Warming enhances primary production and respiration and changes plankton community structure in an estuarine upwelling system

Abstract Marine heatwaves are expected to become more frequent and intense due to global climate change, potentially impacting plankton communities. In the present study, the effects of a simulated short‐term warming event on the structure and functioning of coastal plankton communities were investigated during a mesocosm experiment conducted with water from the Ría de Vigo, a productive estuary strongly influenced by coastal upwelling dynamics from the North‐Western (NW) Iberian Peninsula. A warming event was simulated in triplicate mesocosms by increasing their water temperature at +3°C compared to three other mesocosms, which followed the natural temperature of the Ría. High‐frequency monitoring from sensors immersed in all mesocosms was used to estimate gross primary production (GPP), community respiration ( R ), phytoplankton growth ( μ ), and loss ( L ) rates. Phytoplankton size structure and community composition, photosynthetic efficiency, and inorganic nutrient concentrations were assessed from daily manual sampling of the mesocosms. Warming induced a mismatch between phytoplankton growth and its grazers, as it enhanced μ at the beginning of the experiment, while L only increased after a few days. Warming also significantly enhanced both GPP and R , but the metabolic index (GPP : R ratio) was not significantly affected by warming, as both GPP and R were enhanced to a similar extent. Warming altered the phytoplankton community structure by favoring haptophytes at the expense of diatoms and dinoflagellates. Conducted during phosphorus‐depleted post‐upwelling conditions, this experiment highlights the positive effects of short‐term warming events on the metabolic and trophic functioning of plankton communities from coastal upwelling systems.