the contiguous United
States. To date, the Survey has yielded more than two million
data points. In-depth analyses are being made to advance the rationale
and data base for refinement of nutrient water quality criteria
for the Nation's freshwater lakes.
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ABSTRACT
Data collected by the National Eutrophication Survey (NES) team from
418 eastern lakes were utilized to determine correlations between chlorophyll a,
an indicator of lake productivity, nutrient, and other water quality parameters.
High linear correlations were determined between mean total phosphorus
and mean chlorophyll a levels, especially in lakes with retention times of
greater than 14 days. These basic relationships were compared for populations
of lakes subdivided on the bases of stratification, vegetation dominance
and fishery type. Significant regional differences were noted in the basic
relationships of chlorophyll a to total phosphorus. Correlations determined
for chlorophyll a with phosphorus, Kjeldahl nitrogen, pH and total alkalinity
were positive; those with Secchi disk transparency and nitrogen/phosphorus
ratio were negative.
Relationships between lake "problems," and nutrient or other water
quality parameters were established by comparing historical and field
observational data of general lake conditions with physical, chemical and
biological values obtained from NES sample analyses. The distributions of
lakes with algal blooms, aquatic macrophyte problems, low dissolved oxygen
concentrations, and/or fishkills are presented as functions of mean total
phosphorus and chlorophyll a concentrations. Except for algal blooms,
these lake problems were not consistently associated with extremes of
chlorophyll and total phosphorus levels.
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CONTENTS
Page
Foreword ii
Abstract iii
Introduction 1
Conclusions 3
Materials and Methods 4
Lake Selection 4
Lake Sampling 4
Data Management 4
Results and Discussion 7
Factors Affecting Productivity 7
Productivity Prediction 8
Lake Problems 10
Figures 12
References 20
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INTRODUCTION
In evaluating the impact of eutrophication or nutrient enrichment it is
important to consider the primary uses to which the water body will be put. The
addition of phosphorus to a lake or impoundment may be blessing or bane depending
upon existing nutrient levels and the lake's intended purpose. In general, the
addition of phosphorus to natural waters will increase their productivity. Only
waters with great excesses of nutrients or some superimposed toxic effects would
be expected not to respond to phosphorus enrichment.
Increased productivity may, in some waters, give rise to excessive aquatic
weed growths, algal blooms, fishkills, or reduced dissolved oxygen levels
affecting the nature and distribution of the lake's biotic community. On the
other hand, the manifestations of nutrient enrichment may include increased
fishery productivity, not unwelcome in surface waters managed primarily for
sport and commercial fishing.
The National Eutrophication Survey (NES) was initiated in 1972, to investi-
gate the role of nutrients and their sources in accelerated eutrophication
(aging) of freshwater lakes and reservoirs. Consistent with this objective,
relationships between ambient nutrient concentrations and existing lake conditions
are being examined. The NES staff has brought statistical and computer techniques
to bear upon major segments of the 2% million values generated during the 4-year
sampling program and contained in STORET, (the U.S. Environmental Protection
Agency's water quality data STOrage and RETrieval system). Presently, in-depth
analysis has been conducted for 418 lakes east of the Mississippi River, sampled
at least three times each. Of these, 191 lakes sampled in 1972 represent the
"Northeast States":
Connecticut
Maine
Massachusetts
Michigan
Mi nnesota
New Hampshire
New York
Rhode Island
Vermont
Wisconsin
while 227 lakes sampled in 1973 represent the "Southeast States":
Alabama
Delaware
Florida
Georgia
Illinois
Indiana
Kentucky
Maryland
Mississippi
New Jersey
North Carolina
Ohio
Pennsylvania
South Carolina
Tennessee
Virginia
West Virginia
Key questions revolve about the nature and extent of water quality changes
to be effected per unit change in phosphorus. "Phosphorus" is used here rather
than "nutrient," as the Survey data strongly reinforce the relationship between
phytoplankton productivity and phosphorus. The effects of nitrogen, while
obviously important in all biological systems, are very difficult to quantify,
much less control, in an "open" natural system. Those NES lakes found to be
"nitrogen-limited" in their growth responses were often so because of excessive
phosphorus rather than low nitrogen levels.
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The analysis of NES lake data has sought to answer several basic questions:
(1) What physical, chemical, and hydrologic parameters affect lake
productivity?
