Long-term impacts of climate change on the seasonal trajectories of phytoplankton in a coastal lagoon (western Mediterranean Sea)

In recent decades, oligotrophication and climate change have impacted the structure and functioning of aquatic ecosystems. The combined effects of these two forcing have been studied little, particularly in Mediterranean coastal lagoons, which are considered vulnerable sentinel ecosystems that harbor numerous ecosystem services. Phytoplankton communities are a sensitive indicator of hydroclimatic changes; however there is a lack of knowledge regarding long-term changes in their trends and seasonality. Using time-series analyses from 22 years of environmental monitoring in a large coastal lagoon (Thau lagoon), we examined trends in hydrological and climatic variables, as well as phytoplankton biomass and microphytoplankton abundance. We examined the seasonal patterns of 56 dominant taxonomic units (TUs) of diatoms and dinoflagellates, identifying the primary hydrological and climatic variables that determine changes in their seasonality. Our results reveal an unprecedented increase in seawater temperature of +0.09 °C yr −1 from 1998 to 2019. Diatom abundance decreased significantly over the last decade, with earlier peaks. The three main planktonic diatoms ( Skeletonema , Pseudo-nitzschia and Chaetoceros ), which are dominant in spring and winter, all showed decreasing trends and unprecedented changes in their seasonality for first two. In contrast, benthic diatoms ( Cylindrotheca , Nitszchia , Hantzschia ) and two other summer taxa increased steadily. Seawater temperature and solar radiation appear to be the main drivers of changes in community seasonality. Results from long-term phytoplankton monitoring are essential for identifying future changes in coastal ecosystems, as such changes could have cascading impacts on the entire food web and substantial economic repercussions. • Warming of the lagoon by +0.09 °C yr −1 from 1998 to 2019. • Seawater temperature was the main driver of phytoplankton seasonality. • Planktonic diatom abundance decreased significantly and showed earlier peaks. • Increase in benthic diatoms ( Cylindrotheca ) during the summer. • Skeletonema 's ecological niche reduced to winter.