Extreme weather events alter planktonic communities in boreal lakes

Climate warming has been shown to increase the frequency of extreme weather effects on small lakes by increasing the variability of terrigenic inputs and surface water temperatures. We hypothesized that the effect of thermal variability on boreal plankton depends on dissolved terrigenic matter (i.e., temperature-terrigenic interaction). A two-factor mesocosm (1500-L capacity) experiment consisting of three terrigenic treatment levels (control, [-] runoff, [+] runoff) and three temperature treatment levels (control, warm, and cold) was conducted in triplicate for a total of 27 mesocosms deployed in Lake 302S of the Experimental Lakes Area in Canada. The warming treatment amplified the positive effect of terrigenic amendment on total phytoplankton biomass by stimulating large (>35-mu m Greatest Axial Linear Dimension; GALD) taxa during the 50-d experiment. In comparison, removal of terrigenic matter increased the abundance of smaller (<35-mu m GALD) phytoplankton along with copepods and cladocerans under cold and warm conditions, respectively. We also attempted to corroborate our experimental findings by comparing planktonic communities collected from reference Lake 239 during climatically contrasting summers between 1970 and 2001. Although planktonic communities in Lake 239 also differed significantly between years characterized by cold, wet vs. warm, dry ice-free conditions, their responses ran opposite to those detected during the experiment, highlighting the potential overriding importance of other scale-dependent factors (e.g., fish predation, vertical migration) mediating the effects of climate on lake communities.