Differential sensitivity of planktonic trophic levels to extreme summer temperatures in boreal lakes

The stress–size hypothesis predicts that smaller organisms will be less sensitive to stress. Consequently, climate warming is expected to favour smaller taxa from lower trophic levels and smaller individuals within populations. To test these hypotheses, we surveyed zooplankton communities in 20 boreal lakes in Killarney Provincial Park, Canada during 2005 (an anomalously warm summer) and 2006 (a normal summer). Higher trophic levels had larger responses to warm temperatures supporting the stress–size hypothesis; however, rather than imposing negative effects, higher density and biomass were observed under warmer temperatures. As a result, larger taxa from higher trophic levels were disproportionately favoured with warming, precluding an expected shift towards smaller species. Proportionately greater increases in metabolic rates of larger organisms or altered biotic interactions (e.g. predation and competition) are possible explanations for shifts in biomass distribution. Warmer temperatures also favoured smaller individuals of the two most common species, in agreement with the stress–size hypothesis. Despite this, these populations had higher biomass in the warm summer. Therefore, reduced adult survivorship may have triggered these species to invest in reproduction over growth. Hence, warmer epilimnions, higher zooplankton biomass and smaller individuals within zooplankton populations may function as sensitive indicators of climate warming in boreal lakes.