(2) With what confidence can we predict the changes in lake productivity
resulting from phosphorus enrichment?
(3) Do lake groups established on the bases of stratification, primary
nutrient limitation, fishery type, or aquatic weed dominance differ
with respect to the productivity responses noted in question 2?
(4) At what phosphorus and chlorophyll a (chl a) levels do such problems
as algal blooms, excessive weeds, fishkills and dissolved oxygen
depression appear?
(5) Of what value are the planktonic algae as water quality indicators?
(6) What environmental factors can be altered to eliminate nuisance
algal blooms?
The purpose of this report is to summarize major study findings to date
with respect to ambient nutrient/water quality relations and answer, to the
extent possible, some of the questions posed above.
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CONCLUSIONS
The following conclusions can, be drawn from our study:
»
1. Productivity, as indicated by mean chl a concentrations, is
strongly related to ambient phosphorus levels, especially in lakes with
hydraulic retention time greater than 14 days.
2. Significant regional differences in chl a response per unit
total phosphorus have been discovered; further studies are ongoing to
determine the basis of these differences.
3. No differences in the response of chl a to phosphorus were
noted in comparing populations of lakes divided on the bases of stratification,
vegetation dominance or fishery type.
4. Algal bloom response is dramatic to high ambient phosphorus
levels. No algal blooms were noted in lakes with mean total phosphorus
concentrations less than 19 micrograms (yg) per liter.
5. Aquatic weed "problems" generally occurred at lower phosphorus
levels than did algal blooms. Phosphorus source control is unlikely to
have significant impact upon aquatic macrophyte populations in many
cases.
6. Fishkills v/ere found to be generally unrelated to mean phosphorus
or chl a levels or even to chronic low-oxygen conditions.
7. In the Southeast, many of the oxygen "problems," as defined in
this report, arise from the establishment of trout fisheries in marginal
habitats.
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MATERIALS AND METHODS
LAKE SELECTION
Selection of lakes and reservoirs included in the National Eutrophication
Survey in 1972 and 1973 was limited to lakes 40 hectares or more in surface
area, with mean hydraulic retention times of at least 30 days, and impacted
by municipal sewage treatment plant effluent either directly or by discharge
to an inlet tributary within 40 kilometers of the lake. Specific selection
criteria were waived for lakes of special State interest. As a result, a
broader range of water quality was represented than would have been possible
had the selection criteria been rigidly enforced. Although lakes selected
were not necessarily representative of average conditions existing in the
study area, the relationships observed between ambient nutrients and lake
v/ater quality should not be biased.
LAKE SAMPLING
Sampling was accomplished by tv/o teams, each consisting of a limnologist,
pilot, and sampling technician, operating from pontoon-equipped helicopters.
With few exceptions, each lake was sampled under spring, summer, and fall
conditions. Sampling site locations were chosen to define the character of
the lake water as a whole and to investigate visible or known problem areas,
e.g., algal blooms, sediment or effluent plumes.
The number of sites was limited by the survey nature of the program and
varied in accordance with lake size, morphological and hydrological complex-
ity, and practical considerations of time, flight range, and weather. At
each sampling depth, water samples were collected for oxygen determinations.
Contact sensor packages were used to measure depth, conductivity, turbidity,
pH, dissolved oxygen, and temperature. Fluorometric chlorophyll (chl a)
analyses were performed at the end of each day in a mobile laboratory.
Nutrients and alkalinity were determined by automated adaptations of proce-
dures described in "Methods for Chemical Analysis of Water and Wastes"
(U.S. EPA 1971). Survey methods are detailed elsewhere (U.S. EPA 1974,
1975).
DATA MANAGEMENT
Data collected were stored in STORET and manipulated, as prescribed by
Bliss, Friedland, and Hodsen (1976). Basic calculations for parameters meas-
ured in sampled lakes were performed in such a v/ay as to give equal weight to
each depth sampled at a station, each sampling station sampled on an individual
lake during a sampling round, and each sampling round on an individual lake
during a sampling year.
