Consequences of elevated temperatures for zooplankton assemblages in temperate lakes

Abstract

This paper is a review of direct and indirect effects of elevated temperatures (≥25°C) on freshwater zooplankton in temperate latitudes. Effects of temperature on aspects of behavior, individual growth, development and reproduction, and population dynamics are compared across zooplankton taxa. Thermal responses vary markedly among co-occurring species and among clones within parthenogenetic species. There is strong evidence that elevated temperature can produce a reduction in body size within zooplankton populations. Also, some zooplankton assemblages become increasingly dominated by smaller bodied forms under warm conditions. We hypothesize that certain cold-water zooplankton species show the greatest risk of local extinction during late summer heating events because of their dependence on thermal refugia, and their lack of diapausing or dispersal stages. Indirect effects of elevated temperature mediated by changes in the algal food supply or by altered interspecific interactions are also explored. Elevated temperature can change algal community structure and algal food quality, all of which can affect the outcome of zooplankton interspecific competition. Size-selective predation rates of planktivorous fish are also likely to increase with increasing temperature or when zooplankton are driven from the hypolimnion. The onset of fish feeding is also likely to begin earlier in the season thus reducing the time that large zooplankton are present during the ice-free season. Reductions in the size structure of zooplankton assemblages are likely to alter water clarity, nutrient regeneration and fish abundances.