Dataset from an in situ mesocosm experiment testing the effects of a marine heatwave on a Mediterranean coastal plankton community

Marine heatwaves are projected to increase in frequency and intensity in the Mediterranean Sea under global warming. To assess their effects on the structure and functioning of plankton communities a 20-day in situ mesocosm experiment was conducted in May and June 2019 in the Mediterranean coastal Thau Lagoon (43°24’53’’N, 3°41’16’’E, south of France). A simulated heatwave was applied by increasing the temperature of three mesocosms (HW treatment) by +3 °C for the first 10 days of the experiment, then heating was then turned off for an additional 10 days and the HW mesocosms experienced the same temperature as three other mesocosms (control condition) which remained at natural ambient lagoon temperature for the entire experiment. High-frequency automated sensors in all mesocosms were used to measure dissolved oxygen concentrations and saturations, chlorophyll-a fluorescence, photosynthetically active radiation, salinity, and water temperature. These high-frequency monitoring were complemented by manual daily sampling for nutrients and phytoplankton pigment concentrations. These data allowed the estimation of key functional processes as oxygen concentrations were used to estimate gross primary production (GPP) and community respiration (R), while chlorophyll fluorescence data were used to estimate phytoplankton growth (μ) and loss (l) rates, using the methods of Soulié et al. (2021, 2022). Phytoplankton community composition was assessed based on chemotaxonomic pigment concentrations measured with high performance liquid chromatography and the method of Zapata et al. (2000).