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Mean parameter values for each sampling station were calculated as
follows:
D
Par. = 5 Par./D (Eq. 1)
J tl v
where Par. = mean value for a parameter at the j sampling station
J during a sampling round
Par^. = value for the i^ depth
and D = the number of depths for which a parameter was measured at
the j sampling station during a sampling round
Mean parameter values for each sampling round were calculated as
follows:
S
Par, = 2 Par./S (Eq. 2)
* 0=1 d
where Parfe = mean value for the kth sampling round on a given lake
S = number of sampling sites
Mean lake parameter values for a given sampling year were calculated as
follows:
_ 3
Par = ^ Par,/3 (Ecl- 3)
k=l K
where Par = mean parameter value for a given sampling year
Lake parameter values were calculated only when values were available for
the first, second, and third sampling rounds during a given sampling year from
a lake. Formulas 1, 2, and 3 were used to determine parameter values for total
phosphorus, dissolved phosphorus, dissolved orthophosphorus, total alkalinity,
ammonia-N, nitrate-nitrite-N, and Kjeldahl-N in milligrams per liter (mg/liter),
temperature in degrees Celsius, turbidity in percent transmission, pH in standard
units, Secchi disk in inches and hydraulic retention time in days.
Nitrogen and phosphorus values calculated at the sampling station level
(equation 1) were used to compute nitrogen/phosphorus (N/P) ratios as follows:
N _ ammonia-N + nitrate-nitrate-N /r„ ^
P - total dissolved phosphorus for 1972 lake data; (Eq" 4)
and
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Any total or dissolved phosphorus value for which a corresponding nitrog
enous elemnt was missing was deleted. The N/P ratios thus obtained were then
handled as the other preceding parameters to establish lake means for the
sampling year. A change in laboratory analyses occurred after the 1972 sam-
pling year resulting in substitution of dissolved orthophosphorus for total
dissolved phosphorus. Total phosphorus was considered to better represent the
bioavailable phosphorus pool than does orthophosphorus (Lean 1973) and was
therefore selected as the denominator in equation 5.
Unlike the above parameters where measurements were made at various depths,
only one chl a measurement was made at any individual sampling station during a
sampling round. Therefore,
chl a . = chl a (Eq. 6)
J
where chT a ' ~ mean a concentrati°n in micrograms/1 iter (ug/liter)
3 for the sampling station during a sampling round
chl a = the chl a concentration in ug/liter for an integrated water
sample from the surface to 4.6 meters or to a point just off
the bottom when the depth was less than 4.6 meters
When photic zone determinations were made during t.he 1973 sampling year, the
compensation point (1% of incident light remaining) provided the greatest depth
of integration, if it exceeded 4.6 meters.
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RESULTS AND DISCUSSION
FACTORS AFFECTING PRODUCTIVITY
Those physical, chemical and biological parameters found to be most
highly correlated with lake productivity (as measured by chl a levels) are
listed below in approximate order of importance:
log-log Correlation Coefficient (r)
Northeast Southeast
Total phosphorus 0.74 0.74
Secchi disc transparency -0.74 -0.60
Kjeldahl (organic) nitrogen NA 0.82
Dissolved phosphorus 0.66 NA
Dissolved orthophosphorus NA 0.59
pH 0.49 0.71
Nitrogen/phosphorus ratio -0.51 -0.40
Total alkalinity 0.37 0.46
Ammonia-N 0.48 0.19
NA - Parameter not measured
The relationship of Kjeldahl-N to chl a noted is not unexpected as the
chlorophyll-containing autotrophs are primary sources of the organic nitrogen
in surface waters. The pH response likely reflects increased carbon assimilation
in waters as chl a levels increase. Low N/P ratios have generally been
associated with high phosphorus levels in the NES lakes; the negative corre-
lation of chl a with N/P ratio therefore roughly mirrors the positive chl a
response to total phosphorus. The negative correlation ofchl a with Secchi
disk transparency suggests a positive correlation between suspensoids and
chl a3 and between suspensoids and total phosphorus. The following equation
(after Verduin, et al 1976) provides an estimate of suspensoids:
0 0.12SD
where S is dry weight of suspensoids expressed in grams per cubic meter of
water, SD is the Secchi disk depth in meters, the numerator (2) represents a
coefficient which relates Secchi transparency to the depth of the photic zone,
and the factor 0.12 represents the light extinction per gram dry weight of
suspensoids (in square meters per gram).
Although the Survey lakes share some common features as a result of their
selection criteria, they reflect a broad range of size, depth, type (natural,
artificial, level-enhanced), location, hydraulic retention time (hours to
years), degree of nutrient enrichment, geologic substrate, watershed land use,
and primary water use (drinking water, fisheries, general recreation, indus-
trial , etc.).
