Filamentous green algae inhibit phytoplankton with enhanced effects when lakes get warmer

P>1. Filamentous green algae (FGA) may represent an alternative state in high-nutrient shallow temperate lakes. Furthermore, a clear water state is sometimes associated with the dominance of FGA; however, the mechanisms involved remain uncertain. 2. We hypothesised that FGA may promote a clear water state by directly suppressing phytoplankton growth, mostly via the release of allelochemicals, and that this interaction may be affected by temperature. 3. We examined the relationships between FGA, phytoplanktonic chlorophyll a concentrations and zooplankton in a series of mesocosms (2.8 m3) mimicking enriched shallow ponds now and in a future warmer climate (0 and c. 5 degrees C above ambient temperatures). We then tested the potential allelopathic effects of FGA (Cladophora sp. and Spirogyra sp.) on phytoplankton using several short-term microcosms and laboratory experiments. 4. Mesocosms with FGA evidenced lower phytoplanktonic chlorophyll a concentrations than those without. Zooplankton and zooplankton : phytoplankton biomass ratios did not differ between mesocosms with and without FGA, suggesting that grazing was not responsible for the negative effects on phytoplanktonic biomass (chlorophyll a). 5. Our field microcosm experiments demonstrated that FGA strongly suppressed the growth of natural phytoplankton at non-limiting nutrient conditions and regardless of phytoplankton initial concentrations or micronutrients addition. Furthermore, we found that the negative effect of FGA on phytoplankton growth increased up to 49% under high incubation temperatures. The experiment performed using FGA filtrates confirmed that the inhibitory effect of FGA on phytoplankton may be attributed to allelochemicals. 6. Our results suggest that FGA control of phytoplankton growth may be an important mechanism for stabilising clear water in shallow temperate lakes dominated by FGA and that FGA may play a larger role when lakes get warmer.