USA
S.O.iL.E.C
1994 State of the Lakes Ecosystem
Conference
Background Paper
Nutrients:
Trends and System Response
August 1995
Environment Canada
United States Environmental Protection Agency
EPA 905-R-95-015
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State of the Lakes Ecosystem Conference
Background Paper
NUTRIENTS: TRENDS AND SYSTEM
RESPONSE
Melanie Neilson
Serge L'ltalien
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
August 1995
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Table of Contents
Acknowledgments iv
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 16
List of Figures 20
Figures 21
Nutrient Trends and Response - SOLEC Background Paper
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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 comments
of many reviewers both before and after the State of the Lakes Ecosystem Conference, October
26-28, 1994.
NOTICE TO READER
These Background 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 Background Papers were first released as Working Papers to provide the basis for
discussions at the first State of the Lakes Ecosystem Conference (SOLEC) in October, 1994.
Information provided by SOLEC discussants was incorporated into the these final SOLEC
background papers. SOLEC was intended to provide key information required by managers
to make better environmental decisions.
IV
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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 Background 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 (hypolirnnion). Decomposing filamentous algae (Cladophord) also piled up on
some beaches. Taste and odor problems appeared in drinking water due to blue-green algae.
In the late 1960's, a review of the state of the Lower Lakes by the International Joint
Commission (IJC 1969), 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 hypolirnnion 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
mYd 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, costly 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
the engineering designs that otherwise might not have been included.
Nutrient Trends and Response - SOLEC Background Paper 3
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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 IJC recommended that the phosphorus concentration
guidelines be based on spring open-lake concentrations, since these largely influence summer
phytoplankton biomass (Great Lakes 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
In recognition that phosphorus loading targets had not yet been attained in lakes Erie and Ontario,
a Phosphorus Load Reduction Supplement was added to the Agreement in 1983 which identified
loading reductions of 2,000 and 430 metric tonnes/yr for lakes Erie and Ontario, respectively, still
to be achieved. The allocation of reductions to meet target loads for Lake Erie were further
defined as 300 metric t/yr from Canadian sources, and 1700 metric t/yr from U.S. sources. The
U.S further apportioned load reduction goals by state. At this point, the loadings were expected
to be achieved mainly through non-point source (agricultural) programs.
This paper discusses the progress in controlling phosphorus loads to the lakes and the resulting
responses of each of the Great Lakes. Emphasis is placed on concentrations and long-term trends
of phosphorus, nitrogen and chlorophyll a in the offshore waters and on the hypolimnetic oxygen
depletion in Lake Erie. Some nearshore Areas of Concern are impacted by problems related to
excessive nutrients (Figure 1), but a detailed discussion of these areas is beyond the scope of this
paper.
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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.
Non-point source programs were intended to assist farmers through combined incentive programs,
education and research, and they included conservation farming 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. In the U.S., the non-point source programs have
involved a number of approaches and jurisdictions, including numerous federal grants for
projects. Participation by farmers and other landowners is still voluntary, but many successful
projects have fostered continuing interest in improved agricultural practices. These programs in
the states bordering Lake Erie have reduced phosphorus loadings by approximately 1100 metric
t/yr (of the 1700 metric t/yr targeted reductions). For Lake Ontario, estimated reductions of 404
metric t/yr exceed the 1983 goal of 235 metric t/yr. The Canadian federal and provincial
governments conducted the Soil and Water Environmental Enhancement Program (SWEEP) for
Lake Erie from 1985 through 1993. Results from the SWEEP program indicate that Canada has
met or exceeded its agricultural non-point source phosphorus loading reduction targets for Lake
Erie.
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 (lakes 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.
Nutrient Trends and Response - SOLEC Background Paper
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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 accurate 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 ng/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
loads to Lake Erie are directly related to the amount of precipitation falling in the basin
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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 guideline 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 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).
Nutrient Trends and Response - SOLEC Background Paper
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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 jag/L (Figure 5). 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
(jg/L in 1974 have now been reduced to 3 (ag/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 Cladophora in Lake Ontario decreased by 58% (Painter and Kamaitis
1987).
