Nutrient limitation constrains thermal tolerance in freshwater phytoplankton

Thermal tolerance can depend critically on environmental context (e.g., resource availability and biotic interactions), yet it is often measured only under idealized conditions. Here, we investigated how the concentration of phosphate (a limiting resource for algal growth in freshwater ecosystems) influences the thermal optimum for growth rate in five species of freshwater phytoplankton. We found that low-phosphate concentrations led to a sharp decline in species' thermal optima, by up to 15 degrees C relative to replete conditions, with the magnitude of the decline varying between species. Rapid global environmental change is expected to lead to rising temperatures, while nutrient concentrations in freshwaters are forecast to increase in waterbodies subject to eutrophication and decline in large lentic systems that become warmer and more stratified. Our findings suggest that phytoplankton that experience warming and nutrient limitation concurrently will be more vulnerable to environmental change.