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The relationships of the parameters listed above, as well as some others
were compared in subpopulations of lakes grouped according to primary limiting
nutrient, stratification pattern and vegetation dominance (Lambou, et al.,
1976). Lakes with short retention times were excluded. In many cases the
response relationships were stronger and better defined as the lake groups
became more nearly homogeneous. For example, in northeastern "P-limited"
lakes with hydraulic retention times greater than 14 days, the "coefficient of
determination," (r2) is 0.83 (r = 0.91); therefore 83 percent of productivity
changes could be accounted for by changes in total phosphorus alone. The
degree to which this can be considered cause and effect and the "independence"
of phosphorus activity in general lake productivity is being further examined
by National Eutrophication Survey (NES) staff.
PRODUCTIVITY PREDICTION
A high correlation was found between chl a, averaged over three seasons,
and ambient phosphorus concentrations determined for the same period. The
following table presents levels of chl a as a function of total phosphorus,
predicted by three different equations:
The regression equations used to predict the above chl a levels are:
A. log yg/liter chl a = 1.87 + 0.64 log yg/liter total phosphorus,
based upon all 418 eastern NES lakes.
(r = 0.73)
B. log ug/liter chl a = 1.99 + 0.70 log yg/liter total phosphorus,
based upon all eastern NES lakes with retention times greater than
14 days (318 lakes).
(r = 0.81)
C. log yg/liter chl a = 3.29 + 1.46 log yg/liter total phosphorus,
modified after Jones and Bachmann (1976), based upon 143 lakes of
wide distribution.
Total Phosphorus (yg/liter)
Chl a (yg/liter)
ABC
0.005
0.010
0.015
0.020
0.025
0.050
0.100
2.5 2.4 0.8
3.9 3.9 2.3
5.0 5.2 4.2
6.1 6.3 6.5
7.0 7.4 8.9
10.9 12.0 25.0
17.0 19.5 68.0
(r = 0.95)
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The two NES regression equations (A and B) are based upon phosphorus
and chl a values averaged over three seasons; the Jones and Bachmann
equation (C) is derived entirely from summer values. The inclusion of a
high percentage of "N-limited" lakes, low spring and fall responses, and
the high turbidity levels of many of the waters sampled contribute to the
relatively low overall response relationships reflected in the NES equations.
The relationship of chl a to phosphorus was found to be weak in north-
eastern lakes with short (less than 14 days) hydraulic, retention times
(r = 0.35); the algae appear to be flushed from the lake system before
maximum densities can be achieved. A potential problem is temporarily
"solved" by exporting it downstream. The short-term weak response seen in
northeastern lakes improved dramatically in analysis of similar lakes in
the Southeast.
If the eastern United States is bisected by an east-west line formed
by the Roanoke, New, and Ohio Rivers, the States north of this line show
significantly higher average summer chl a to total phosphorus response
relationships (0.41 pg chl a/mg total phosphorus) than those States south
of the line (0.29 yg chl a/mg total phosphorus). Preliminary analyses
suggest that much of the phosphorus in the southern waters is associated
with inorganic suspended materials and not readily bioavailable. The chl a
to total phosphorus response average for those lakes of Secchi disk light
penetration <30 inches appears to be only about 60 percent of that found
for lakes of greater than 70-inch Secchi disk. Florida lakes represent an
exception to the low southern lake response, perhaps because the overwhelming
preponderance of large man-made impoundments in the southern lake group
does not extend to Florida. Further analyses are ongoing to establish the
bases for such regional response differences.
In the Northeast the response of chl a to total phosphorus was signifi-
cantly higher in those lakes with a high nitrogen/ phosphorus (N/P) ratio
called "P-limited", than in those with low N/P ratios, called "N-limited."
An intermediate or "transition" range was identified within which neither
phosphorus nor nitrogen exerted independent growth control. Southeastern
lakes, whether considered "P-limited" or "N-limited" on the basis of algal
assay or N/P ratio, showed no differences in chl a responses to increased
phosphorus levels. This is consistent with the findings of Schindler and
his co-workers (Schindler 1977) studying Canadian Shield lakes. Productivity
was more highly correlated with total phosphorus levels than with inorganic
nitrogen levels, even in "N-limited" lakes.