Cladophora growth rates have been modelled as a function of light, temperature, and phosphorus
by Auer and Canale (1982), and Painter and Jackson (1989). Based on model projections,
estimated SRP concentrations in the nearshore areas of Lake Ontario and Lake Erie are sufficient
to sustain Cladophora growth. Cladophora growth in the Upper Lakes is a localized problem
responding to local inputs. SRP will have to be monitored carefully 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. Using the most restrictive of the
many proposed trophic status indicators in the literature (Forsberg and Ryding 1980), Rast and
Lee (1978) have suggested that chlorophyll a concentrations below 2.0 ug/L are indicative of
oligotrophic conditions. Using this criteria, the Upper Lakes have been oligotrophic at least since
1980 (the 1989 value for Lake Michigan notwithstanding, Figure 6). This is consistent with the
goals of phosphorus reduction programs, as outlined in the GLWQA. Reductions in chlorophyll
concentrations in the offshore waters of 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.
Nutrient Trends and Response - SOLEC Background Paper 9
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Eutrophication and/or undesirable algae, however, continued to present problems in 18 of the 43
areas in the Great Lakes identified by the IJC as having the worst problems (see Figure 1).
Remedial action plans to address these problems are being developed individually for each of
these "Areas of Concern".
3.2 Lake Erie Dissolved Oxygen Depletion
Figure 7 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 limbatd), 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).
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.
The contribution of nitrogen and phosphorus from septic systems is unknown, but may be
significant in some areas. 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 8). 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).
10
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However, nitrate may be a predisposing factor in several diseases in fish (Colt and Armstrong
1981).
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. When the N:P ratio exceeds 29, there is a shift in dominance from blue-green to green
algae and diatoms (Smith 1983). Hartig et .al (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 (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. 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.
Nutrient Trends and Response - SOLEC Background Paper
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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, stressed by both salmonid stocking (top-down) and phosphorus
abatement (bottom-up) management strategies. Johannsson et al (1991) compared nearshore and
offshore zooplankton in Lake Ontario between 1981 and 1988 and reported that zooplankton
abundance had declined as of 1983-84, particularly in the eastern basin, but the species
composition did not change appreciably. They suggested these changes could have resulted from
a reduced phytoplankton food supply, as a result of lower phosphorus concentrations, combined
with continued levels of predation by alewife.
Changes in phytoplankton abundance and species in Lake Erie from 1970 through the mid-1980s
were also consistent with the expected impacts of reduced nutrient loadings (Makarewicz and
Bertram 1991). For example, the mean algal biomass during this period declined by 65% (from
3.4 g/m3 to 1.18 g/m3); the nuisance blue-green algae Aphanizomenon flos-aquae decreased 89%
(from 2 g/m3 to 0.22 g/m3); and the number of dominant eutrophic diatom species decreased in
the western basin, whereas the number of dominant mesotrophic species increased (from 1 to 4).
Not all changes in the lower food web are attributable to changes in phosphorus concentrations.
For example, zooplankton standing stocks in the nearshore region of Lake Michigan declined 10-
fold during 1982-84, although phosphorus concentrations in the offshore waters had not declined
appreciably (Evans 1986). Predation by yellow perch, which confine themselves to the nearshore
regions, was suggested as the likely cause. In Lake Erie, the recovery of the walleye fishery and
the introduction of a new salmonid fishery also have had a cascading effect on trophic structure.
As top-level predators increased in abundance, forage fish abundance decreased, perhaps
contributing to the establishment of the large predaceous spiny water flea by 1985, and allowing
larger zooplankton to dominate the community structure. Grazing pressure from these larger
zooplankton appears to have caused a further decrease in algal abundance (Makarewicz and
Nutrient Trends and Response - SOLEC Background Paper 13
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Bertram 1991).
Although the desired reductions in phosphorus have been achieved, which have lead to positive
changes in the plankton communities, the question remains, "What level of fishery can be
sustained by the resultant food base?" Zooplankton comprise almost the entire diet of alewife,
and are a significant component of the diet of smelt. A reduction in the abundance of
zooplankton, therefore, could only result in reduced populations of alewife and smelt. The Great
Lakes Fishery Commission reports 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 their prey was the subject of
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 (NYDEC). 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 Fishery Commission, recommended reduction of predator numbers to stabilize the
predator/prey balance. In response, both OMNR and NYDEC 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 was 35% lower than 1991 levels. In 1994,
additional stocking reductions were expected to bring about a total reduction of 47% in predator
demand from the 1991 level. OMNR and NYDEC 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 also encouraged 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; a decline in alewife abundance and a
resurgence of walleye, whitefish and yellow perch), the participants at the International Joint
Commission's Food Web II 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. Water quality and fisheries agencies are coming to recognize the
need to act on these recommendations.
