Evaluation of factors related to increased zooplankton biomass and altered species composition following impoundment of a Newfoundland reservoir

Zooplankton biomass and species composition were monitored in Cat Arm Lake and Reservoir on Newfoundland's Great Northern Peninsula from 1983 to 1993. Zooplankton biomass increased approximately 19-fold in the oligotrophic reservoir following impoundment in 1984, relative to biomass in the preexisting lake. Microcrustaceans Cyclops scutifer and Daphnia dubia, either rare or absent from Cat Arm Lake prior to impoundment, were consistently measurable components of the zooplankton community by 1993. Similar changes elsewhere have been attributed to both increased water retention time and enhanced phytoplankton biomass, factors whose effects are usually interdependent. In Cat Arm, there were no increases in either phytoplankton biomass or primary productivity during the first 3 years of impoundment, and natality of the dominant cladoceran, Daphnia catawba, did not increase. Summer water retention time increased from preimpoundment levels of 4 days in 1983 to 338 days in 1993. Zooplankton biomass was significantly correlated with water retention time (Spearman's r(s) = 0.72, p = 0.04) and showed no significant correlation with phytoplankton biomass, primary productivity, nutrient concentrations, pH, colour, or epilimnetic temperature. Changes in the zooplankton community in this subarctic system can thus be attributed most directly to a decrease in losses due to washout.