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Lake Erie Oxygen Depletion Controversy
J. Great Lakes Res. 8(4): 719-722 Internat. Assoc. Great Lakes Res., 1982
LAKE ERIE OXYGEN DEPLETION CONTROVERSY
J. Barica Aquatic Ecology Division National Water Research Institute Canada Centre for Inland Waters P. O. Box 5050 Burlington, Ontario, Canada L7R 4A6
ABSTRACT. A summary of a special workshop held at the Canada Centre for Inland Waters, Burlington, Ontario, 2-3 December 1981, is presented. The purpose of the workshop was to air some differences of opinion regarding the response of the hypolimnetic anoxia of Lake Erie to phosphorus controls. The workshop confirmed the validity of the 1978 U.S.-Canadian Great Lakes Water Quality Agreement's goals on restoring year-round aerobic conditions in the hypolimnion ofthe central basin of Lake Erie through phosphorus loading reduction. The author served as workshop chairman and convenor. ADDITIONAL INDEX WORDS: Hypolimnetic oxygen, phosphorus loading.
ture. Charlton suggested that eutrophicationrelated changes in the oxygen depletion rate were too small to be recognized and, hence, the Water Quality Agreement goal may not be realistic. Charlton's suggestions were recently challenged by F. Rosa and N. M. Burns (unpublished working draft) who reassessed the available historic data. They introduced four corrections (adjustments) to reduce the data for the different years to standard conditions. The National Water Research Institute (NWRI, Canadian Department of Environment) decided to air this controversy through a scientific debate. A special workshop was held at the Canada Centre for Inland Waters (CCIW), Burlington, Ontario, 2-3 December 1981, with representation from both Canadian and U.S. organizations, (12 speakers and over 30 observers and participants). Prior to the workshop, both Charlton's papers and Rosa and Burns' latest document with new calculations were sent for review to the participants. The workshop was organized in three sessions:
One of the specific goals of the Canada/ U.S. Great Lakes Water Quality Agreement of 1978 is restoration of year-round aerobic conditions in the bottom waters of the central basin of Lake Erie through an overall reduction of phosphorus (P)loading to 11,000 t yr- I (Annex 3, Section A). This goal reflects a generally accepted axiom that lake anoxia is primarily a function of organic production. Dobson and Gilbertson (1971) presented 1929-1970 data on oxygen depletion in the hypolimnion of the central basin of Lake Erie (uncorrected) which showed a significant trend (0.075 mgL-lmo-lyr-l) with increasing P loading over the period under study. In recent years, however, several scientists have questioned this axiom and suggested that factors other than organic production govern hypolimnetic oxygen consumption. Papers by M. N. Charlton (1980a,b) are perhaps most challenging in this context. Charlton's interpretation of hypolimnetic oxygen data from central Lake Erie suggests that historic increases in the apparent depletion were not as great as those presented by Dobson and Gilbertson, and variations that did occur were strongly related to variations in hypolimnion thickness. Charlton's oxygen depletion rates were only 0.02 mg L-I mo- I , if uncorrected, and zero if corrected for hypolimnion thickness and tempera-
1. State-of-the-art presentations by invited speakers on factors controlling oxygen depletion (Theme: What do we know in 1981 about lake oxygen depletion?). 2. Debate between Rosa/ Burns and Charlton on 719
720
J. BARICA
their interpretations of Lake Erie hypolimnetic oxygen depletion data followed by open discussion among all participants (Theme: What is right?). 3. Discussion of specific questions related to manageability of Lake Erie oxygen recovery through phosphorus loading reduction (Theme: Is restoration of year-round aerobic conditions in the Lake Erie hypolimnion an achievable objective?).
