Canada
• J-<« -Ci • V> •
State of The Lakes Ecosystem Conference
Nutrients:
Trends and System Response
October 1994
Environment Canada
^nvironmental Protection Agency
EPA905-D-94-001d
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State of the Lakes Ecosystem Conference
NUTRIENTS: TRENDS AND SYSTEM
RESPONSE
Melanie Neilson
Serge L'Italian
Violeta Glumac
Don Williams
Environment Canada
Environmental Conservation Branch
Burlington, Ontario
Paul Bertram
Great Lakes National Program Office
United States Environmental Protection Agency
Chicago, Illinois
July 1994
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Table of Contents
Acknowledgments iii
EXECUTIVE SUMMARY 1
1.0 INTRODUCTION 3
2.0 STATUS AND TRENDS FOR FISH AND WILDLIFE HEALTH 5
2.1 Reductions in Historic Loadings 5
2.2 Current Status 6
3.0 SYSTEM RESPONSE 9
3.1 Soluble Reactive Phosphorus and Algal Growth 9
3.2 Lake Erie Dissolved Oxygen Depletion 10
3.3 Nitrate-plus-nitrite 10
4.0 WHERE DO WE GO FROM HERE? 13
5.0 REFERENCES 15
List of Figures 18
Figures 19
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ACKNOWLEDGEMENTS
The authors would like to acknowledge the contributions of the following in developing this
cluster paper: Kevin McGunagle, International Joint Commission-Regional Office, for providing
us with the latest phosphorus loading estimates; Scott Painter, National Water Research Institute,
for developing the section on Cladophora; Peter Yee, Environmental Services Branch, for over-
basin precipitation records for each lake; Len Kamp, Monitoring and Systems Branch, for
information on the SWEEP program; Phil Smith, Ontario Ministry of Natural Resources, for
providing us with Ontario stocking information; and Tom Nalepa, Great Lakes Environmental
Research Laboratory, for 1991 Saginaw Bay data. In addition, we are grateful for the review
comments provided by Ora Johannsson, Gary Sprales, Murray Charlton, C.H. Chan, Ken Kuntz
and Hugh Dobson.
NOTICE TO READER
These Working Papers are intended to provide a concise overview of the status of conditions
in the Great Lakes. The information they present has been selected as representative of the
much greater volume of data. They therefore do not present all research or monitoring
information available. The Papers were prepared with input from many individuals
representing diverse sectors of society.
The Papers will provide the basis for discussions at SOLEC. Readers are encouraged to
provide specific information and references for use in preparing the final post-conference
versions of the Papers. Together with the information provided by SOLEC discussants, the
Papers will be incorporated into the SOLEC Proceedings, which will provide key information
required by managers to make better environmental decisions.
Nutrient Trends and Response - SOLEC working paper
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EXECUTIVE SUMMARY
Reductions in annual phosphorus loadings have been achieved in all five Great Lakes, with
current loads clearly below the target loads of the 1978 Canada-U.S. Great Lakes Water Quality
Agreement for lakes Superior, Huron and Michigan, and at or near the target limits for lakes Erie
and Ontario. Phosphorus concentrations are likewise below the expected open-lake
concentrations that reflect achievement of loadings limits for the Upper Lakes. In Lake Ontario,
expected phosphorus concentrations have been achieved for several recent years. In central and
eastern Lake Erie, phosphorus concentrations have been achieved, but some annual fluctuations
around the objective still exist In western Lake Erie, annual phosphorus concentrations are
highly variable, although spring averages below the objective have been reported in at least two
recent years.
Soluble reactive phosphorus (SRP) represents that fraction of the total phosphorus which is
directly available to the primary producers (plants and algae). Generally, for all of the Great
Lakes, spring SRP trends followed those of total phosphorus concentrations. The decline in SRP
concentrations has resulted in noticeable changes, both nearshore and offshore. The nearshore
effects were observable in the reduction in Cladophora growth. In the offshore, concentrations
of chlorophyll a (an indirect measure of productivity) indicate that the Upper Lakes have
remained oligotrophic, while the Lower Lakes (particularly Lake Ontario) are tending towards
oligotrophic conditions.
Nitrate-plus-nitrite is also an important nutrient in water systems. Major sources of nitrogen to
the lakes are agricultural runoff, municipal sewage treatment plants and atmospheric deposition.
Increasing levels of nitrate-plus-nitrite have been reported in the Great Lakes for the past two
decades, particularly in Lake Ontario. Current open lake concentrations do not create a public
health concern, as they are at least 20 times lower than the guideline for protection of drinking
water (10 mg/L). The combination of reductions in phosphorus concentrations and increases in
nitrogen concentrations have served to not only reduce the total quantity of algae in the water
(i.e., reduced chlorophyll and Cladophora levels), but also to shift the species composition away
from nuisance blue-greens and toward more desirable, and historically prevalent, diatoms.
Phosphorus controls appear to have been successful in lowering the loadings into the lakes, and
consequently reducing, to varying extents, the resultant open lake concentrations of total and
soluble reactive phosphorus. However, the goal of establishing year-round aerobic conditions in
the hypolimnion of Lake Erie's Central Basin has not been realized. It has been determined that
lake phosphorus loads would have to be reduced to about 5000 metric t/y to achieve the desired
effect on oxygen. There is also evidence to suggest that there were brief periods of anoxia in
some areas of the Central Basin of Lake Erie for hundreds of years, prior to European
colonization and the onset of cultural eutrophication. Perhaps intermittent anoxia is an inherent
property of the basin, and management to achieve a state where anoxia does not occur is not a
realistic goal for lake managers.
Nutrient Trends and Response • SOLEC working paper
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What impact have nutrient controls had on the food web? Zooplankton are the pivotal trophic
level in lake ecosystems, responding to both salmonid stocking (top-down) and phosphorus
abatement (bottom-up) management strategies. In Lake Ontario, zooplankton composition has
not changed systematically or substantially throughout the 1980's, although zooplankton
abundance declined as of 1983-84, particularly in the eastern basin. This has been attributed to
a combination of stable fish predation and reduced food supply.
