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 (Pmax : Chl a) may be positive, nonsignificant, or negative, depending on nutrient availability. The magnitude of Pmax : 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 Pmax : 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.

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