Warm spring and summer water temperatures in small eutrophic lakes of the Canadian prairies: potential implications for phytoplankton and zooplankton

Shallow, polymictic lakes with low heat storage capacity are especially susceptible to warmer spring conditions, predicted for a changing climate. In these lakes, atmosphere to water mass heat transfer is efficient as a result of high wind exposures and large surface areas relative to volumes. We examined effects of warmer water temperatures on phytoplankton and zooplankton abundance and species composition in three small eutrophic lakes (5.2-14.9 ha) of the Canadian prairies in Winnipeg, Manitoba, Canada, over two open-water seasons with contrasting spring weather conditions, i.e. 2005-a normal spring and 2006-a warm spring. Warmer spring and summer water temperatures were associated with decreased water transparency, increased phytoplankton biomass, increased relative filamentous cyanobacteria biomass and shifts in dominant genera from Aphanizomenon to Anabaena and Planktothrix. Zooplankton responded strongly; abundance of Daphnia (D. pulicaria, D. ambigua and D. parvula) decreased while rotifers, Skistodiaptomus oregonensis and Bosmina longirostris increased in abundance. Of several factors influencing phytoplankton dynamics, total dissolved nutrients [nitrogen (N), phosphorus (P) and N:P] and water column stability did not show important changes between years. In contrast, water temperature [described as the metric degree-days (degrees C day)] was related to changes in phytoplankton and % cyanobacteria biovolume. Daphniid abundance showed a significant negative relationship with an increase in filamentous cyanobacteria biomass and, thus we suggest, was indirectly associated with increased water temperatures.