Zooplankton comprise almost the entire diet of alewife, and are a significant component of the
diet of smelt The reduction in abundance of zooplankton, therefore, could only result in lower
production of alewife and smelt Alewife and smelt biomass indices have declined since the
early 1980's. At the same time, the numbers of hatchery-reared salmon and trout stocked in
Lake Ontario steadily increased from just over 1 million in 1972 to approximately 8.2 million
in 1984. As the stocked fish continued to grow and accumulate, the total weight of salmon and
trout reached a peak in 1986 and, since then, has remained high. Alewife and smelt populations
in Lake Ontario are under stress from both ends of the food chain. How much stocking is too
much?
In an attempt to restore the balance between stocked predators and prey in Lake Ontario, the two
agencies responsible for fisheries management agreed to revise their Lake Ontario fish stocking
plans, using a two-year phased-in approach, beginning in 1993. For 1993, the two agencies set
a target for predator demand which is 35% lower than 1991 levels. In 1994, additional stocking
reductions are expected to bring about a total reduction of 47% in predator demand from the
1991 level.
Fisheries and water quality management strategies have evolved independently in the Great
Lakes. In general, these strategies operate from different ends of the management spectrum —
bottom-up (phosphorus control) and top-down (massive fish stocking). A more ecologically
oriented approach, which recognizes the interactions between fisheries and water quality, will be
required to manage the Great Lakes.
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1.0 Introduction
In the 1960's, severe degradation of the Lower Lakes and several embayment areas of the Upper
Lakes aroused public concern. Enormous algal blooms were frequently observed and normal
aquatic life disappeared from waters adjacent to densely industrialized and populated areas. In
the Central Basin of Lake Erie, Saginaw Bay and Green Bay, bacterial decomposition of large
quantities of algae that had settled to the sediment surface lead to anoxia (lack of oxygen) in the
bottom waters (hypolimnion). Decomposing filamentous algae (Cladophora) also piled up on
some beaches. Taste and odor problems appeared in drinking water due to blue-green algae.
Forage fish, mostly alewives, were dying in large numbers in lakes Michigan and Ontario.
In the late 1960's, a review of the state of the Lower Lakes by the International Joint
Commission (IJC), based on the results of special studies by Canada and the U.S., identified
eutrophication as a problem due to excessive inputs of nutrients. Phosphorus was subsequently
identified as the key nutrient controlling eutrophication. If cultural eutrophication of the Great
Lakes was to be reversed, phosphorus would need to be controlled. The major sources of
phosphorus were municipal and industrial wastes, and urban and agricultural runoff. In 1972 the
United States and Canada signed the Great Lakes Water Quality Agreement (GLWQA). The
GLWQA focused on reducing phosphorus inputs to the lakes in order to:
a) substantially eliminate nuisance algal growth in the Lower Lakes and the International
Section of the St Lawrence River;
b) restore year-round aerobic conditions in the hypolimnion of Lake Erie's Central Basin;
and
c) maintain Lake Superior's and Lake Huron's oligotrophic state.
Municipal wastes containing phosphate detergents contributed 70% of total inputs of phosphorus.
Programs that were implemented to reduce phosphorus loads to the Great Lakes included
improving major municipal wastewater treatment facilities (those discharging more than 3800
mVd or 1 million gallons/d) so their effluents contained no more than 1 mg P/liter, limiting P
content in household detergents used in the Great Lakes Basin, requiring industries to remove P
from their discharges to the maximum extent practicable, and controlling P loadings from
agricultural operations.
Restrictions on the P content of household detergents was considered one of the most effective
early actions that could be taken to reduce phosphorus loadings. The Canadian federal
government chose to implement such restrictions in 1972, and several of the U.S. states did so
soon thereafter. The U.S. federal government also encouraged engineering solutions to
phosphorus removal from sewage. U.S. federal grants to states and local municipalities for
construction or upgrading of sewage treatment plants was a highly visible, cosily part of a
nationwide program to improve the level of sewage treatment However, in many cases around
the Great Lakes, the targeted load reductions forced the consideration of phosphorus removal in
Nutrient Trends and Response - SOLEC working paper 3
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the engineering designs that otherwise might not have been included.
Updated phosphorus loading targets for each lake were incorporated into the renegotiated
GLWQA in 1978. These target loads were based on achieving average annual in-lake
phosphorus concentrations [guidelines] (Vallentyne and Thomas 1978), as shown in Table 1.
These loading values were subsequently confirmed (Phosphorus Management Strategies Task
Force 1980), and were incorporated into the revised GLWQA in 1978. Later, the Aquatic
Ecosystem Objectives Committee of the UC recommended that the phosphorus concentration
guidelines be based on spring open-lake concentrations, since these are what determine summer
phytoplankton biomass (Science Advisory Board 1980).
Table 1. Phosphorus target loads (metric t/yr) and spring total phosphorus
guidelines (ug/L).
Basin Phosphorus Target Load Guideline
Lake Superior 3400 5
Lake Michigan 5600 7
Lake Huron 4300 5
Lake Erie 11000
Western Basin 15
Central Basin 10
Eastern Basin 10
Lake Ontario 7000 10
A Phosphorus Load Reduction Supplement was added to the Agreement in 1983 in recognition
that additional efforts needed to be directed at non-point sources (such as agricultural inputs) in
the Lower Lakes. Specifically, this Supplement called for an additional loading reduction from
non-point sources of 2,0001 and 430 metric tonnes/yr for lakes Erie and Ontario, respectively.
This paper discusses the progress in controlling phosphorus loads to the lakes and the resulting
responses of each of the Great Lakes. Concentrations and long-term trends of phosphorus,
nitrogen and chlorophyll a, as well as hypolimnetic oxygen depletion in Lake Erie, will be
discussed.
