Moderate weather extremes alter phytoplankton diversity—A microcosm study

Moderate weather extremes are predicted to increase in frequency and intensity as a function of climate change. These extremes can result in sudden changes in temperature, light and nutrient levels, which may strongly affect the phytoplankton community in many lakes. We conducted a microcosm experiment with eight freshwater algal species, representing different functional groups, to investigate the interactive effects of these environmental variables on a phytoplankton community typical of an oligo‐mesotrophic temperate lake. We manipulated temperature (6, 16 and 26°C), light (18, 56, 132 μmol m −2  s −1 ) and soluble reactive phosphorus (0, 1, 5 μg/l) concentrations in a 3 × 3 × 3 factorial design and analysed treatment effects on algal biomass, species diversity (SDiv) and functional diversity (FDiv). Strong temperature variations affected phytoplankton biomass and diversity more strongly than moderate changes in nutrient and light levels. In particular, SDiv and FDiv decreased significantly with increasing temperatures. Our results support earlier findings that lake warming favours heterotrophic bacteria, relative to phytoplankton. We conclude that sudden temperature increases as they typically occur in temperate lakes during heat waves may decrease phytoplankton diversity, leading to altered ecosystem functioning. In contrast, heavy rain events usually characterised by decreasing levels of temperature and light intensities, concurrent with increases in nutrient concentrations, may sustain high phytoplankton diversity.