The Stability of Temperate Lakes Under the Changing Climate

There is a collective prediction among ecologists that climate change will enhance phytoplankton biomass in temperate lakes. Yet there is noteworthy variation in the structure and regulating functions of lakes to make this statement challengeable and, perhaps, inaccurate. To generate a common understanding on the trophic transition of lakes, I examined the interactive effects of climate change and landscape properties on phytoplankton biomass in 12,644 lakes located in relatively intact forested landscapes. Chlorophyll-a (Chl-a) concentration was used as a proxy for phytoplankton biomass. Chl-a concentration was obtained via analyzing Landsat satellite imagery data over a 28-year period (1984-2011) and using regression modelling. The most common lake trophic state was oligotrophic (median Chl-a \u3c 2.6 μg L-1), while the least common was hyper-eutrophic (median Chl-a \u3e 56 μg L-1). Lake volume was the most important factor in determining the present trophic state of the lakes. The majority of the lakes (91.6%) did not show a change in trophic state over an almost 3-decade long sampling period; only 4.0% of the lakes became more eutrophic, and 4.4% of the lakes became more oligotrophic. Lakes with smaller volumes were further responsive to temperature (warmer lakes were more eutrophic), while lakes with larger volumes were more responsive to precipitation (wetter lakes were more oligotrophic). Early warning indicators of change in trophic state were examined in the patterns of the residuals of the time series of Chl-a once non-stationary and stationary trends were removed. Remarkably, the majority (56.5%) of the lakes showed patterns in the residuals that were not defined by a single trophic metric but fluctuated among different trophic states. There was an unexpected instability among some lakes as they switched between oligotrophic and eutrophic states (12.5%) or were transitioning from eutrophic towards oligotrophic states (23.4%), or from oligotrophic towards eutrophic states (20.6%). The complex responses of phytoplankton biomass to climate change suggests that our ability to predict the future trophic state of lakes will be limited but enhanced if we recognize that lakes and their catchments will be both impacted by climate change