1 The allocation of reductions to meet target loads for Lake Erie are further defined as 300 metric t/yr from
Canadian sources, and 1700 metric t/yr from U.S. sources. Lake Ontario was never apportioned.
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2.0 Status and Trends of Total
Phosphorus in the Great Lakes
2.1 Reductions in Historic Loadings
The combination of phosphorus-detergent restrictions and improved sewage treatment facilities
was successful in reducing phosphorus loadings. By 1985, 85% of the 40 largest municipal
discharges in the Great Lakes basin were in compliance with the 1 mg/L phosphorus limit,
including the 9 largest dischargers.
In order to address agricultural non-point sources for Lake Erie, the Canadian federal and
provincial governments initiated the Soil and Water Environmental Enhancement Program
(SWEEP; 1985-1993). Its intention was to assist farmers through combined incentive programs,
education and research, and included conservation fanning practices, installation of structural soil
erosion control measures and environmentally appropriate animal waste handling practices.
Cropping and tillage practice changes were expected to account for most of the phosphorus
reductions from agricultural non-point sources. Results from the SWEEP program, to date,
indicate that Canada has met or exceeded its agricultural non-point source phosphorus loading
reduction targets for Lake Erie.
The non-point source phosphorus reduction programs in the U.S. have involved a number of
approaches and jurisdictions, including numerous federal grants for projects to demonstrate
conservation tillage and no tillage practices. Participation by fanners and other landowners is
still voluntary, but many successful projects have fostered continuing interest in improved
agricultural practices.
Estimates of phosphorus loadings from tributaries, municipal and industrial point sources,
atmospheric sources and the connecting channels have been calculated for each lake on an annual
basis by the Regional Office of the International Joint Commission, and reported by the Water
Quality Board. Results for 1976 to 1991 are presented in Figure 2. Figure 3 presents the trends
in open lake total phosphorus concentrations for the period 1971 to 1992 measured during spring
cruises conducted by Environment Canada Cakes Superior, Huron/Georgian Bay and Ontario) and
the Great Lakes National Program Office of U.S. EPA (lakes Michigan and Erie). Nutrient
concentrations are usually greatest in early spring, and concentrations at this time determine the
limits of algal growth during the summer.
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2.2 Current Status
Reductions in annual phosphorus loadings have been achieved in all five Great Lakes, with
current loads clearly below the target loads of the 1978 Agreement for lakes Superior, Huron and
Michigan, and at or near the target limits for lakes Erie and Ontario.
Phosphorus concentrations are likewise below the expected open lake concentrations that reflect
achievement of loadings limits for the Upper Lakes. In Lake Ontario, expected phosphorus
concentrations have been achieved for several recent years. In Central and Eastern Lake Erie,
phosphorus concentrations have been achieved, but some annual fluctuations around the objective
still exist. In Western Lake Erie, annual phosphorus concentrations are highly variable, although
spring averages below the objective have been reported in at least two recent years.
Lake Superior: Since 1985, loadings have been below the target (3400 metric t/y).
During the period of record, 1983 to 1992, open lake concentrations were always well
below the 5 ug/L guideline. The most recent data for the lake indicate that higher
concentrations are found only in the Duluth-Superior Harbor region in the western arm
of the lake.
Lake Michigan: Since 1981, loadings have been below target (5600 metric t/y).
Tributary loadings, which account for the bulk of the load, have varied over the period
of record. Atmospheric loadings, however, appeared to decline an order of magnitude
between 1980 and 1981. Beginning in 1981, more acurate estimates of atmospheric
loadings became available through U.S. and Canadian atmospheric monitoring networks,
and subsequent total load estimates have reflected the better atmospheric data. Since
1976, lakewide total phosphorus concentrations have consistently been below the 7 ug/L
guideline, with concentrations of approximately 5 ug/L.
Lake Huron: With the exception of 1982 and 1985, loadings have been below target
(4300 metric t/y) since 1981, also reflecting the better atmospheric data available. In
1985, tributary loadings doubled, coincident with record (1900-1993) average Lake Huron
basin precipitation (National Oceanic and Atmospheric Administration, NOAA). Open
lake total phosphorus levels have remained below the 5 ug/L guideline from 1980 to
1991, except in 1987 when 5.5 ug/L was observed. Localized problems persist in
Saginaw Bay (see Figures 4a and 4b) and along the Ontario shore of southern Lake
Huron.
Lake Erie: Phosphorus loadings demonstrated a general decreasing trend during the
period 1976-1991. Municipal loadings showed a decrease between 1976 and 1981, but
have remained fairly constant since then. While Lake Erie receives the largest municipal
load of any of the Great Lakes (Dolan 1993), 100% of the largest plants are in
compliance with the 1 mg/L effluent limitations. For the decade 1981-1991, loadings
were equal to or below the target except for the years 1982,1984 and 1990. Phosphorus
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loads to Lake Erie are directly related to the amount of precipitation falling in the basin
since the major phosphorus inputs come from the tributaries to the lakes. Thus 1990, the
wettest year on record (NOAA) for Lake Erie, caused an approximate doubling of
tributary loads, resulting in the highest recorded phosphorus load since 1980.
In the Western Basin, average spring phosphorus concentrations continue to be highly
variable, subject to sampling locations, influence of seasonal tributary loadings and
sediment resuspension from storms. During the period 1983-1985, average spring
phosphorus concentrations were typically 20-25 ug/L, although in 1984 one survey
averaged 69.3 ug/L. During 1990 and 1992, spring averages were reduced to 12.2 and
10.9 ug/L, respectively, but the 1991 average (27.5 ug/L) demonstrates the continuing
variable nature of the Western Basin.
Since 1970, average spring concentrations in the Central Basin have generally declined,
dropping below the guideline of 10 ug/L during 1988-1990. Concentrations were slightly
above the guidelines during 1991-92. However, annual fluctuations are common, in part
due to the influence of resuspended sediments from storms, but average concentrations
remain around the guideline.
