Effects of Impoundment and Diversion on the Crustacean Plankton of Southern Indian Lake

Planktonic crustaceans in Southern Indian Lake were surveyed in 1972 before impoundment, during 1975 when water levels rose above the recorded high level, and then annually from 1977 to 1980 after full impoundment and diversion. Synoptic data were collected in each of these years in midsummer during July or August from a set of 53 stations covering all regions of the lake. Vertical hauls were made at each station using twin nets of 77-μm mesh and 25-cm mouth diameter. In 1972, the crustacean plankton fauna of Southern Indian Lake was composed mainly of 15 copepod and 15 cladoceran species. At least 25 of these species were truly pelagic, while 5 were littoral. Cyclopoids comprised 46% of total crustacean abundance, calanoids 33%, and cladocerans 21%. Dominant species within these three groups were Cyclops bicuspidatus thomasi (37.6% of total abundance), Diaptomus ashlandi (20.1%), and Chydorus sphaericus (8.9%). Total abundance averaged 76 individuals (ind)∙L−1, comparable to more southerly waters such as Lake Ontario (80 ind∙L−1) and Lake Winnipeg (53–108 ind∙L−1) (K. Patalas. 1975. Int. Ver. Limnol. Verh. 19: 504–511). Throughout the lake, total crustacean abundances varied from 10 ind∙L−1 near the Churchill River inflow to between 100 and 200 ind∙L−1 in a few well protected areas. From 50 to 100 ind∙L−1 occurred within the main body of Southern Indian Lake. In 1975, an unusually high water year, lake mean plankton abundance was 61 ind∙L−1. Following diversion in 1976, no dramatic changes in species composition were observed. However, the mean abundance of crustaceans decreased to 40–46 ind∙L−1 during the period 1977–80. Regional zooplankton responses were varied. No significant changes occurred in areas adjacent to the Churchill River inflow, with pre- and post-diversion densities being 35 and 36 ind∙L−1, respectively. In the main water bodies north of the diversion route, abundances declined by 60% and biomass by 50%. A 2–3 °C drop in northern basin water temperatures, related to diversion, reduced growth rates by approximately 20% and resulted in a 60% decline in crustacean production. Not all groups of crustaceans responded similarily to impoundment and diversion. Lakewide average numbers of cladocerans declined from 16 to 4 ind∙L−1 and the area of their distribution was reduced particularly in northern regions of the lake following impoundment. Cyclopoids declined from 35 to 16 ind∙L−1 but showed no change in their distribution. These reductions were related to decreased water temperatures, lower midsummer chlorophyll a concentrations, and decreased water transparencies. The mean abundance of calanoids as a group did not change, but the abundance and distribution of individual species were variably altered. Smaller calanoid species showed either no change or a decrease in their numbers and distribution. Larger species, e.g. Limnocalanus macrurus, Senecella calanoides, and Diaptomus sicilis, were significantly more abundant and widespread following diversion. Similar increases in Mysis relicta were also observed. These large species, preferred food items for both whitefish and cisco, are cold stenotherms, inhabiting deeper water layers. Their increased abundance is likely associated with decreased water transparency offering better protection against predatory fish, decreased water temperature creating more favorable conditions, and increased water depth enlarging the volume of deeper waters suitable for these species.