Idiosyncratic patterns of chlorophyll-a anomalies in response to marine heatwaves in the Adriatic Sea (Mediterranean Sea) over the last two decades.

In the open ocean, marine heatwaves (MHWs) have been associated to a decline of Chlorophyll-a (Chl-a) concentration in tropical and temperate areas while, at higher latitudes, they seem to enhance phytoplankton productivity. Currently, uncertainties remain on the outcomes of MHWs on primary production in coastal and heterogenous marine regions. We analyzed long-term modeled satellite-derived data on sea surface temperature and Chl-a concentration in the Adriatic Sea (Mediterranean Sea), a semi-enclosed basin where coastal and open-sea environmental conditions co-occur, to explore Chl-a responses to MHWs. We found that both low and high Chl-a anomalies were strictly dependent on MHWs, although following direct or inverse relationships in different areas, as a consequence of regional-scale heterogeneities in nutrient availability, riverine inputs, circulation and geomorphology. Along the west coast and shallow areas of the North and Central Adriatic, high MHWs frequency, duration and intensity corresponded to high frequency of Chl-a peaks and/or increased intensity and duration of low Chl-a anomalies, suggesting pronounced fluctuations with intense phytoplankton blooms alternating to extremely low production events. Conversely, in offshore and deeper areas, especially in the South Adriatic, MHWs frequency, duration and intensity inversely correlated with Chl-a anomalies, indicating a possible reduction of phytoplankton biomass and a decline of organic matter flow towards the sea floor. Prolonged MHWs may therefore drive shifts in primary production with possible ecosystem-wide effects in both coastal and pelagic areas. These multifaceted MHW-Chl-a interactions observed in the Adriatic Sea emphasize the need for context-specific assessments in environmentally complex marine regions to develop management strategies addressing ecological and socioeconomic issues arising from the unrelenting increase of temperature anomalies.