Ocean warming, marine heatwaves and phytoplankton biomass: long-term trends in the Northern Adriatic Sea

In the present study, we investigated the trends of Sea Surface Temperature (SST), marine heatwaves (MHWs) and phytoplankton biomass, in terms of chlorophyll-a (chl-a), in the Northern Adriatic Sea (NAS), and the potential effects of ocean warming onto chl-a. Deseasonalized trends, calculated to assess the general tendencies without the effect of seasonality, showed a general increase of SST, and a decrease of chlorophyll-a concentration in almost the entire NAS. The strongest increase in SST was found along the eastern coast and in offshore areas, whereas the most pronounced decrease in chl-a was observed along the western coast, especially near the Po River delta. Seasonally, these trends were generally more marked in spring and summer. Spatial and seasonal variability in MHWs mean values and trends were also observed across the basin. A different response of chl-a to SST anomalies was highlighted over time, with negative correlations spreading throughout the NAS at subsequent time (one month later), together with positive correlations in eutrophic lagoonal areas. Different case studies and cluster analysis were used to assess the effects of ocean warming, also related to MHWs, on phytoplankton biomass. The relationships varied based on the background trophic conditions: in oligotrophic regions, marine heatwaves and extreme heat conditions led to reduced chlorophyll-a concentrations, while the same conditions in eutrophic areas, such as the western coast and lagoons, caused an increase in phytoplankton biomass. Our results indicated that MHWs and SST increases, are among the factors that are affecting the phytoplankton communities of the NAS.