WINTER LIMNOLOGY - A COMPARISON OF PHYSICAL, CHEMICAL AND BIOLOGICAL CHARACTERISTICS IN 2 TEMPERATE LAKES DURING ICE COVER

The winter dynamics of several chemical, physical, and biological variables of a shallow, polymictic lake (Opinicon) are compared to those of a deep, nearby dimictic lake (Upper Rock) during ice cover (January to early April) in 1990 and 1991. Both lakes were weakly inversely thermally stratified. Dissolved oxygen concentration was at saturation (11-15 mg l(-1)) in the top 3 m layer, but declined to near anoxic levels near the sediments. Dissolved oxygen concentrations in the deep lake were at saturatino in most of the water column and approached anoxic levels near the sediments only. Nutrient concentrations in both lakes were fairly high, and similar in both lakes during ice cover. Total phosphorus concentrations generally ranged between 10-20 mu g l(-1), NH4-N between 16-100 mu g l(-1), and DSi between 0.9-1.9mgl(-1); these concentrations fell within summer ranges. NO3-N concentrations were between 51-135 mu l(-1) during ice cover, but occured at trace concentrations (<0.002 mu l(-1)) during the summer. The winter phytoplankton community of both lakes was dominated by flagellates (cryptophytes, chrysophytes) and occasionally diatoms. Dinoflagellates, Cyanobacteria and green algae were poorly represented. Cryptophytes often occured in fairly high proportions (20-80%) throughout the water column, whereas chrysophytes were more abundant just beneath the ice. Zooplankton population densities were extremely low during ice cover (compared to maximum densities measured in spring or summer) in both lakes, and were comprised largely of copepods.