Heat is driving mortality events in the Mediterranean

An international team of researchers led by the Institut de Ciències del Mar (ICM-CSIC) has found evidence that heatwaves in the Mediterranean Sea between 2015 and 2019 were the cause of recurring mass mortality events. The experts warn that these extreme conditions could become the norm, impacting all regions of the Mediterranean Basin.

The researchers discovered that populations of dozens of species – including sponges and corals – were affected by hotter water temperatures along thousands of kilometers of Mediterranean coastline. 

“Specifically, the impacts of mortalities were observed between the surface and 45 meters’ depth, where the recorded marine heat waves were exceptional, affecting more than 90% of the Mediterranean surface and reaching temperatures of more than 26ºC,” explained study co-author Joaquim Garrabou.

The experts found that some of the populations that were most affected by the heatwaves were keystone species, which are organisms that are central to the functioning of their ecosystems. The Mediterranean heatwaves were particularly damaging to Posidonia oceanica meadows and coral reefs.

According to the experts, this is the first study to assess the effects of mass mortalities on a Mediterranean scale over five consecutive years. The massive project involved more than 30 research groups from 11 countries. This made it possible to examine the impacts of extreme warming on marine organisms throughout the entire basin. 

“Unfortunately, the results of the work show that the Mediterranean Sea is experiencing an acceleration of ecological impacts associated with climate change, posing an unprecedented threat to the health and functioning of its ecosystems,” said study co-authors Cristina Linares and Bernat Hereu.

According to Professor Alfonso Ramos, mass mortality events in the Mediterranean are equivalent to the bleaching events also observed consecutively in the Great Barrier Reef, suggesting that these episodes are already the norm rather than the exception.

In order to reach more effective management decisions during the ongoing climate crisis, the researchers are urging for stronger coordination and cooperation at regional, national and international levels.

The study is published in the journal Global Change Biology.

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By Chrissy Sexton, Earth.com Staff Writer