In order to understand the effects of nutrient and food web controls in lakes Michigan, Ontario
14
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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.
Drinking water impairment (taste and odor problems), loss of fish habitat, and the production and
edibility of fish are all potential issues of concern related to zebra mussels.
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; Holland 1993;
Leach 1993), and they have enhanced water clarity in shallow waters, where they are found in
greatest numbers (Charlton 1994). The diversion of plankton from pelagic to benthic food
pathways by zebra mussels could also affect the biomagnification of toxic organic contaminants
through higher trophic levels (Bruner et al 1994), and could result in increased concentrations
of PCBs and other contaminants in desired sport fish. The combination of the clearing effect and
the potential for higher concentrations of contaminants in fish species has particular direct
implications for the walleye fishery in Lake Erie. There are currently a number of initiatives,
such as the binational Lake Erie Trophic Transfer project, that are beginning to address the
impact of zebra mussels on the Great Lakes ecosystem.
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 effectively manage the Great Lakes.
Nutrient Trends and Response - SOLEC Background Paper 15
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5.0 References
Auer, M.T. and R.P. Canale. 1982. Ecological studies and mathematical modeling of Cladophora
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.
Bruner, K.A., S.W. Fisher and P.P. Landrum. 1994. The role of zebra mussel, Dreissena
polymorpha, in contaminant cycling: II. Zebra mussel contaminant accumulation from algae and
suspended particles, and transfer to the benthic invertebrate, Gammarus fasciatus. J. Great Lakes
Res. 20(4):735-750.
Charlton, M.N. 1980. Oxygen depletion in Lake Erie: has there been any change?
Can. J. Fish. Aquat. Sci. 37: 72-81.
Charlton, M.N., I.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.
Colt, J.E. and D.A. Armstrong. 1981. Nitrogen toxicity to crustaceans, fish and mollusks.
Proceedings of the Bio-Engineering Symposium for Fish Culture, American Fisheries Society,
Fish Culture Section Publ. 1:34-47
Delorme, L.D. 1982. Lake Erie oxygen: the prehistoric record. Can J. Fish. Aquat. 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.
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. J. Fish. Aquat. Sc. 43: 154-
159.
16
-------
Forsberg, C. and S. Ryding. 1980. Eutrophication parameters^nd trophic state indices in 30
Swedish waste-receiving lakes. Arch. Hydrobiol. 89: 189-207.
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 polymorpha) in the Great Lakes region. Can. J, Fish. Aquat. Sci.
48: 1381-1388.
Hartig, J.H. and J.E. Gannon. 1986. Opposing phosphorus and nitrogen trends in the
Great Lakes. Alternatives (13): 19-26.
Hartig, J.H., J.F. Kitchell, D. Scavia, and S.B. Brandt. 1991. Rehabilitation of Lake
Ontario: the role of nutrient reduction and food web dynamics. Can J. Fish. Aquat. Sci. 48: 1574-
1580.
Hebert, P.D.N., B.W. Muncaster, and G.L. Mackie. 1989. Ecological and genetic studies on
Dreissena polymorpha (Pallas): a new mollusc in the Great Lakes. Can. J. Fish. Aquat. Sci.
46:1587-1591.
Holland, R.E. 1993. Changes in planktonic diatoms and water transparency in Hatchery Bay. Bass
Island area, Western lake Erie since the establishment of the zebra mussel. J. Great Lakes Res.
19(3):617-624.
International Joint Commission. 1969. International Lake Erie Water Pollution Board
and the International Lake Ontario-St. 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, O.E., E.L. Mills and R. O'Gorman. 1991. Changes in the nearshore and
offshore zooplankton communities in Lake Ontario: 1981-1988. Can. J. Fish. Aquat. Sci. 48:
1546-1557.
Leach, J.H.. 1993. Impacts of the zebra mussel (Dreissena polymorpha) 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.
Nutrient Trends and Response - SOLEC Background Paper 17
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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. Aquat. Sci. 48: 2051-2060.
Makarewicz, J.C. and P. Bertram. 1991. Evidence for the restoration of the Lake Erie ecosystem:
water quality, oxygen levels and pelagic function appear to be improving. Bioscience 41(4): 216-
223.
Nicholls, K.H., and G.J. Hopkins. 1993. Recent changes in Lake Erie (north shore)
phytoplankton: cumulative impacts of phosphorus loading reductions and the zebra mussel
introduction. J. 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. Aquat. 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 G.F. 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, MN.
