Phytoplankton thermal response curves in natural communities of different origins

Thermal response curves have been increasingly used to estimate phytoplankton thermal sensitivity to predict the effect of climate change on species performance and persistence. However, little is known about the shape of thermal responses at the community level, where species composition and diversity might modify thermal dependence. We tested how performance (growth rates) and standing stock (maximum biomass) respond to thermal gradients in natural freshwater phytoplankton communities. Five microcosm experiments were conducted using two systems (one tropical and one subtropical) sampled at different time points. Communities from the subtropical system were additionally exposed to different nutrient levels. We found that thermal performance curves showed a general unimodal (or ascendant) shape as previously reported for populations. Interestingly, maximum biomass showed a similar (unimodal) trend, which has been proposed theoretically but not shown experimentally. Additionally, lower nutrient availability decreased the thermal optimum and thermal sensitivity of growth rate and maximum biomass. Overall, our results showed that thermal gradients and temperature x nutrient interactions significantly affect performance and biomass, indicating that thermal dependence is not masked by the community context. These results are key for scaling up the effects of temperature changes to higher levels of biological organization.