Summary
The influence of frequent water-column mixing on phytoplankton succession 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 species 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 Opinicon. Lightly-silicified and spindle-shaped diatoms (Rhizosolenia and single-celled Fragilaria) were more abundant in the dimictic lake, whereas heavily-silicified diatoms (Aulacoseira) were more abundant in the frequently-mixed lake. Zooplankton taxa and their seasonal patterns were similar in both lakes, but populations were considerably denser in the frequently-mixed lake. The results of canonical correspondence analysis and detrended correspondence analysis (multivariate ordination techniques) indicated that, although phytoplankton growth was influenced by similar physical and chemical variables in both lakes, the influence 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 epilimnion of a typical dimictic lake, such as Upper Rock Lake.