Evidence of warming effects on phytoplankton productivity rates and their dependence on eutrophication status

Using 31‐yr data from measurements in a lake that has experienced change in eutrophication status, I showed that the effects of global warming on chlorophyll a (Chl a )‐normalized maximum rates of photosynthesis (P max : Chl a ) may be positive, nonsignificant, or negative, depending on nutrient availability. The magnitude of P max : Chl a change in response to warming showed hyperbolic relationships with phosphorus concentrations; it was positive and constant when total phosphorus (TP) in the lake water exceeded 22 µg P L −1 (eutrophic conditions) but was negative when TP was lower (nutrient‐poor conditions), indicating direct negative effects of warming on primary productivity (PP) under phosphorus scarcity. Vertically integrated PP responses corroborate those of P max : Chl a . These data also showed long‐term seasonal variations in the sensitivity of phytoplankton productivity to temperature. The observed hyperbolic curves strongly suggest that the "limiting‐nutrient cell quota"‐based mechanism reported so far only in laboratories (by studies analyzing temperature‐nutrient effects on microalgal growth or photosynthesis) operates in nature and plays a key role in determining phytoplankton response to warming of waters. The present findings provide insight on how phytoplankton productivity may respond to future warming in lakes of differing eutrophication status.