In the Eastern Basin, phosphorus concentrations declined from greater than 20 ug/L in the
early 1970's to below the objective of 10 ug/L in 1987. Spring concentrations remained
below the guideline through 1990, but slightly exceeded it in 1991 and 1992.
Lake Ontario: Phosphorus loadings decreased from about 15,000 metric t/y in 1972 to
the target of 7,000 metric t/y in 1981. Since that time, annual loadings have fluctuated
near the target, but were below targeted limits only during the years 1983,1988 and 1989.
Prior to 1983, loadings from the Niagara River were comparable to those from all other
tributaries combined. Tributary loadings have declined such that Niagara River loadings
now dominate.
Lakewide concentrations of total phosphorus have decreased significantly over the past
20 years. Levels exceeded 20 ug/L during 1971-1977, but have declined to be at or
below the guideline of 10 ug/L since 1986. During 1991 and 1992, mid-lake spring total
phosphorus concentrations were actually below 10 ug/L. In 1991/92, values above the
recommended guideline were only found in very confined regions along the shoreline, a
sharp contrast to conditions observed in 1980 (Figures 4a, 4b).
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3.0 System Response
3.1 Soluble Reactive Phosphorus and Algal Growth
Soluble reactive phosphorus (SRP) represents that fraction of the total phosphorus which is
directly available to the primary producers (plants and algae). Generally, for all of the Great
Lakes, spring SRP trends followed those of total phosphorus concentrations. In the Upper Lakes
(Superior, Huron and Michigan), concentrations of SRP have fluctuated mildly, never exceeding
2 ug/L. Larger variations were observed in both the Central and Eastern Basins of Lake Erie;
however, concentrations have remained below 5 ug/L since 1974. The greatest decline in open
lake concentrations occurred in Lake Ontario, where SRP concentrations as high as 15 ug/L in
1974 have now been reduced to 3 ug/L.
The decline in phosphorus concentrations, especially SRP, has resulted in noticeable changes,
both nearshore and offshore. Nearshore effects are observable in the reduction in Cladophora
growth. During periods of excessive nutrient enrichment, large odoriferous masses of decaying
Cladophora created problems along the shoreline of the Lower Lakes. Between 1972 and 1983,
however, the amount of Cladophera in Lake Ontario decreased by 58% (Painter and Kamaitis
1987).
Cladophora growth rates as a function of light, temperature, and phosphorus have been modelled
by Auer and Canale (1982), and Painter and Jackson (1989). Prior to phosphorus management
efforts in Lake Ontario, Cladophora growth rates would have exceeded 80% of maximum,
whereas present growth rates are predicted to be about 60% of maximum. Further phosphorus
controls in Lake Ontario will be necessary to eliminate nearshore Cladophora problems. In Lake
Erie, where SRP concentrations declined from approximately 10 ug/L to about 1-2 ug/L,
Cladophora growth potential has been reduced from about 80% of maximum to 40%.
Cladophora growth in the Upper Lakes is a localized problem responding to local inputs. SRP
concentrations in the Upper Lakes range from 0.6 to 1.4 ug/L and little change is apparent over
time. Depending on temperature and SRP concentration, the Cladophora growth rate can range
from 20 to 40% of maximum. SRP will have to be monitored carefully, especially in areas such
as Georgian Bay, to ensure that a Cladophora problem does not arise. Jackson and Hamdy
(1982) have suggested that a 1 ug/L increase in total phosphorus could result in nuisance growths
in the Thirty Thousand Islands area of Georgian Bay.
Green plants and algae contain chlorophyll, a pigment that is easily measurable and thus can be
used to estimate the quantity of algae in the water. The chlorophyll data reflect the offshore
responses to the reductions in phosphorus loadings and spring concentrations. It is measured in
the summer, at the peak of the primary production of a lake. Rast and Lee (1978) have
suggested that chlorophyll a concentrations below 2.0 ug/L are indicative of oligotropbic
conditions. Dobson et al (1974), however, have suggested an oligotrophy/mesotrophy threshold
Nutrient Trends and Response - SOLEC working paper 9
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of 4.4 ug/L. Using the latter criteria, all of the Great Lakes exhibited oligotrophic conditions
during the period 1980-1992 (see Figure 5). Using Rast and Lee's criteria, the Upper Lakes have
been oligotrophic at least since 1980 (the 1989 value for Lake Michigan notwithstanding);
however, reductions in chlorophyll concentrations in lakes Erie and Ontario indicate a trend from
mesotrophy toward oligotrophy over the period 1980-1990. Chlorophyll changes in the offshore,
combined with the nearshore Cladophora trends, indicate that the GLWQA goal of "reduction
in the present level of algal biomass to a level below that of a nuisance condition" has been
achieved.
Eutrophication and/or undesirable algae, however, continued to present problems in 18 of the 43
areas in the Great Lakes identified by the LTC as having the worst problems (see Figure 1).
Remedial action plans are being developed individually for each of these "Areas of Concern",
to address these problems.
3.2 Lake Erie Dissolved Oxygen Depletion
Figure 6 illustrates that dissolved oxygen concentrations in the bottom waters (hypolimnion) of
the Central Basin of Lake Erie have continued to decline during the summer season throughout
the period 1987 to 1991 (Bertram 1993). Charlton et al (1993) observed a similar pattern as far
back as 1979. Episodes of anoxia in the late summer continue to exist in some areas of the
Central Basin. At fall overturn, oxygenated waters again extend from surface to bottom.
However, bottom dwelling invertebrates, such as the mayfly (Hexagenia limbata), are sensitive
to low oxygen concentrations, and even short periods of anoxia quickly kill the organisms. Prior
to 1953, mayflies were the most abundant species in the benthic community of the Western Basin
(Reynoldson and Hamilton 1993). However, two particularly long warm calm spells in both 1953
and 1955 produced anoxic conditions in the Western Basin, and mayflies have been essentially
absent since.
