Gorgonians belong to the large group of Cnidaria which includes, among other species, the corals, sea anemones or jellyfish. They colonize the bottom of seas and oceans all over the planet, from shallow coastal areas to underwater canyons, from temperate and tropical areas to polar areas.
Endowed with a more or less rigid horny or calcareous axis, they can form dense stands which structure the bottom, forming veritable "animal forests" which offer refuges to a large number of marine species.
One of the most beautiful underwater landscapes in the Mediterranean
In the Mediterranean, between the surface and 100 meters deep, 5 main species of gorgonians can be encountered. One of them, the purple gorgonian (Paramuricea clavata), form remarkable populations both for their colors and for the size of the colonies, which can exceed one meter in height.
It is one of the most beautiful underwater landscapes in the western Mediterranean, popular with both amateur divers and photographers. The colonies present are either male or female and all come from a ciliated larva (the planula). Their growth is slow (2 to 3 cm/year maximum) and their age reaches several decades.
“Animal forests” weakened by human activities…
Stands of purple gorgonians, like other gorgonian stands, are fragile and subject to pressures caused by human activities in the coastal zone. In the Mediterranean, these populations are also regularly impacted by the consequences of climate change.
The anchoring of pleasure boats (fishermen, divers) and the laying of fishing nets can thus cause the uprooting of these fragile colonies or cause injury (necrosis) to the living tissue covering the horny axis. The exposed areas are then colonized by a set of epibiont organisms that jeopardize the survival of the colony.
Hypersedimentation linked to coastal developments, changes in waterways and other discharges of sediments into the marine environment also constitute a threat to gorgonian populations.
… and by climate change
Along with the increase in human activities in the coastal zone, climate change has manifested itself, in recent decades, in the appearance of thermal anomalies in the water column linked to periods of absence of wind. (mistral on the coasts of Provence).
The consequence is a sinking of the “hot water” surface layers (temperature > 22°C) for long periods (several weeks). These warm waters are then at the origin of more or less significant mortality episodes within the populations of gorgonians usually exposed to colder waters (around 13 to 15°C).
In 1999, a large thermal anomaly has thus affected gorgonian populations in the western Mediterranean, from Spain to Italy. Apart from gorgonians, around twenty other species (sponges, bivalve molluscs, bryozoans, ascidians) were also affected.
This thermal anomaly was characterized by the presence for a month of a column of hot water (23 to 24°C) up to 40 to 60 meters deep. Other events of this type were then observed in 2003, 2006 or 2009, affecting the gorgonians to varying degrees.
If the increase in temperature is at the origin of these episodes of mortality, the mechanisms are multiple. They integrate the intervention of physiological processes, modifications of the microbiota associated with gorgonians, sometimes with the appearance of pathogenic agents, but also genetic factors conferring greater or lesser resistance to these thermal stresses.
Taking into account the major climate changes underway, a change in the geographical and bathymetric distribution of these structuring and heritage species is thus expected in the decades to come.
Discovering a unique deep stand
Although the distribution of gorgonian populations is quite well known down to about fifty meters in the Mediterranean, it is much less so deeper down.
A singular settlement was thus discovered a few decades ago off the Blue Coast (north harbor of Marseille) between 50 and 60 meters deep. This stand occupies a vast rocky plateau extending over nearly 2500 hectares.
It is characterized by a high density of gorgonians, but above all by the presence of giant colonies (between 1,50 and 1,80 meters in height). The presumed age of such colonies is probably around a century.
This gigantism and this unique concentration of colonies of Paramuricea clavata were the subject of two studies whose objectives were to understand the origin of these particular characteristics (REFUCLIM program) and to address more precisely their distribution (GIGOR program). The results seem to show that apart from their morphology these gorgonians are genetically unique and clearly distinct from those encountered less deeply.
The surrounding environment is very special. The gorgonians are not only regularly subjected to strong inputs of organic matter linked to the proximity of the Rhône plume, but the bottom currents are generally weak there. The combination of these two parameters allows gorgonians to reach exceptional sizes.
A refuge from global warming?
These stands of Paramuricea clavata constitute a remarkable heritage, protected for the moment from the warming of Mediterranean waters. Unfortunately, they are locally impacted by fishing and more particularly by recreational fishing. Indeed, the vast rocky plateau occupied by these giant gorgonians is a place of concentration of sea bream in the fall, fish coveted by amateur fishermen.
Up to 180 boats were counted there in one day (on October 29, 2016 following the count carried out by the blue coast marine park) during the sea bream gathering period. Boat anchors and fishing lines lead to the uprooting of many giant colonies. The management of the frequentation of this zone, included in a Natura 2000 area, therefore seems essential if we want to combine the survival of a unique natural heritage with the practice of human activities.
More generally, the study of deep gorgonian populations constitutes a major line of research in the years to come, not only to initiate appropriate protection measures, but also to better understand their connections with the populations occupying the most shallow depths.
One of the challenges is in particular to know in which case these deep stands can constitute refuge populations vis-à-vis global warming.
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