Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea.

Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015-2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning. Overall, we show that increasing the resolution of empirical observation is critical to enhancing our ability to more effectively understand and manage the consequences of climate change.

Mass mortality hits gorgonian forests at Montecristo Island.

Mediterranean gorgonian forests are species-rich habitats, and like many other marine habitats they are threatened by anthropogenic disturbances and mass mortality events. These mortality events have often been linked to anomalies in the temperature profiles of the Mediterranean region. On 5 September 2017, colonies of the gorgonians Eunicella singularis and Eunicella cavolini exhibited rapid tissue loss, down to a depth of 30 m along the steep cliffs of Montecristo Island, Tuscan Archipelago National Park, Tyrrhenian Sea, Italy. Interestingly, Montecristo has previously been identified as a reference site for the ecological quality assessment of the western Mediterranean benthic assemblages on rocky bottoms. The observed mortality event occurred during a period of increased sea temperature. By utilising a combination of high-resolution oceanographic analysis, forecast models and citizen science initiatives, we propose that an early warning system for the concomitance of heat waves and mortality events can be put in place. A temperature-based coral disease surveillance tool could then be established for the entire Mediterranean Sea. Such a tool would allow for the timely study of mass mortality phenomena and the implementation of prompt mitigation and/or restoration initiatives. Finally, this specific mortality event, in a Marine Protected Area, offers a unique opportunity to monitor and assess the resilience of gorgonian populations and associated benthic assemblages in the absence of other, more directly, anthropogenic disturbances such as pollution and land runoff.