Comparison of the chl a phosphorus relationships in stratified and
non-stratified, phytoplankton dominated and macrophyte dominated, coldwater
fishery and warmwater fishery lakes revealed no significant response
differences by lake population.
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LAKE PROBLEMS
The frequency with which such lake problems as algal blooms, fishkills,
dissolved oxygen depressions, and excessive weed growths occur has been
examined both as a function of ambient phosphorus concentrations and chl a
levels.
The criteria for placing lakes in problem categories were as follows:
(1) Aquatic weeds - Lakes which exhibited extensive reaches of submerged
or floating higher aquatic plants, with histories of recurrent
weed problems, and/or reported to be problem lakes in this regard,
based upon field observations, historical information, and
contact with State and local personnel.
(2) Algal blooms - Lakes in which readily observable color or turbidity
changes were attributed to high phytoplankton concentrations as
recorded by field limnologists or reported by State personnel.
(3) Dissolved oxygen depression - Lakes in which dissolved oxygen
levels of less than 4 mg/liter (a) were present in the hypolimnion
of thermally stratified lakes managed for trout, (b) extended into
the epilimnion of warm-water fishery lakes, or (c) were present
under thermally mixed conditions, regardless of lake use.
(4) Fishkills - Lakes which exhibited sudden large-scale fish deaths
reported by State or local personnel; lakes with annual occurrences
such as spring shad die-offs or kills due to known releases of
toxic pollutants were not included.
Examination of the distribution of lake problems by phosphorus or chl a
level is enlightening (Figs. 1-8). Aquatic weeds may be present in high
concentrations at extremely low phosphorus concentrations. It is unlikely
that source control of this nutrient will significantly reduce such macrophyte
populations. Many submerged aquatic plants are highly competitive for nutri-
ents at low levels in the water (except perhaps carbon), or survive quite
well on nutrients absorbed through their root systems. The bulk of aquatic
weed problems occur at lower phosphorus and chl a levels than do algal blooms.
A probable explanation for this is shading by high phytoplankton densities
present at elevated nutrient levels; the reduced penetration of light into
the water discourages submerged aquatic weed growth.
Phytoplankton algae, however, respond dramatically to phosphorus, as
evidenced by the distribution of algal blooms relative to phosphorus levels
in northeastern and southeastern lakes. Algal blooms were recorded in 70 of
the northeastern and 124 of the southeastern NES lakes. No blooms were noted
in lakes in which the mean total phosphorus concentration was less than
19 ng/liter (log P = 1.23), However, blooms were recorded in lakes with mean
chl a concentrations below 1 pg/liter. This apparent paradox may be partially
explained by the nature of the sampling operation. It was noted that surface
films of high algal concentration were, on rare occasion, swept aside by
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downdraft from the helicopter (turbine engine-equipped helicopters are not
shut down while on sampling station) to reveal highly transparent waters
beneath. Values of chl a obtained under these unique circumstances would
likely be low, while the highly visible surface film of algae would invariably
be noted as a bloom.
Twenty percent of the algal blooms recorded occurred with corresponding
mean chl a levels of 10 yg/liter or less.
The distribution of reported fishkills presented in Figs. 7 and 8 is
rather remarkably unrelated to either phosphorus concentrations above
25 yg/liter (log P = 1.40) or chl a levels. Following up this surprising
finding, the distribution of reported fishkills and that of recorded
dissolved oxygen problems were also found to be unrelated. Fishkills, with
some exceptions, result from low dissolved oxygen conditions accompanying
the decomposition of organic materials, as during the collapse of an algal
bloom. The NES data suggest, however, that such events are precipitous,
and bear little relationship to the chronic low oxygen conditions noted in
many Survey lakes. The chronic oxygen problem lakes were found to be
nearly free of recorded fishkills (though not free of fish). Only 1 of
27 fishkills occurred in a lake with recorded oxygen problems. A NES
limnologist was at hand to observe the simultaneous anoxic conditions of
the singular incident.
In contrast to the northeastern lakes, lakes in the Southeast showed
few dissolved oxygen problems at high levels of phosphorus or chl a. The
bulk of the dissolved oxygen problems in the latter group appeared at the
low ends of the respective ranges. More than one-half (17 of 30) of the
low oxygen problems noted in 1973 data appeared in southeastern lakes
managed as "coldwater" (trout) fisheries. These habitats must be considered
marginal for coldwater fisheries and compare with only five such fisheries
free of oxygen problems. In general, the lakes considered.to be marginal
as coldwater fisheries would not have been considered problems were they
being managed as warmwater (bass, pike, walleye, etc.) fisheries. The
majority of the oxygen problems noted in the Southeast resulted from creating
trout fisheries by stocking waters in artificial impoundments which are
only marginal for them. However, it should be noted that these fisheries
are.making use of habitat in the lake which is generally too cold to support
warmwater fish.