RESULTS There were 12 presentations addressing the theme of what is currently known about lake oxygen depletion, particularly Lake Erie oxygen depletion. The paleolimnological information presented by G. P. Harris (McMaster University) showed that Lake Erie has become more eutrophic since the clearing the forests in 1850. L. D. Delorme (NWRI) concluded from ostracode remains that Lake Erie has been a productive lake for the past several hundred, if not thousands, of years and has had low oxygen conditions. As related information regarding understanding the process, J. A. Mathias (Freshwater Institute, Canadian Department of Fisheries and Oceans) presented an analysis of shallow eutrophic lakes under ice cover. Oxygen depletion rates were governed primarily by the ratio of the lake volume to sediment area. R. W. Cornett (Atomic Energy of Canada) elaborated on the mechanisms of the hypolimentic oxygen depletion in general and concluded that respiration at the sediment surface is not influenced by the changes in rates of sedimentation (or surface water productivity). Hence, the total rate of oxygen depletion would not respond significantly to changes in the phosphorus loading. R. A. Vollenweider and L. L. Janus (NWRI) presented a new statistical model for predicting hypolimnetic oxygen depletion rates in lakes in general (based on analysis of 21 different lakes). Applied to Lake Erie, the model output would predict that hypolimnetic oxygen depletion rates increase parabolically with increasing chlorophyll levels, and conversely, that considerable reduction in chlorophyll (to 1 or 2 IJ-g L-I chlorophyll a, annual mean) would be required to reduce average hypolimentic depletion rates substantially. Because a certain decrase of hypolimnetic oxygen depletion rates is likely to occur in response to the current phosphorus control program, the late summer
conditions may become better than at present. It is unlikely, however, that a year-round 4 to 5 mg L-l dissolved oxygen (DO) concentration could be achieved for the whole basin. A more likely level would be 2 to 4 mg L-I which would imply that, in years of unfavourable meteorological conditions, temporary anoxia could still occur in localized areas. D. C. Lam (NWRI) presented observed and simulated Lake Erie results for 1967 through 1978, demonstrating that variations in meteorological conditions control hypolimnetic thickness which in turn causes large variations of hypolimnetic oxygen concentrations, masking the effect of changing phosphorus loading. H. A. Regier (U niversity of Toronto) presented historic data on Lake Erie fish and fish guts content, and fish catch data for cisco, lake whitefish, blue pike, lake trout, walleye, and rainbow smelt in the central basin. Populations in the central basin declined in a fashion that was inconsistent with population records from other areas of the lake not severely affected by oxygen depletion. Fish catch distribution data suggested that degradation of the central basin fishery progressed measurably between 1948 and 1956. N. A. Thomas (U .S. Environmental Protection Agency) and C. E. Herdendorf (Center for Lake Erie Area Research, the Ohio State University) reported on recent changes in total phosphorus, chlorophyll, and hypolimnetic oxygen depletion rates in Lake Erie. The changes indicate a general improvement in lake quality, including the oxygen conditions, in the past 3 years (1978-80). The inlake concentration of phosphorus has decreased 33 percent, and chlorophyll a 18 percent between 1974-80. Their findings were in agreement with D. DiToro's (Manhattan College) assessment based on a model of Lake Erie. Using a model, a significant improvement in the central basin oxygen regime was predicted to have occurred during the 1978-1980 period in response of the lake to the P loading reduction in early 1970. DiToro also suggested that, based on his model predictions, the local area of anoxia (defined as 0.5 mg L-I DO) of Lake Erie should have significantly diminished over the same years. F. M. Boyce and G. N. Ivey (NWRI) presented preliminary findings of their studies on currents and physical processes. They demonstrated the occurrence of downward entrainment of thermocline water which increased both temperature and the oxygen content of the hypolimnion. H. F. Dobson then re-stated his belief that the present
LAKE ERIE OXYGEN DEPLETION CONTROVERSY program of phosphorus removal would improve dissolved oxygen conditions, suggesting that after significant reduction of P loading from Cleveland and Detroit, higher oxygen levels in Lake Erie would be seen in the 1980 decade. Session 2 was a debate between Rosa and Burns on one side and Charlton on the other. Discussion focussed primarily on the justification of the data correction procedures used. Rosa and Burns calculated the rates using only data from an area in the center of the basin to eliminate considerable horizontal variability across the basin. Their second correction was based on Burns (1976) mesolimnion exchange model and corrected for the variable downward entrianment of oxygen. Variations in temperature were minimized by adjusting all data to a standard temperature of 10° C using a QIO
4
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y'- 0.019 x +1.7, r - 0.48 (insign.)
