This is the main conclusion of a study in collaboration with the ICM-CSIC that has been published recently in the journal Global Change Biology.
A new study in which the Institut de Ciències del Mar (ICM-CSIC) has participated has unveiled that 80% of the coral reefs in the Calanques National Park, in Marseille (France), are in a very fragile condition due to the exceptional marine heat wave recorded in the northwestern Mediterranean during the 2022 summer, which especially affected organisms living above 30 meters deep.
Specifically, the study results, recently published in the prestigious journal Global Change Biology, show an unprecedented impact on the populations of two very important coral species from an ecological point of view: the red gorgonian (Paramuricea clavata) and the red coral (Corallium rubrum), which provide essential complex habitats for a great diversity of associated fauna.
To carry out the study, researchers conducted censuses in 148 populations located between 0 and 40 meters deep. Thanks to this, they were able to see that 80% of the colonies analyzed show signs of mortality, especially above 30 meters depth in both species.
"This is mainly due to climate change, which is causing a significant increase in the frequency, extent and intensity of marine heat waves. These are both abnormally long periods of warm water temperatures and shorter periods of extremely high temperatures," explains the leading author of the study, Tristan Estaque, from Septentrion Environment.
The 2022 marine heat wave
Last summer, the northwestern Mediterranean area experienced one of the most severe marine heat waves since satellite data on seawater surface temperature became available some 40 years ago. This heat wave lasted from June to September, with temperatures in some locations reaching 3 to 5 degrees above normal. For example, some areas of the French and Balearic coasts reached 30 degrees at the surface.
In addition, according to the study, record temperatures were reached in the Calanques National Park not only at the surface, but also at greater depths. Thus, for example, it reached 27.4 degrees at 20 meters depth, the highest figure in Les Calanques in the last two decades. These temperature conditions affected, in addition to the two coral species studied, other organisms such as sponges, mollusks, other invertebrates and even limestone algae.
"These are benthic species that contribute to the formation of the coralligenous habitat, one of the most emblematic and diverse in the Mediterranean," states the ICM-CSIC researcher Joaquim Garrabou, who also participated in this study, which is the first to assess the impact of the exceptional marine heat wave of 2022.
This work is part of the activities of the T-MEDNet network, coordinated from the ICM-CSIC to monitor the effects of climate change on Mediterranean coastal ecosystems. Currently, more than 50 research and management teams of marine protected areas from 11 Mediterranean countries are participating.
In addition to monitoring the state of ecosystems, for more than 20 years this network has been promoting the acquisition of temperature series down to 40 meters depth. Thus, for example, it has been possible to detect that during 2022 a record was broken considering the number of days with temperatures above 25 degrees at different depths. In this regard, the scientific team assures that "this is a particularly critical situation", since this temperature is the maximum that the coral species studied can withstand.
As for the recovery of the organisms, the authors of the study warn that it is "extremely difficult" considering the high level of impact and the low population dynamics of the impacted species. In this sense, they state that "the mortality episode observed in 2022 will probably lead to the collapse of the shallowest populations and could relocate the upper limit of distribution from 10 to 30 meters depth".
"The 2022 mortality episode is new and dramatic evidence of the accelerating process of climate change effects, not only in the Mediterranean but also in the ocean in general. This process is leaving us with new scenarios with marine habitats that are rapidly transforming", concludes the scientific team, which against this backdrop calls for decisive action to buffer and reverse the effects of climate change.