The impact of climate forcing on the phenology of ecological processes in the North Temperate Lakes

Recent studies have shown that physical processes and biological communities are impacted to varying degrees by global climate change. Differences in sensitivity to climate forcing within and among these elements of an ecosystem have lead to the decoupling of processes previously linked in the seasonal timing of their occurrence (phenology). The decoupling of key elements of food webs could have lasting implications in ecosystem functioning (e.g. the decoupling of climate sensitive algal abundance and less sensitive zooplankton grazing of algae in lakes). To investigate the effects of climatic drivers on physical features and biological processes, we estimated the long-term trends in ice phenology and thermal structure, using 15-30-year time series of measurements for eleven lakes monitored by the North Temperate Lakes Long-Term Ecological Research (NTL-LTER) program. Lakes coherently responded to regional climatic warming, showing declining ice cover duration, prolonged growing season, and increasingly warmer waters. We further compared the sensitivity of physical processes to climate forcing in lakes as a function of morphometric features, trophic states, and dissolved organic carbon concentrations. We then examined whether these changes in thermal structure and ice phenology have yielded changes in the seasonal timing of each lake’s plankton community composition and abundance, modeling our predictions after the PEG (Plankton Ecology Group) model of plankton succession in freshwater. We found that the timing of biological processes (e.g. the timing of peak chlorophyll-a concentration, zooplankton biomass and individual zooplankton species abundance) were highly variable among years.  Further investigation into the metrics used to define the timing of biological processes and into other aquatic systems with long-term time series will be required to better assess the impact of climate forcing on the phenology of a lake’s ecology.