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• • •
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value of 2. Their fourth correction term was to account for the effect of variable hypolimnion thickness by standardizing all calculations to a thickness of 4.15 m. In contrast, Charlton corrected the rates for hypolimnetic thickness and temperature only. Basic uncorrected rates presented .by both parties yield approximately the saine regression line (slope 0.02 mgL-l mo- 1). The controversy arises from different corrections applied to the basic rates. Different corrections would yield different regression lines and lead to different interpretations. While Rosa and Burns' data show an increasing trend with time, Charlton's do not (Fig. 1). This is the core of the controversy. Rosa/Burns' interpretation suggests that the hypolimnion depletion rates have been steadily increasing over the past 50
CHARLTON
ROSA/BURNS
721
•
•
y - 0.021 x +1.74 ,r- 0.47(P<0.OS)
UNCORRECTED
!C(
a: 0,+---+-----1----+---+----+---+---+----+---+---+---+---+--+-----1
z
o
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.
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1
CORRECTED O+--~---r--~--_r_----,---,._--+--,._----,---,._-_..--~-__r-----I 1920 1930 1940 1950 1960 1970 1980 1920 1930 1940 1950 1960 1970 1980
FIG. 1. Different interpretations ofLake Erie-CentralBasin hypolimnetic oxygen depletion data by Rosa/Burns (left) and Charlton (right). Solid line, 1929-1979(80); broken line, 1949-1979(80). Note differences in significance levels.
722
J. BARICA
years (with the exception perhaps of the last 3 years where some decline is visible). They concluded that P loadings affect dissolved oxygen depletion and that P loading reduction would lead to improvement of the dissolved oxygen conditions. According to Charlton's interpretation, the oxygen depletion rates fluctuate according to physical factors such as the thickness and temperature of the hypolimnion and the trophic state. Hence, the effect of P loading reduction on oxygen depletion would be minimal and the goal of year-round aerobic conditions, as stipulated in the Great Lakes Water Quality Agreement, may be unrealistic and unachievable.
interaction of physical and biochemical processes. The concensus was that, without the control measures implemented within the last decade, the lake would be much worse now. Thus, control measures have had a beneficial effect, but prediction of future hypolimnion oxygen conditions cannot be made with absolute confidence. The goals of the agreement do not explicitly provide for the variability of the natural environment. Further predictions should take much greater care to incorporate this phenomenon. Efforts should be made to supplement the objectives of the Agreement in order to explain the associated uncertainties.
CONCLUSIONS
ACKNOWLEDGMENTS
The workshop confirmed the desireability of a Lake Erie phosphorus reduction program and validated the feasibility of the 1978 Great Lakes Water Quality Agreement's goal of restoring aerobic conditions to the bottom waters of the central basin on a year-round basis. No revisions in the present strategy were recommended. The presentations and discussions also provided a valuable synthesis of up-to-date information. Although there are still differences of opinion regarding the degree to which phosphorus control will affect hypolimnetic anoxia, it is clear that, given present levels of understanding, the oxygen conditions in the lake are related to the degree to which phosphorus inputs are controlled. Most of the participants agreed that restoration of year-round aerobic conditions in the central basin hypolimnion was feasible, but caution was advised as to the magnitude of the level of P reduction needed to achieve it and the time required for the lake to respond. Low dissolved oxygen concentrations (below that necessary to support fish) are likely to occur annually over the same part of the lake basin and anoxia may occur from time to time as a result of an unpredictable
The author wishes to thank Dennis Gregor, Environment Canada, Ontario Region, for his help as conference co-chairman. The secretarial support of Dina Paolini and Jackie Major of the National Water Research Institute is also gratefully acknowledged. REFERENCES Burns, N. M. 1976. Oxygen depletion in the Central and Eastern Basins of Lake Erie, 1970. J. Fish. Res. Board Can. 3:512-519. Charlton, M. N. 1980a. Oxygen depletion in Lake Erie: has there been any change? Can. J. Fish. Aquat. Sci. 37:72-81. _ _ _ _ . 1980b. Hypolimnetic oxygen consumption in lakes: discussion of productivity and morphometry effects. Can. J. Fish. Aquat. Sci. 37:1531-1539. Dobson, H. H., and Gilbertson, M. 1971. Oxygen depletion in the hypolimnion of the Central Basin of Lake Erie, 1929-1970. In Proc. 14th Conf. Great Lakes Res, pp. 743-748. InternaL Assoc. Great Lakes Research. Great Lakes Water Quality Agreement of 1978. International Joint Commission, Great Lakes Regional Office, Windsor, Ontario.
