2023-09-19 18:46:59
DECRYPTION – Last year’s marine heatwaves caused massive mortality among these fragile animals.
When the word “gorgon” is mentioned, the image of this evil mythological creature with snake hair appears. The one that populates the waters of the Mediterranean is much more graceful… and much more vulnerable. A study published on the scientific journal website Global Change Biology looks back at the catastrophic impact of the marine heatwaves of 2022 on red gorgonians, which are found particularly in coastal waters.
Distant cousins of corals, gorgonians belong to the animal kingdom. « These are colonies made up of several polyps (organisms) with eight flexible tentacles, explains Bastien Mérigot, lecturer at the University of Montpellier and co-author of the study. They obtain their food by filtering nutrients in the water and create a skeleton which is adorned with red, yellow or white tissue depending on’they belong to one or the’another of the three main species found in the Mediterranean. » This gives it the appearance of a shrub.
Read alsoCuriously, deep-sea fish do not suffer from underwater heatwaves
Franco-Spanish study focuses on red gorgonian populations (Paramuricea clavata) of the Marseille Calanques National Park, hit last year by a mass mortality event. These episodes are not new: those of 1999, 2003 or 2006 are remembered. But last year’s is « one of the strongest ever recorded in the region », explain the authors of this publication.
Waters above 25°C for a month
The cause is the marine heat waves recorded in the Mediterranean throughout the summer of 2022. The average temperature of surface waters was between 1.3°C and 2.6°C above the average (1982-2011 ). In the creeks, « the threshold of 25 °C has been exceeded for 32 days in a row at 5 meters deep », notes Bastien Mérigot, who is also a researcher at UMR Marbec, a research unit on marine biodiversity. This heat even reached the depths: it was more than 25°C at a depth of 30 meters for three days.
Also read: The surface of the seas and oceans is overheating
The gorgons were « dramatically affected » by these heatwaves, according to the study, the result of the observation of 26 different sites (and thousands of organisms). Between the surface and 20 meters depth, more than 80% of the colonies were partially or entirely necrotic: their colored envelope had disappeared, leaving only a rickety and whitish skeleton exposed. « On some sites, the gorgonians were all dead », notes Bastien Mérigot. In deeper waters (between 20 and 30 meters) « the impact was only slightly lower, affecting more than 65 % des colonies », also note the researchers. According to them, this « impact was 142 % higher than during the 2003 mass mortality event ».
Several hypotheses have been formulated in previous studies to explain the deleterious effect of heat on gorgonians: a reaction to oxidative stress, exhaustion due to thermal shock which would make them unable to feed, the decline of a bacteria which they use to defend themselves against others microbes…
An entire ecosystem threatened
The fact is that « When a gorgon is completely necrotic, it does not grow back. Recolonization can only take place thanks to other populations further away or deeper (during spawning, the gorgonians emit gametes into the water, and the larva will settle on the ground) », eexplains the author of the study. He also points out that the growth of gorgonians is very low, of the order of 1 to 5 cm per year. « Rebuilding gorgon populations affected by the 2022 heatwave will be extremely difficult, if not impossible », therefore conclude the scientists.
With their disappearance, an entire ecosystem is threatened: gorgonians in fact serve as shelter for many molluscs and fish, such as sea bream or sea bream.
Gorgonians are home to a number of species of molluscs and fish. Adobe Stock
After an extraordinary summer in 2022, Mediterranean waters were once again exceptionally warm in 2023 (with 28.4°C on average recorded on July 24, a record). « The warming trend in the Mediterranean is + 0,4 °C per decade over the period 1982-2020, explains Thibault Guinaldo, researcher in spatial oceanography at the National Center for Meteorological Research (Météo-France/CNRS), in Lannion. Thus, global warming causes an increase in frequency and intensity of marine heat waves. » We must not forget the influence of the natural variability of the system, he nevertheless reminds us: “ Mistral and tramontane conditions can, for example, limit this warming locally, without changing the underlying trend. »
According to the IPCCon a global scale, « marine heat waves (…) have doubled in frequency and become more durable, more intense and more widespread ». UN climate experts predict up to 50 times more marine heatwaves by 2100, depending on the greenhouse gas emissions scenario.
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