Reynoldson, T.B. and A.L. 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 phytoplankton. Science 221: 669-671.
Stevens, R.J.J. and M.A. Neilson. 1987. Response of Lake Ontario to reductions in
phosphorus load, 1967-82. Can. J. Fish, and Aquat. Sci. 44(12): 2059-2068.
Ten Winkel, E.H., and C. Davids. 1982. Food selection by Dreissena polymorpha Pallas
(Mollusca: Bivalvia). Freshwat. 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 III, A Technical
Group to Review Phosphorus Loadings to the Parties of the Great Lakes Water Quality
18
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Agreement of 1972. Printed by the International Joint Commission, Windsor, Ontario. 84pp.
Vollenweider, R.A. and L.L. 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 Background Paper 19
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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. Soluble Reactive Phosphorus levels in the Great Lakes, 1968-1992.
Figure 6. Trends in mean summer chlorophyll a, 1974 - 1992.
Figure 7. Hypolimnetic oxygen concentrations (meanj+standard deviation) in the Central
Basin of Lake Erie, 1987 through 1991. [Bertram, 1993]
Figure 8. Spring mean nitrate-plus-nitrite trends for open lake, 1968 - 1992.
20
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:Severn Sound
Collingwood Harbour
Fox River and (Sreen Say (
; Sheboygi
Milwaukee Estuary
Grand Calumet River
and Indiana Harbor Canal
Oswego River
Cuyahoga River
. Slack River
Data Source: Intemation Joint Commission, 1991.
Figure 1. Areas of Concern with eutrophication - or undesirable algae-related impairments
-------
,n
1984 1989 1918 1990
1984 1966 1966 191
•Mr
Data Source: Great Lakes Water Quality Board Report to the International Joint Commission, (1977 -1987)
1990 Loadings, personal communications (Dave Dolan, IJC-RO).
Legend
d]Connecting Channels
• 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).
-------
1071 1974 1977 19KJ 1«3 t9J» 1989 1992
-------
* V « ; *,-,.. *"
•r
*• -
«
* *« * * «
•* - » ^ *
Legend , *
•0.000 - 0.005 « I
*0.005 - 0.007 ^ ' "*.
0.007-0.010 * .
•0.010-0.012 ».*
•0.012-0.015 **
•0.015+ * **»
Figure 4a. 1980/1983 Spring total phosphorus concentrations.
-------
-t
* % V
f; )l»| ';,J;-|->;'; * *» " * 4
Legend jp jii^t'* ^ * #
»- ^ *
.
0.000 - 0.005 <>,*
0.005-0.007 = * * I •
•
0.007-0.010 J * -T"^-» *
• 0.010-0.012
.0.012-0.015
.0.015 +
Data Source: Environmental Conservation Branch, Environment Canada: Lakes Superior, Huron and Ontario.
Great Lakes National Program Office, US EPA: Lakes Michigan and Erie.
Wisconsin Department of Natural Resources: Green Bay.
Figure 4b. 1991 /1992 Spring total phosphorus concentrations.
-------
. . , , , , »_••»,»•••
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Data Sovce: Environmental Cmservalion. Environrnent Canada
Great Lakes National Program Office, USEPA
Figure 5. SRP levels in the Great Lakes, 1968 -1992.
-------
7.0 -i
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3-H — s — i — i — i—i— i — i — r-^^-^TT" i ^ i T-
YMT
EastffnBwn
Figure 6. Trends in mean summer chlorophyll_a, 1974 -1992.
Data Source: Environmental Conservation Branch, Environment Canada
Great Lakes National Program Office, US EPA
-------
DISSOLVED OXYGEN CONCENTRATIONS
LAKE ERIE CENTRAL BASIN
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27
JUNE
Figure 7.
JULY AUG SEPT
Julian Day
Hypolimnetic dissolved oxygen concentrations (mean + standard deviation) in the central basin of Lake Erie, 1987
through 1991. (Bertram, 1993)
-------
0.50
0.40-
0.30
I
OJO
0.10
0.00
1974 1977 1MO 1983 1988 1989 1992 1
1974 1977 1980 1983 1988 1969 1992
tar
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0.00
1>74 1977 1980 1883 1889 1989 1992 1974 1977 1980 1983 1988 1919 1992
tar *M
Oital Buin Elltem BMn
Figure 8. Spring mean nitrate-plus-nitrite trends for open lake, 1968 -1992.
Data Source: Environmental Conservation Branch, Environment Canada
Great Lakes National Program Office, US EPA
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