In 1989, the rate at which dissolved oxygen was depleted throughout the summer (corrected for
hypolimnion temperature and thickness, vertical mixing and seasonal effects) was the lowest
measured for 20 years. This would suggest that, under some weather conditions, the hypolimnion
may be capable of sustaining aerobic waters for the entire season. However, the depletion rates
for 1990 through 1992 were more typical of the rates calculated for the late 1970s and early
1980s. This is not unexpected, given the lake morphometry and variability in the weather. In
general, reduced dissolved oxygen depletion rates seem to be associated with lower spring total
phosphorus levels (Bertram 1993), suggesting that phosphorus loading reduction strategies are
producing the desired effect in Lake Erie. Some lapse of time between achievement of
phosphorus loading targets and the maintenance of aerobic conditions in the Central Basin was
predicted at the time that the loading targets were determined (DiToro and Connolly 1980).
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3.3 Nitrate-plus-nitrite
Nitrate-plus-nitrite is also an important nutrient in water systems. Major sources of nitrogen to
the lakes are agricultural runoff, municipal sewage treatment plants and atmospheric deposition.
Increasing levels of nitrate-plus-nitrite have been reported in the Great Lakes for the past two
decades (Stevens and Neilson 1987; Williams 1992), particularly in Lake Ontario (Figure 7).
Current open lake concentrations do not create a public health concern, as they are at least 20
times lower than the guideline for protection of drinking water (10 mg/L).
Atmospheric deposition is suspected of being the major cause of nitrogen increases in the Upper
Lakes because of their large surface-area-to-drainage-basin ratio, lower population densities and
relatively fewer municipal and industrial dischargers. It has been estimated that 58% of the total
nitrogen load to Lake Superior is due to precipitation (Hartig and Gannon 1986; Bennett, 1986).
In Lake Erie, increased use of chemical fertilizers and gaseous emission of nitrogen compounds
within the drainage basin are believed to be the major causes. Nitrogen fertilizer sales in the
Lake Erie basin increased by roughly 50% between 1974 and 1980, continuing an increasing
trend which began at least as early as 1970 (Richards and Baker, 1993).
Changes in the ratio of nitrogen to phosphorus (N:P ratio) can affect algal species composition.
Under phosphorus-rich conditions, when nitrogen may be limited, blue-green algae have a
competitive advantage because they can utilize ("fix") nitrogen directly, whereas other types of
algae cannot Blue-green algae composed much of the "nuisance algae" referred to in the
GLWQA. Smith (1983) has observed that, when the N:P ratio exceeds 29, there is a shift in
dominance from blue-green to green algae and diatoms. Hartig etal (1991) have postulated that
it is likely that until about 1982-83 (when the N:P ratio crossed the 29:1 threshold), Lake
Ontario's summer phytoplankton biomass was actually limited by nitrogen. This would explain
why chlorophyll a levels only began to respond to further phosphorus reductions after this time.
The combination of reductions in phosphorus concentrations and increases in nitrogen
concentrations have served to not only reduce the total quantity of algae in the water (Le.,
reduced chlorophyll and Cladophora levels), but also to shift the species composition away from
nuisance blue-greens and toward more desirable, and historically prevalent, diatoms. This shift
will likely cause a change in zooplankton species and density. Trends in increasing nitrogen
compounds in the Great Lakes may warrant continued monitoring, but they do not appear to be
cause for alarm at this time.
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4.0 Where Do We Go From Here?
Phosphorus controls appear to have been successful in lowering the loadings into the lakes, and
consequently reducing, to varying extents, the resultant open lake concentrations of total and
soluble reactive phosphorus. The desired response in algal biomass, as indicated by chlorophyll
a concentrations, was finally observed in Lake Ontario in 1985 when a 50% decrease occurred.
However, the goal of establishing year-round aerobic conditions in the hypolimnion of Lake
Erie's Central Basin has not been realized. Vollenweider and Janus (1981) determined that lake
phosphorus loads would have to be reduced to about 5000 metric t/y for the desired effect on
oxygen and that, consequently, the goal of year-round aerobic conditions in Lake Erie should be
reconsidered. There is also evidence to suggest that there were brief periods of anoxia in some
areas of the Central Basin of Lake Erie for hundreds of years, prior to European colonization and
the onset of cultural eutrophication (Charlton 1980; Delorme 1982; Reynoldson and Hamilton
1993). Perhaps intermittent anoxia is an inherent property of the basin, and management to
achieve a state where anoxia does not occur is not a realistic goal for lake managers.
What impact have nutrient controls had on the food web? Zooplankton are the pivotal trophic
level in lake ecosystems, responding to both salmonid stocking (top-down) and phosphorus
abatement (bottom-up) management strategies. Johannsson et al (1991) compared nearshore and
offshore zooplankton abundance and composition in Lake Ontario between 1981 and 1988 and
reported that, although composition had not changed systematically or substantially though the
1980's, zooplankton abundance had declined as of 1983-84, particularly in the eastern basin.
They attributed these changes to a combination of stable fish predation and reduced food supply.
In Lake Michigan, Evans (1986) reported that zooplankton standing stocks in the nearshore
region declined 10-fold during 1982-84, to 3% of their 1975-81 average level. As a similar
change was not observed in the offshore waters, predation by yellow perch (which confine
themselves to the nearshore regions) was suggested as the likely cause.
Having achieved the desired reductions in phosphorus, which then lead to reduced zooplankton
production, begs the obvious question: given the extensive fish stocking programs, especially in
lakes Ontario and Erie, is there sufficient food base to sustain these fish populations?
Zooplankton comprise almost the entire diet of alewife, and are a significant component of the
diet of smelt. The reduction in abundance of zooplankton, therefore, could only result in lower
production of alewife and smelt The Great Lakes Fishery Commission report that alewife and
smelt biomass indices have declined since the early 1980* s (GLFC 1992). At the same time, the
numbers of hatchery-reared salmon and trout stocked in Lake Ontario steadily increased from just
over 1 million in 1972 to approximately 8.2 million in 1984. As the stocked fish continued to
grow and accumulate, the total weight of salmon and trout reached a peak in 1986 and, since
then, has remained high. Alewife and smelt populations in Lake Ontario are under stress from
both ends of the food chain. How much stocking is too much?
