Temperature-dependent responses to light and nutrients in phytoplankton.

Nutrients and light are major resources controlling growth, biomass, and community structure of phytoplankton. When looking at those resources individually, resource uptake and biochemical transformation, and thereby also the demand for resources, have been shown to be temperature-dependent. However, there is still a lack of understanding of how temperature controls the response to multiple resources, although simultaneous limitation by multiple resources is common for single species and whole communities. We conducted a multifactorial, gradient-design experiment growing four freshwater phytoplankton species under 125 combinations of temperature, light, and nutrients (5 × 5 × 5 levels). In three of four species, we found evidence for an interactive effect of light and nutrients on growth that was modulated by temperature. The effect of high-level supply of both resources on algal growth rate generally exceeded the sum of their individual effects. Conversely, the lowest growth rates occurred not necessarily at the lowest level of both resources but at the most extreme light:nutrient supply ratios (either only light or nutrients were at highest supply level but the other resource remained at low supply). These interactive light-nutrient effects were modulated by temperature, resulting in highest growth rates when both resources and temperature were highest. Our study demonstrates that temperature modulates the magnitude of the interactive light-nutrient effect on phytoplankton growth. Consequently, these findings highlight the importance of considering temperature to understand the limitation by multiple resources and show that growth responses would be over- or underestimated when these interactions are not taken into account. Our results provide a first indication that the resource-dependent growth of phytoplankton will change in a warming world when considering multiple resources.