LAKE ERIE OXYGEN DEPLETION CONTROVERSY
J. Barica Aquatic Ecology Division National Water Research Institute Canada Centre for Inland Waters P. O. Box 5050 Burlington, Ontario, Canada L7R 4A6
ABSTRACT. A summary of a special workshop held at the Canada Centre for Inland Waters, Burlington, Ontario, 2-3 December 1981, is presented. The purpose of the workshop was to air some differences of opinion regarding the response of the hypolimnetic anoxia of Lake Erie to phosphorus controls. The workshop confirmed the validity of the 1978 U.S.-Canadian Great Lakes Water Quality Agreement's goals on restoring year-round aerobic conditions in the hypolimnion ofthe central basin of Lake Erie through phosphorus loading reduction. The author served as workshop chairman and convenor. ADDITIONAL INDEX WORDS: Hypolimnetic oxygen, phosphorus loading.
ture. Charlton suggested that eutrophicationrelated changes in the oxygen depletion rate were too small to be recognized and, hence, the Water Quality Agreement goal may not be realistic. Charlton's suggestions were recently challenged by F. Rosa and N. M. Burns (unpublished working draft) who reassessed the available historic data. They introduced four corrections (adjustments) to reduce the data for the different years to standard conditions. The National Water Research Institute (NWRI, Canadian Department of Environment) decided to air this controversy through a scientific debate. A special workshop was held at the Canada Centre for Inland Waters (CCIW), Burlington, Ontario, 2-3 December 1981, with representation from both Canadian and U.S. organizations, (12 speakers and over 30 observers and participants). Prior to the workshop, both Charlton's papers and Rosa and Burns' latest document with new calculations were sent for review to the participants. The workshop was organized in three sessions:
One of the specific goals of the Canada/ U.S. Great Lakes Water Quality Agreement of 1978 is restoration of year-round aerobic conditions in the bottom waters of the central basin of Lake Erie through an overall reduction of phosphorus (P)loading to 11,000 t yr- I (Annex 3, Section A). This goal reflects a generally accepted axiom that lake anoxia is primarily a function of organic production. Dobson and Gilbertson (1971) presented 1929-1970 data on oxygen depletion in the hypolimnion of the central basin of Lake Erie (uncorrected) which showed a significant trend (0.075 mgL-lmo-lyr-l) with increasing P loading over the period under study. In recent years, however, several scientists have questioned this axiom and suggested that factors other than organic production govern hypolimnetic oxygen consumption. Papers by M. N. Charlton (1980a,b) are perhaps most challenging in this context. Charlton's interpretation of hypolimnetic oxygen data from central Lake Erie suggests that historic increases in the apparent depletion were not as great as those presented by Dobson and Gilbertson, and variations that did occur were strongly related to variations in hypolimnion thickness. Charlton's oxygen depletion rates were only 0.02 mg L-I mo- I , if uncorrected, and zero if corrected for hypolimnion thickness and tempera-
1. State-of-the-art presentations by invited speakers on factors controlling oxygen depletion (Theme: What do we know in 1981 about lake oxygen depletion?). 2. Debate between Rosa/ Burns and Charlton on 719
720
J. BARICA
their interpretations of Lake Erie hypolimnetic oxygen depletion data followed by open discussion among all participants (Theme: What is right?). 3. Discussion of specific questions related to manageability of Lake Erie oxygen recovery through phosphorus loading reduction (Theme: Is restoration of year-round aerobic conditions in the Lake Erie hypolimnion an achievable objective?).