The issue of restoring the balance between stocked predators and prey was the subject of
Nutrient Trends and Response - SOLEC working paper 13
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discussion at a series of public meetings held in 1992 by the agencies responsible for the fish
stocking program in Lake Ontario: the Ontario Ministry of Natural Resources (OMNR) and New
York State Department of Environmental Conservation (DEC). There emerged consensus that
alewives be maintained as the dominant forage species, so that a diverse fishery could be
maintained. A scientific task force, established by the Lake Ontario Committee of the Great
Lakes Fisheries Committee, recommended reduction of predator numbers to stabilize the
predator/prey balance. In response, both OMNR and New York DEC agreed to revise their Lake
Ontario fish stocking plans, using a two-year phased-in approach, beginning in 1993. For 1993,
the two agencies set a target for predator demand which is 35% lower than 1991 levels. In 1994,
additional stocking reductions are expected to bring about a total reduction of 47% in predator
demand from the 1991 level. OMNR and DEC set targets to stock 5.1 million fish in Lake
Ontario in 1993, and 4.5 million in 1994. To further reduce the demand for food by predators
already in the system, OMNR is also encouraging increased harvesting of salmon and trout
Having seen changes in Lake Michigan's food web now starting to appear in Lake Ontario (eg.,
changes in zooplankton species and standing stock; decline in alewife abundance and a
resurgence of yellow perch), the participants at the International Joint Commission's Food Web
n Workshop, which focussed on Lake Ontario (Hartig et al 1991), recommended that water
quality and fisheries agencies: (1) standardize monitoring techniques and establish and maintain
compatible, long-term, limnological data sets, (2) cooperate on research (eg. controlled,
mesoscale, whole-system experiments) designed to quantify the rates (eg. growth, predation, etc.)
of food web interactions (emphasis must be placed on an interdisciplinary approach that explicitly
accounts for time and spatial scale effects), and (3) promote initiatives which quantify the impact
of changes in food web dynamics on reduction of toxic substances levels in Great Lakes fishes.
In order to understand the effects of nutrient and food web controls in lakes Michigan, Ontario
and Erie, research should be focussed on quantifying fluxes of energy, and collecting more
information on feeding habits and rates for important species so that food web models can be
constructed. Of course, this will be complicated by the recent invasion by zebra mussels (and,
in Lake Erie, quagga mussels), and their associated impacts on water quality and the food web.
Zebra mussels filter-feed all particles, including large chain-forming diatoms, and even some
relatively large zooplankton organisms (Ten Winkle and Davids, 1982: Maclsaac et al, 1991).
Recent studies in Lake Erie indicate that zebra mussels have caused reductions in phytoplankton
biomass (Nicholls and Hopkins 1993; Hebert et al 1989; Griffiths et al 1991; Leach 1993), while,
at the same time enhancing water clarity in the shallow waters, where they are found in greatest
numbers (Charlton 1994). This clearing effect has direct implications for walleye, which prefer
to dwell in murkier waters. Loss of fish habitat, production and edibility are potential issues of
concern related to these invaders.
In conclusion, fisheries and water quality management strategies have evolved independently in
the Great Lakes. In general, these strategies operate from different ends of the management
spectrum — bottom-up (phosphorus control) and top-down (massive fish stocking). A more
ecologically oriented approach, which recognizes the interactions between fisheries and water
quality, will be required to manage the Great Lakes.
14
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5.0 References
Auer, M.T. and R.P. Canale. 1982. Ecological studies and mathematical modeling of Cladovhora in Lake Huron:
3.. The dependence of growth rates on internal phosphorus pool size. J. Great Lakes Res. 8(1): 93-99.
Bennett, E.B. 1986. The nitrifying of Lake Superior. Ambio. 15(5): 272-275.
Bertram, P.E. 1993. Total phosphorus and dissolved oxygen trends in the Central
Basin of Lake Erie. 1970-1991. J. Great Lakes Res. 19(2): 224-236.
Charlton, M.N. 1980. Oxygen depletion in Lake Erie: has there been any change?
Can. J. Fish. AquaL Sci. 37: 72-81.
Charlton, MJS[., J.E. Milne, W.G. Booth, and F. Chiocchio. 1993. Lake Erie offshore
in 1990: restoration and resilience in the Central Basin. J. Great Lakes Res. 19(2): 291-309.
Charlton, M.N.. 1994. The case for research on the effects of zebra mussels in Lake Erie: Summary of information
from August and September 1993. Environment Canada, Lakes Research Branch, NWRI Contribution No. 94-02.
Delorme, LJD. 1982. Lake Erie oxygen: the prehistoric record. Can J. Fish. AquaL Sci. 39: 1021-1029.
DiToro, D.M and J.P. Connolly. 1980. Mathematical models of water quality in large
lakes. Part 2: Lake Erie. Report EPA-600/3-30-065, U.S. Environmental Protection Agency, Office of Research and
Development, Duluth, MN.
Dobson, H.F.H., M. Gilbertson and P.G. Sly. 1974. A summary and comparison of
nutrients and related water Quality in lakes Erie. Ontario. Huron and Superior. J. Fish. Res. Board Can. 31:731-738.
Dolan, D. 1993. Point source loadings of phosphorus to Lake Erie: 1986-1990. J.
Great Lakes Res. 19(2): 212-223.
Evans, M.S. 1986. Recent major declines in zooplankton populations in the inshore
region of Lake Michigan: probable causes and implications. Can. I. Fish. AquaL Sc. 43: 154-159.
Great Lakes Fisheries Commission. 1992. Signs of Change in the Lake Ontario
Ecosystem. Prepared by the Lake Ontario Committee.
Great Lakes Science Advisory Board. 1980. Report of the Aquatic Ecosystem
Objectives Committee. International Joint Commission, Windsor, Ontario. 127 pp.
