TEMPORAL AND SEASONAL TRENDS IN NUTRIENT DYNAMICS AND BIOMASS MEASURES IN LAKES MICHIGAN AND ONTARIO IN RESPONSE TO PHOSPHORUS CONTROL

Results from long-term ecological monitoring studies on Lake Michigan (1983-92) and Lake Ontario (1981-92) were compared with regard to changes in phosphorus loads. In Lake Ontario, total phosphorus (TP) loads decreased from 14 000 t . yr(-1) in 1970 to 7500 t . yr(-1) in 1981, and correspondingly, midlake TP concentration decreased from 25 to 16 mu g . L(-1). From 1981 to 1991 TP loads remained around 7500 t . yr(-1); however, TP concentration continued to decline from 16 to 10 mu g . L(-1). Similarly, mean summer particulate organic carbon (POC), chlorophyll a (CHLa), and nitrate utilization rate decreased by approximately 40, 20, and 50%, respectively. Conversely, si[ica utilization rates increased markedly after 1983. In Lake Michigan, TP loads also decreased by around 50% from 1974 to 1990 (2000 t . yr(-1)); however, TP concentrations at our 100-m station in the southern basin increased during the study period from around 4 to 8 mu g . L(-1). There were no distinct trends in CHLa or nutrient utilization patterns; however, POC levels decreased sharply after 1987. Overall, silica utilization rates in Lake Michigan were 50% higher than in Lake Ontario (14.7 vs. 9.6 mu g . L(-1). d(-1)), whereas nitrate utilization rates were only half (1.4 vs. 4.1 mu g . L(-1). d(-1)).