RESULTS There were 12 presentations addressing the theme of what is currently known about lake oxygen depletion, particularly Lake Erie oxygen depletion. The paleolimnological information presented by G. P. Harris (McMaster University) showed that Lake Erie has become more eutrophic since the clearing the forests in 1850. L. D. Delorme (NWRI) concluded from ostracode remains that Lake Erie has been a productive lake for the past several hundred, if not thousands, of years and has had low oxygen conditions. As related information regarding understanding the process, J. A. Mathias (Freshwater Institute, Canadian Department of Fisheries and Oceans) presented an analysis of shallow eutrophic lakes under ice cover. Oxygen depletion rates were governed primarily by the ratio of the lake volume to sediment area. R. W. Cornett (Atomic Energy of Canada) elaborated on the mechanisms of the hypolimentic oxygen depletion in general and concluded that respiration at the sediment surface is not influenced by the changes in rates of sedimentation (or surface water productivity). Hence, the total rate of oxygen depletion would not respond significantly to changes in the phosphorus loading. R. A. Vollenweider and L. L. Janus (NWRI) presented a new statistical model for predicting hypolimnetic oxygen depletion rates in lakes in general (based on analysis of 21 different lakes). Applied to Lake Erie, the model output would predict that hypolimnetic oxygen depletion rates increase parabolically with increasing chlorophyll levels, and conversely, that considerable reduction in chlorophyll (to 1 or 2 IJ-g L-I chlorophyll a, annual mean) would be required to reduce average hypolimentic depletion rates substantially. Because a certain decrase of hypolimnetic oxygen depletion rates is likely to occur in response to the current phosphorus control program, the late summer
conditions may become better than at present. It is unlikely, however, that a year-round 4 to 5 mg L-l dissolved oxygen (DO) concentration could be achieved for the whole basin. A more likely level would be 2 to 4 mg L-I which would imply that, in years of unfavourable meteorological conditions, temporary anoxia could still occur in localized areas. D. C. Lam (NWRI) presented observed and simulated Lake Erie results for 1967 through 1978, demonstrating that variations in meteorological conditions control hypolimnetic thickness which in turn causes large variations of hypolimnetic oxygen concentrations, masking the effect of changing phosphorus loading. H. A. Regier (U niversity of Toronto) presented historic data on Lake Erie fish and fish guts content, and fish catch data for cisco, lake whitefish, blue pike, lake trout, walleye, and rainbow smelt in the central basin. Populations in the central basin declined in a fashion that was inconsistent with population records from other areas of the lake not severely affected by oxygen depletion. Fish catch distribution data suggested that degradation of the central basin fishery progressed measurably between 1948 and 1956. N. A. Thomas (U .S. Environmental Protection Agency) and C. E. Herdendorf (Center for Lake Erie Area Research, the Ohio State University) reported on recent changes in total phosphorus, chlorophyll, and hypolimnetic oxygen depletion rates in Lake Erie. The changes indicate a general improvement in lake quality, including the oxygen conditions, in the past 3 years (1978-80). The inlake concentration of phosphorus has decreased 33 percent, and chlorophyll a 18 percent between 1974-80. Their findings were in agreement with D. DiToro's (Manhattan College) assessment based on a model of Lake Erie. Using a model, a significant improvement in the central basin oxygen regime was predicted to have occurred during the 1978-1980 period in response of the lake to the P loading reduction in early 1970. DiToro also suggested that, based on his model predictions, the local area of anoxia (defined as 0.5 mg L-I DO) of Lake Erie should have significantly diminished over the same years. F. M. Boyce and G. N. Ivey (NWRI) presented preliminary findings of their studies on currents and physical processes. They demonstrated the occurrence of downward entrainment of thermocline water which increased both temperature and the oxygen content of the hypolimnion. H. F. Dobson then re-stated his belief that the present
LAKE ERIE OXYGEN DEPLETION CONTROVERSY program of phosphorus removal would improve dissolved oxygen conditions, suggesting that after significant reduction of P loading from Cleveland and Detroit, higher oxygen levels in Lake Erie would be seen in the 1980 decade. Session 2 was a debate between Rosa and Burns on one side and Charlton on the other. Discussion focussed primarily on the justification of the data correction procedures used. Rosa and Burns calculated the rates using only data from an area in the center of the basin to eliminate considerable horizontal variability across the basin. Their second correction was based on Burns (1976) mesolimnion exchange model and corrected for the variable downward entrianment of oxygen. Variations in temperature were minimized by adjusting all data to a standard temperature of 10° C using a QIO
4
-
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~
,..
!..J
•
y'- 0.019 x +1.7, r - 0.48 (insign.)
3
•
~
-
~!.Jll=
• • •
y -0.02 x +1.6, r-0.63,(P<0.OS)
E
W
•
.. ~
y'-0.014X+2.19. r-0.27(insign)
2
C)
value of 2. Their fourth correction term was to account for the effect of variable hypolimnion thickness by standardizing all calculations to a thickness of 4.15 m. In contrast, Charlton corrected the rates for hypolimnetic thickness and temperature only. Basic uncorrected rates presented .by both parties yield approximately the saine regression line (slope 0.02 mgL-l mo- 1). The controversy arises from different corrections applied to the basic rates. Different corrections would yield different regression lines and lead to different interpretations. While Rosa and Burns' data show an increasing trend with time, Charlton's do not (Fig. 1). This is the core of the controversy. Rosa/Burns' interpretation suggests that the hypolimnion depletion rates have been steadily increasing over the past 50
CHARLTON
ROSA/BURNS
721
•
•
y - 0.021 x +1.74 ,r- 0.47(P<0.OS)
UNCORRECTED
!C(
a: 0,+---+-----1----+---+----+---+---+----+---+---+---+---+--+-----1
z
o
I-
W ...J
4
y'-0.034x+1.0, r-0.82, (P<0.01)
•
0W
C
3
Z
W
~ ~
Y'-~~:06~~~: •
2
~1
00
.