Griffiths, R.W., D.W. Schloesser, J.H. Leach, and W.P. Kovalak. 1991. Distribution and dispersal of the zebra mussel
(Dreissena polvmorpha) in the Great Lakes region. Can. J. Fish. AquaL Sci. 48: 1381-1388.
Hartig, J.H. and JE. Gannon. 1986. Opposing phosphorus and nitrogen trends in the
Great Lakes. Alternatives (13): 19-26.
Hartig, J.H., J J. Kitchell, D. Scavia, and S.B. Brandt. 1991. Rehabilitation of Lake
Ontario: the role of nutrient reduction and food web dynamics. Can J. Fish. AquaL Sci. 48: 1574-1580.
Nutrient Trends and Response - SOLEC working paper 15
-------
Hebert, PD.N., B.W. Muncaster, and GJL Mackie. 1989. Ecological and genetic studies on Dreissena polymorpha
(Pallas): a new mollusc in the Great Lakes. Can. J. Fish. AquaL Sci. 46:1587-1591.
International Joint Commission. 1969. International Lake Erie Water Pollution Board
and the International Lake Ontario-SL Lawrence River Water Pollution Board. Pollution of Lake Ontario and the
international section of the St Lawrence River.
Jackson, M.B. and Y.S. Hamdy. 1982. Projected Cladophora growth in southern Georgian Bay in response to
proposed municipal sewage treatment plant discharges to the Mary Ward Shoals. J. Great Lakes Res. 8(1): 153-163.
Johannsson, OJE., EX. Mills and R. O'Gorman. 1991. Changes in the nearshore and
offshore zooplankton communities in Lake Ontario: 1981-1988. Can. J. Fish. AquaL Sci. 48: 1546-1557.
Leach, J.H.. 1993. Impacts of the zebra mussel (Dreissena polvmoryha) on water quality and fish spawning reefs
in western Lake Erie. In Zebra Mussels: Biology, Impact and Control, ed. T.F. Nalepa and D.W. Schloesser, pp.
381-397. Lewis Publishers Inc., Ann Arbor.
Maclsaac, H.J., W. G. Sprules, and J.H. Leach. 1991. Ingestion of small-bodied zooplankton by zebra mussels
(Dreissena polymorpha): can cannibalism on larvae influence population dynamics? Can. J. Fish. AquaL Sci. 48:
2051-2060.
Nicholls, K.H., and G.J. Hopkins. 1993. Recent changes in Lake Erie (north shore) phvtoplankton: cumulative
impacts of phosphorus loading reductions and the zebra mussel introduction. Joum. Great Lakes Res., 19(4): 637-647.
Painter, D.S. and G. Kamaitis. 1987. Reduction of Cladophora biomass and tissue
phosphorus in Lake Ontario. 1972-1983. Can. J. Fish. AquaL Sci. 44: 2212-2215.
Painter, D.S. and M.B. Jackson. 1989. Cladophora internal phosphorus modeling: Verification. J. Great Lakes Res.
15(4): 700-708.
Phosphorus Management Strategies Task Force. 1980. Phosphorus Management for
the Great Lakes. Final Report. International Joint Commission, Windsor, Ontario. 129 pp.
Rast, W. and GP. Lee. 1978. Summary analysis of the North American OCED
Eutrophication Project: nutrients, loading-lake response relationships and trophic site indices. Report EPA-600/3-78-
008, U.S. Environmental Protection Agency, Duluth, MR
Reynoldson, T.B. and AX. Hamilton. 1993. Historic changes in populations of
burrowing mayflies (Hexagenia limbata) from Lake Erie based on sediment tusk profiles. J. Great Lakes Res. 19(2):
250-257.
Richards, R.P. and D.B. Baker. 1993. Trends in nutrient and suspended sediment
concentrations in Lake Erie tributaries. 1975-1990. J. Great Lakes Res. 19(2): 200-211.
Smith, V.H. 1983. Low nitrogen to phosphorus ratios favor dominance by blue-green
algae in lake phvtoplankton. Science 221: 669-671.
Stevens, RJJ. and M.A. Neilson. 1987. Response of Lake Ontario to reductions in
phosphorus load. 1967-82. Can. J. Fish, and Aquatic Sci. 44(12): 2059-2068.
Ten Winkel, E.H., and C. Davids. 1982. Food selection by Dreissena polvmoroha Pallas (Mollusca: Bivalvia).
16
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FreshwaL Biol. 12:553-558.
Vallentyne, J.R. and N.A. Thomas, co-chairs. 1978. Fifth year review of Canada
-United States Great Lakes Water Quality Agreement. Report of Task Group ffl, A Technical Group to Review
Phosphorus Loadings to the Parties of the Great Lakes Water Quality Agreement of 1972. Printed by the
International Joint Commission, Windsor, Ontario. 84pp.
Vollenweider, R.A. and Li. Janus. 1981. The OECD cooperative program in
eutrophication: Canadian contribution. Scientific Series #131, National Water Research Institute, Inland Waters
Directorate, Environment Canada, Burlington, Ontario, Canada.
Williams, D J. 1992. Great Lakes water quality, a case study. In: Dunnette and
O'Brien [Eds.] The Science of Global Change: The Impact of Human Activities on the Environment American
Chemical Society Symposium Series 483, Washington, pp. 207-223.
Nutrient Trends and Response - SOLEC working paper 17
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LIST OF FIGURES
Figure 1. Areas of Concern with eutrophication- or undesirable algae-related impairments.
Figure 2. Total phosphorus loadings to the Great Lakes (metric tonnes/year).
Figure 3. Spring mean total phosphorus trends for open lake, 1971 - 1992.
Figure 4a. 1980/1983 Spring total phosphorus concentrations.
Figure 4b. 1991/1992 Spring total phosphorus concentrations.
Figure 5. Trends in mean summer chlorophyll a, 1974 - 1992.
Figure 6. Hypolimnetic oxygen concentrations (mean+standard deviation) in the Central
Basin of Lake Erie, 1987 through 1991. [Bertram, 1993]
Figure 7. Spring mean nitrate-plus-nitrite trends for open lake, 1968 - 1992.
