Trout stocking in the mid-1960s eliminated the calanoid copepod Hesperodiaptomus arcticus and other largebodied crustaceans such as Gammarus lacustris, Daphnia middendorffiana, and Daphnia pulex from many alpine lakes in the Rocky Mountain Parks of Canada. H. arcticus frequently dominates the plankton communities of fishless lakes, preying on rotifers and nauplius larvae. Following the extirpation of H. arcticus, rotifers and small-bodied cyclopoid copepods dominate the zooplankton assemblages of alpine lakes.We studied the zooplankton community of Snowflake Lake, Banff National Park, from 1966 to 1995. H. arcticus was eliminated following stocking of the lake with trout in the 1960s. It failed to become reestablished after the disappearance of the fish population in the mid-1980s. Several species of rotifers and small-bodied crustaceans, species originally rare or absent from the plankton, became abundant following fish stocking and remained so after the fish population declined.In 1992, we reintroduced H. arcticus to Snowflake Lake. The H. arcticus population grew exponentially for 4 yr, but had not reached stable densities typical of unmanipulated alpine lakes by 1995. By 1994, however, even the small population of Hesperodiaptomus was beginning to suppress populations of rotifers, copepod nauplii, and large diatoms. Because H. arcticus is omnivorous, a simple model of cascading trophic interactions did not predict the outcome of trophic manipulations in this alpine lake.

The seasonal abundance and composition of photosynthetic picoplankton (0.2-2 pm) was compared among five oligotrophic to mesotrophic lakes in Ontario. Epilimnetic picocyanobacteria abundance followed a similar pattern in all lakes; maximum abundance (2-4 x lo5 cells . ml-1)

The influence of frequent water-column mixing on phytoplankton succes sion and species composition was studied by comparing plankton dynamics in two lakes (Lake Opinicon and Upper Rock Lake) which have similar water chemistry, but differ widely in their basin morphometries: Opinicon is shallow and frequently-mixed during the ice-free season, whereas nearby Upper Rock is deep and dimictic. The spe cies composition and the seasonal succession of phytoplankton were broadly similar in both lakes over the three years of study. The spring phytoplankton in both lakes consisted of diatoms that were replaced in the summer and fall by cyanobacteria. Diatom peaks in Opinicon preceded similar peaks in Upper Rock. Loss of diatoms from the phytoplankton in Upper Rock appeared to be related to sedimentation; in contrast, zooplankton herbivory may have resulted, partly, in loss of diatoms in Opi nicon. Lightly-silicified and spindle-shaped diatoms (Rhizosolenia and single-celled Fragilaria) were more abundant in the dimictic lake, whereas heavily-silicified di atoms (Aulacoseira) were more abundant in the frequently-mixed lake. Zooplankton taxa and their seasonal patterns were similar in both lakes, but populations were con siderably denser in the frequently-mixed lake. The results of canonical correspond ence analysis and detrended correspondence analysis (multivariate ordination techni ques) indicated that, although phytoplankton growth was infl uenced by similar phys ical and chemical variables in both lakes, the infl uence of chemical variables was stronger in Upper Rock. A frequently-mixed system, such as Lake Opinicon, appeared to provide a similar environment for plankton development to a well-mixed epilim nion of a typical dimictic lake, such as Upper Rock Lake.

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 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 saturation 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 #g I -I , NH4-N between 16-100/zg 1 -~ , and DSi between 0.9-1.9 mg 1-~; these concentrations fell within summer ranges. NO3-N concentrations were between 51 -135 Ft, g 1-1 during ice cover, but occurred at trace concentrations (<0.002 ltg I-l ) 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 occurred 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.

The dynamics of chrysophyte populations and the onset of encystment in relation to several physical, chemical, and biological variables were studied in two Canadian lakes, Lake Opinicon (LO) and Upper Rock Lake (URL). LO is shallow and polymictic during the ice‐free season, whereas URL is deep and dimictic. Seasonal patterns of many limnological variables, as well as species composition and seasonal dynamics of chrysophyte assemblages, were broadly similar in both lakes. Three groups of chrysophytes were recognized: 1) taxa that were more abundant during ice cover and/or early spring and occasionally in late fall (Dinobryon spp. Ehrenberg and Synura spinosa Korsh), 2) taxa that seemed to prefer moderately warm water proglena americana Calkins and Ochromonas sp. Wyssotzki) and had maximum densities in late summer and occasionally in the fall, and 3) taxa that occurred in low numbers and did not exhibit any discernible seasonality in the two lakes (Mallomonas spp. Perty). Encystment rarely occurred during ice cover in both lakes. Stomatocyst concentrations were highest shortly after ice breakup (late April or early May), declined to trace levels by June, and remained at those levels until the end of the sampling season (late November). Encystment rates (cyst concentrations) were higher in dimictic URL. Encystment in both lakes coincided with concurrent changes in physical and chemical variables and chrysophyte vegetative cell densities, but it was not possible to pinpoint the exact conditions responsible for triggering encystment. Only a small proportion of chrysophytes encysted. Cyst production within a lake may vary from year to year.