The mean total phosphorus level at which dissolved oxygen problems
were found was 10 times as high in warmwater fisheries as in trout-managed
lakes (209 yg/liter vs. 22 yg/liter). That these problems exist at such
very different ranges of nutrient enrichment clearly points up the need to
evaluate water quality "suitability" with respect to its primary beneficial
uses.
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P-LIMITED LAKES
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Log P-Total (pg/1)
Log Chi a (|ig/1)
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TRANSITION LAKES
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Log P-Total (pg/1)
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N- LIMITED LAKES
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Lo^ P-Total (jig/1)
Log Chi a. (pg/1)
Figure 1. Distribution of aquatic weed problems by phosphorus and chloro-
phyll a levels in northeastern NES lakes (problem lakes shaded).
12
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P-LI M IT ED LAKES
28
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Figure 2. Distribution of aquatic weed problems by phosphorus and chloro-
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13
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Lik. *• J",
P-UMITED LAKES
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Figure 3. Distribution of algal blooms by phosphorus and chlorophyll a
levels in northeastern NES lakes (problem lakes shaded).
14
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P-UMITED LAKES
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Figure 4. Distribution of algal blooms by phosphorus arid chlorophyll
levels in southeastern NES lakes (problem lakes shaded).
15
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P-LIMITED LAKES
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N- LIMITED LAKES
20
Si
1
-EL
« 16
Log P-Total (jig/1)
Log Chi a (pg/1)
Figure 5. Distribution of dissolved oxygen problems by phosphorus and
chlorophyll a levels in northeastern NES lakes (problem lakes shaded).
16
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P-UMITED LAKES
Q
28
24
20
16
12
8
4
0
R3
ii
u
cm o* r>
Log P-Total (ug/H
Log Chi .a ipg/ll
TRANSITION LAKES
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Log P-Total (pg/l)
N-LIMITED LAKES
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Log P-Total (iig/l)
28
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© 12
to
«
E 8
a
Z
4
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Log Chi a. ('iig/l)
IBS
Mi
^JwL
Log Chi jb (|ig/l)
Figure 6. Distribution of ciissolved oxygen problems by phosphorus and
phyll a levels in southeastern NES iakes (problem lakes shaded).
n r» n
chloro-
17
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P-LIMITED LAKES
20
18
£ 16
Id 14
_ 12
_el
: el
CO CM 1X3 ' O OO
o •— •" cm esj cm
Log P-Total (pg/1)
Log Chi a (jig/1)
-QL
TRANSITION LAKES
201
¦ 18
16
re 14
W 12
o
OJ
.a
E
3
z
_EL
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cvi rsi
Log P-Total (pg/1)
Lon Chi a (lig/1)
j=£
N- LIMITED LAKES
20
18
16
14
12
10
8
6
Z 4
2
XL 0
1/5
V
0)
J3
£
p ^ 00
rsi eg rvj
ex rg oj
Log P-Total (pg/1)
Log Chi a (pg/1)
Figure 7. Distribution of reported fishkills by phosphorus and chlorophyll a.
levels in northeastern NES lakes (problem lakes shaded.). ~
18
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P-LIMITED LAKES
fljiil
11:!
CM IA
o
a>
A
E
28
24
20
16
12
8
ss
H
S
IB
man
3b.
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Log P-Total lgg/1)
Log Chi a^ (|ig/D
TRANSITION LAKES
28
24
20
16
12
8
~1-n
fN cn r» n m
to e
rsj ri
rj n
Log P-Total (pg/l)
Log Chi a (pg/l)
N-LIMITED LAKES
11
BB
« AO N (D
'' ' N N
Log P-Total lpg/1)
28
24
20
16
12
8
4
0
OS
ran
"Till
(O o
n n m
Log Chi a (pg/l)
Figxore 8. Distribution of reported fishkills by phosphorus and chlorophyll a
levels in southeastern NES lakes (problem lakes shaded).
19
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