18
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,7
Fox River and Green Bay
Milwaukee Estuary
Sheboygan * £ V / / J Saginaw River £ ^f
&*"*^^:\ and Bay *
' ', '.V
Y?*A
l, Severn Sound
Collingwood Harbour
BayofQuintc^^
Port Hope ^ i
Toronto ^-"~,
*
St. Lawrence
Grand Calumet River '
and Indiana Harbor Canal
} I Hamiltoiy
^ Harbour*
j0 Oswego River
Rouge Riverf
Maumee-RiverW^.
"
Cuyahoga River
Black River
Data Source: Internation Joint Commission, 1991.
Figure 1. Areas of Concern with eutrophication - or undesirable algae-related impairments
-------
0 -"'*""l"l"
187$ 1978 1980 1982 18B4 1986 1988 1890
Year
1976 1978 1980 1982 1884 1986 1988 1990
1976 1978 1980 1982 1984 1986 1988 1990
Y»ar
1976 1978 1980 < 1982 1984 1986 1988 1990
Your
1976 1978 1980 1982 1984 1986 1988 1990
Year
Data Source: Great Lakes Water Quality Board Report to the International Joint Commission, (1977 -1987)
1990 Loadings, personal communications (Dave Dolan, IJC-RO).
Legend
Connecting Channels
HI Municipal Loadings
| [Atmospheric and
Industrial Loadings
• Tributary Loadings
•^—Proposed TP guideline
(Phosphorus Management
Strategies Task
Force, 198O)
Figure 2. Total phosphorus loadings to the Great Lakes (metric tonnes/ year).
-------
Proposed TP guideline
(Phosphorus Management
Strategies Task
Force;i980)
30.0-
25.0-
20.0-
fl,0
10.0
"
JMIMHI
0.0-4-
19711973197519771979196119831985198719891991
Year
30.0 -i
25.0-
JO.O
^15.0-
100
5.0
0,0
30.0-
25.0-
200
15.0-
1(10-
5.0-
0.0
300
li iliiiiiiin
19711973197519771979198119831885198719891991
Year
1971,15731975197? 1973198119831985198719881991
19711873197519771979198119831985198719891981
Year
•19711973187519771979196119831985198719891991
Yaar
Eastern Basin
19711973197519771979198119831985198719891991
Year
Central Basin
Data Source: Environmental Conservation Branch, Environment Canada
Great Lakes National Program Office, US EPA
Figure 3. Spring mean total phosphorus trends for open lake, 1971 - 1992.
-------
Legend
[] <0.005 mg/L
o.oos - o.oo?
0.007-0.010
0.010-0.012
0.012-0.015
> 0.015 mg/L
Flno data available
guideline = 0.005 mg/L
guideline = 0.005 mg/L
guideline =t>.010 mg/L
/
Data Source Environmental Conservation Branch, Environment Canada. Lakes Superior, Huron and Ontario
Greal Lakes National Program Office, US EPA Ukej Michigan and Erie
Figure 4a. 1980/1983 Spring total phosphorus concentrations.
-------
guideline = 0.005 mg/L
guideline = 0.010 mg/L
'
Legend
|J <0.005 mg/L
I ] 0.005 - 0.007
• 0.007 - 0.010
| 0.010-0.012
m 0.012-0.015
I > 0.015 mg/L
Flno data available
Dili Source Environment*] Conicrvition Branch, Environment Canada l-akc-i Supenor. Huron and Ontano
Or«l Ltkei Nation»l Progrmm Office, US EPA: Lakes Michigan and Ene
Wisconsin LVpartment of Natural Resources Orecn Bay
Figure 4b. 1991 /1992 Spring total phosphorus concentrations.
-------
70
60
50
10
1.0
0.0
III
1874 1976 1978 1880 1982 1964 19M 1968 1MO 1992
Veir
7.0
8.0
50
20
1.0
0.0
.hliil.ll
1974 1978 1978 1980 1982 1964 1968 1968 1990 1962
Vur
7.0
60
50
j4.0
3 3.0
iO-
1.0
0.0
1974 1978 1978 1160 1M2 1864 1966 1968 1990 1982
YMT
7.0 T
70
60
5.0
4.0
j
a
330
2.0
1.0
0.0
1974 1978 1878 1960 1982 1984 1986 1988 1990 1992
VMI
1974 1978 1978 I960 1962 1984 1966 1969 1990 1992
Eastvn Basin
1974 1978 1971 1980 1982 1984 1968 1988 1990 1992
YMI
CeotrU Buln
Data Source: Environmental Conservation Branch, Environment Canada
Great Lakes National Program Office, US EPA
Figure 5. Trends in mean summer chlorophyll_a, 1974 - 1992.
-------
o
DD
>
X
O
0
>
O
(0
CO
1Z
10
8
6
4
2
0
. • 1987 * 1988 A 1989 * 1990 • 1991
•T i • T
1 !l ,
i r T T
fj fl * I
>
: IM it a
I iJ AT
I. .Iff. . J- . .
150 175 200 225 250 275
I II IMF 1 IIIIY 1 AHfS 1 «!FPT 1
Julian Day
Figure 6. Hypolimnetic dissolved oxygen concentrations (mean i S.D.) in the central basin
of Lake Erie, 1987 through 1991. [Bertram, 1993]
-------
CIO
ooo
1974 1977 I960 1983 19M 1969 1992
v*M
O.M
040
OH)
04.1
a*)
aio
000
974 1877 1900 1963 19M 19M 1B92 „
000
1966 1971 1974 1977 1960 1983 1986
YMI
EutMnBuki
Data Source: Environmental Conservation Branch, Environment Canada
Great Lakes National Program Office, US EPA
1974 1977 I960 19M 1986 1989 1992
VMI
C«ntf*J Basin
Figure 7. Spring mean nitrate-plus-nitrite trends for open lake, 1968 - 1992.
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