Abstract
The effects of climate change on lake ecosystems are often complex. We examined how phytoplankton in two neighbouring, pre-alpine, large oligotrophic lakes with similar catchments and land uses are likely to respond to climate change. We hypothesised that (i) while their climates and landscape filters were relatively similar, differences in in-lake biological, physical and chemical filters would influence the phytoplankton responses to climate and (ii) direct effects of warming on phytoplankton dynamics and productivity would be relatively minor compared to indirect effects, especially those influencing the lakes' mixing regimes. We combined (i) dynamic modelling of the physical forcing of the lakes under climate change, (ii) multi-year field sampling of relevant biological, physical and chemical variables and (iii) bioassay experiments, to test our hypotheses. Water temperatures have warmed over recent decades in one lake, but not in the other. The warming lake showed evidence of incomplete mixing and phytoplankton layering in winter 2009, while the other lake did not. Such changes influenced the phytoplankton phenology, and incomplete winter mixing is common in similar deep, temperate lakes. Inhibited winter mixing and related indirect effects of climate warming appear to be key early drivers of climate change effects on the phytoplankton of deep, temperate lakes.