y -0.012x+2.47, r-0.34 (insign)
y-0.035 x +1.0 , r- 0.89.(p
1
CORRECTED O+--~---r--~--_r_----,---,._--+--,._----,---,._-_..--~-__r-----I 1920 1930 1940 1950 1960 1970 1980 1920 1930 1940 1950 1960 1970 1980
FIG. 1. Different interpretations ofLake Erie-CentralBasin hypolimnetic oxygen depletion data by Rosa/Burns (left) and Charlton (right). Solid line, 1929-1979(80); broken line, 1949-1979(80). Note differences in significance levels.
722
J. BARICA
years (with the exception perhaps of the last 3 years where some decline is visible). They concluded that P loadings affect dissolved oxygen depletion and that P loading reduction would lead to improvement of the dissolved oxygen conditions. According to Charlton's interpretation, the oxygen depletion rates fluctuate according to physical factors such as the thickness and temperature of the hypolimnion and the trophic state. Hence, the effect of P loading reduction on oxygen depletion would be minimal and the goal of year-round aerobic conditions, as stipulated in the Great Lakes Water Quality Agreement, may be unrealistic and unachievable.
interaction of physical and biochemical processes. The concensus was that, without the control measures implemented within the last decade, the lake would be much worse now. Thus, control measures have had a beneficial effect, but prediction of future hypolimnion oxygen conditions cannot be made with absolute confidence. The goals of the agreement do not explicitly provide for the variability of the natural environment. Further predictions should take much greater care to incorporate this phenomenon. Efforts should be made to supplement the objectives of the Agreement in order to explain the associated uncertainties.
CONCLUSIONS
ACKNOWLEDGMENTS
The workshop confirmed the desireability of a Lake Erie phosphorus reduction program and validated the feasibility of the 1978 Great Lakes Water Quality Agreement's goal of restoring aerobic conditions to the bottom waters of the central basin on a year-round basis. No revisions in the present strategy were recommended. The presentations and discussions also provided a valuable synthesis of up-to-date information. Although there are still differences of opinion regarding the degree to which phosphorus control will affect hypolimnetic anoxia, it is clear that, given present levels of understanding, the oxygen conditions in the lake are related to the degree to which phosphorus inputs are controlled. Most of the participants agreed that restoration of year-round aerobic conditions in the central basin hypolimnion was feasible, but caution was advised as to the magnitude of the level of P reduction needed to achieve it and the time required for the lake to respond. Low dissolved oxygen concentrations (below that necessary to support fish) are likely to occur annually over the same part of the lake basin and anoxia may occur from time to time as a result of an unpredictable
The author wishes to thank Dennis Gregor, Environment Canada, Ontario Region, for his help as conference co-chairman. The secretarial support of Dina Paolini and Jackie Major of the National Water Research Institute is also gratefully acknowledged. REFERENCES Burns, N. M. 1976. Oxygen depletion in the Central and Eastern Basins of Lake Erie, 1970. J. Fish. Res. Board Can. 3:512-519. Charlton, M. N. 1980a. Oxygen depletion in Lake Erie: has there been any change? Can. J. Fish. Aquat. Sci. 37:72-81. _ _ _ _ . 1980b. Hypolimnetic oxygen consumption in lakes: discussion of productivity and morphometry effects. Can. J. Fish. Aquat. Sci. 37:1531-1539. Dobson, H. H., and Gilbertson, M. 1971. Oxygen depletion in the hypolimnion of the Central Basin of Lake Erie, 1929-1970. In Proc. 14th Conf. Great Lakes Res, pp. 743-748. InternaL Assoc. Great Lakes Research. Great Lakes Water Quality Agreement of 1978. International Joint Commission, Great Lakes Regional Office, Windsor, Ontario.