Contrasting effects of compositional shifts and resource fluctuations on phytoplankton nutrient uptake under warming

Understanding how global warming-induced changes in phytoplankton affect nutrient cycling is essential for assessing climate change impacts in freshwater environments. The present study used a multispecies model to explore the effect of temperature on nutrient uptake efficiency and its link with community structure and resource use-related traits. Simulations were run under four different annual temperature patterns (baseline to +3°C), along various nutrient loads representing a nitrogen-to-phosphorus (N:P) ratio gradient, and under constant and fluctuating supply. Warming led to more balanced annual nutrient uptake and increasing mean uptake efficiency. Under constant loads, this increase depended on the N:P ratio and on compositional shifts. In communities exhibiting high species turnover under warming, increase in nutrient uptake was constrained by higher minimum resource requirements (R*) during the nutrient-limited summer period. Higher fluctuations in nutrient loads enhanced warming-induced increases in uptake efficiency, independently of shifts in R*, also increasing annual net growth time. These results suggest that warming impacts on nutrient uptake can be traced back to multiple mechanisms: trait-mediated changes in competitive ability, and changes in the length of the growth period. Thus, the relative importance of community reassembly and environmental variability might have substantial influence on phytoplankton-mediated ecosystem responses to climate change.