Responses of Zooplankton Populations to Four Decades of Climate Warming in Lakes of Southwestern Alaska

Zooplankton support a variety of important ecosystem processes in lakes but our understanding of how these organisms respond to ongoing climate change is superficial. Aquatic ecosystems have been exposed to substantial climate warming in the last century, and this trend is certain to continue, especially in northern latitudes. We analyzed a long time series of a variety of physical attributes of Lake Aleknagik, southwest Alaska, to quantify warming trends between 1963 and 2009, and to assess zooplankton community responses to variation in thermal conditions. Our analyses demonstrate clear trends toward earlier spring ice break-up, increases in average epilimnetic temperatures and an 18.5% increase in summer epilimnetic degree days since 1963. We estimated the magnitude and direction of changes in zooplankton production and end of season densities in response to changes in thermal environment, then explored whether the climate-driven responses in production are common in five large lakes that are distributed across a coastal-interior landscape gradient. Although we detected no clear directional changes in zooplankton densities or production rates, there were clear correlations between these variables and interannual variation in thermal conditions. The positive effects of earlier spring ice break-up and increase in summer degree days were strongest on the production and density responses of Daphnia and Bosmina, and the lagged effect of temperature was important and negative only for calanoid copepods. There was a negative effect of sockeye escapement to Lake Aleknagik in the previous year on all taxa. There were lake-specific effects of changes in the thermal environment on almost all taxa and small gradients in the magnitude of responses across the landscape, but no systematic patterns of landscape control.