Moderate and extreme warming under a varied resource supply alter the microzooplankton–phytoplankton coupling in North Sea coastal communities

Abstract

Rising temperature is one of the most visible effects of global change on Earth; however, it is barely known how moderate or extreme warming events impact the trophic interactions and the energy transfer in food webs. Combining a mesocosm approach and two‐point dilution incubations, we quantified how natural plankton assemblages respond to moderate and extreme warming (+6°C vs. +12°C above ambient temperature), covering a nitrogen‐to‐phosphorus gradient from nutrient‐saturated to limited conditions. We addressed how both drivers altered the community structure and mediated the phytoplankton growth (μ) and microzooplankton grazing (m) rates. Moderate and extreme warming effects on the microzooplankton–phytoplankton relationship differed and were mediated by time. This trophic interaction was weakened due to μ outpacing m regardless of the warming treatment at the middle of the experiment. By contrast, after the acclimation period, the trophic interaction was strengthened by increased grazing under extreme warming. The variable grazing pressure found at different temporal scales only under extreme warming could be due to a decreased microzooplankton grazing pressure with increasing temperature when prey biomass is low, and vice versa. Also, it could be a consequence of a switch toward mixotrophy or that the temperatures experienced by grazers were suboptimal compared to their prey. Finally, we found that temperature was the main driver whereas resource availability played a minor role in this trophic interaction. As climate change will intensify in the future, food webs could be less productive but more efficient, and thus, potentially support a higher secondary production.