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Mass mortality events associated with marine heat waves could be the new normal in the Mediterranean

The increase in the intensity and frequency of marine heat waves could be reconfiguring the main coastal marine ecosystems of the Mediterranean

From 2015 to 2019, the Mediterranean Sea experienced a series of marine heat waves that affected all regions of the basin and caused recurrent mass mortality events throughout the analyzed period. This is one of the main conclusions of a study published in the journal Global Change Biology in which a research team from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona participates. According to the conclusions, the populations of nearly 50 marine species (corals, sponges and macroalgae, among others) were affected by these events along thousands of kilometers of Mediterranean coasts, from the Alborán Sea to the coasts of the Proxim East.

"Unfortunately, the results of the work show that the Mediterranean Sea is experiencing an acceleration of the ecological impacts associated with climate change, which poses an unprecedented threat to the health and functioning of its ecosystems," Cristina Linares and Bernat Hereu explain. members of the Department of Evolutionary Biology, Ecology and Environmental Sciences of the UB and the IRBio, and co-authors of the new work.

"The impacts of mortality were observed between the surface and 45 meters deep, where the marine heat waves recorded were exceptional, affecting more than 90% of the Mediterranean and reaching temperatures of more than 26ºC in some areas », explains Joaquim Garrabou, researcher at the ICM-CSIC and one of the authors of the study.

In the work, led by the Institute of Sea Sciences (ICM-CSIC), the Center for Advanced Studies of Blanes (CEAB), the Mediterranean Institute for Advanced Studies (IMEDEA), the Spanish Institute have also participated of Oceanography (IEO), the University of Alicante (UA), the University of Seville (US), among other institutions.

The key species, the most affected

Some of the species most affected by episodes of mass mortality in the Mediterranean are key to maintaining the functioning and biodiversity of the main coastal habitats. For example, the Posidonia oceanica meadows or the coral populations, which make up some of the most emblematic habitats of the Mediterranean.

This is the first study to assess the effects of mass mortalities on a Mediterranean-wide scale over a period of five consecutive years. In total, more than 30 research groups from 11 countries have participated, which has made it possible to observe the incidence and severity of mortality in different regions of the Mediterranean basin. In fact, the most complete picture to date of the impacts of extreme warming events on Mediterranean organisms and ecosystems has been obtained.

From the exception to the norm

The climate crisis is seriously affecting marine ecosystems around the world and the Mediterranean is no exception. Specifically, the associated marine heat waves are causing episodes of massive mortality in all the coastal ecosystems of this basin as a result of the increased frequency, intensity and extent of the climate crisis.

"Given this scenario, it is essential to know the relationship between the different biological responses of marine biodiversity and the different levels of exposure to heat", points out professor Free Espinosa, from the University of Seville. For their part, David Díaz and Emma Cebrián, researchers from the IEO and the CEAB, respectively, regret that, until now, "the high variability of the responses observed between species and populations at very different spatial and temporal scales has undermined our ability to explore this relationship».

Now, thanks to the temporal and spatial resolution considered, it has been possible to demonstrate that there is a significant positive relationship between the duration of heat waves and the incidence of mortality events.

"The mass mortality events in the Mediterranean are equivalent to the bleaching events also observed consecutively in the Great Barrier Reef, which suggests that these episodes are now the norm rather than the exception," explains Professor Alfonso Ramos, from the University of Alicante.

For all this, the scientific community urges to strengthen coordination and cooperation at regional, national and international scale, as has been done in this work, to achieve more effective management decisions capable of dealing with the climate emergency.

This research has been carried out thanks to the support of the European projects H2020 MERCES, H2020Futurmares, InterregMED MPA-Engage, and the HEATMED National Plan project (RTI2018-095346-B-485 I00).

Reference article:

Garrabou, J.; Gómez-Gras, D. et al. «Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea». Global Change Biology, juliol de 2022. Doi: doi.org/10.1111/gcb.16301