Thermophilization but weak phosphorus-affinity shifts in freshwater phytoplankton communities

Eric Jiang, Yiping Zuo 

Climate warming and nutrient enrichment are reshaping many freshwater lakes, yet their relative roles in driving directional changes in phytoplankton communities across broad geographic gradients remain poorly understood. Using data from lakes repeatedly surveyed by the U.S. National Lakes Assessment in 2012, 2017, and 2022, we examined whether warming and phosphorus enrichment produced comparable shifts in phytoplankton community composition, environmental affinities, and cell size structure. Temporal compositional change increased with the magnitude of temperature change but was not significantly associated with changes in phosphorus concentration. Communities showed thermophilization, shifting toward taxa associated with warmer conditions as temperature increased, primarily through gains of warmer-affinity taxa. In contrast, phosphorus-affinity shifts were not significantly related to phosphorus change. Average community-level phytoplankton cell size declined with warming but did not respond to phosphorus change. These results suggest that temperature change drive more consistent taxonomic and functional reorganization than phosphorus change in freshwater phytoplankton communities.