Dataset from an in situ mesocosm experiment evaluating the effects of a simulated marine heatwave on the structure and composition of the plankton food web in a Mediterranean coastal lagoon

Coastal ecosystems are particularly affected by marine heatwaves (MHW) which are predicted to become more frequent and stronger by the end of the century. These extreme events potentially impact marine organisms including plankton communities and their dynamics.  The present study aims to provide a more comprehensive understanding of how each plankton food web component and their interactions will respond to a simulated MHW. For this study, an in situ mesocosm experiment was carried out from May 24th to June 12th, 2019, in the Thau lagoon on the French Mediterranean coast (43.414699N, 3.688125E). Two conditions were applied in triplicate. One condition simulated a MHW of + 3 °C above the natural lagoon temperature for 10 days, followed by a return to the natural lagoon temperature for the next 10 days (MHW condition), while the other condition maintained at natural lagoon temperatures throughout the experiment (control condition). Over the 20 days of experiment, the sampling of plankton communities in both conditions was conducted daily or every two days to study their responses to a simulated MHW.  The abundance of virus-like (hereafter referred to as virus), heterotrophic bacteria, cyanobacteria, pico- and nanophytoplankton eukaryote population were identified using flow cytometer in cells·mL-1. Phytoplankton and ciliates abundances (cells·mL-1) and taxonomic composition were identified by an inverted microscope. Abundance of heterotrophic nanoflagellates was counted by an epifluorescence microscope in cells·mL-1. It should be noted that in some of the related papers, some groups of the same organisms have been grouped within a single group (example, two groups of virus-like).