A few days ago, a man from Blanes (Girona) reported the presence of a dead dolphin at S’Abanell beach. It was of the species Stenella coeruleoalba, one of the most common in the Mediterranean Sea. In order to determine the cause of death, the animal was transferred to the Faculty of Veterinary Medicine of the Universitat Autònoma de Barcelona (UAB).

After performing the necropsy, the body was taken to the OBSEA Underwater Observatory in Vilanova i la Geltrú (Barcelona), where a group of researchers from the Institute of Marine Sciences (ICM-CSIC), the UAB’s Faculty of Veterinary Medicine, the Catalan Ministry of Territory and Sustainability and the SARTI-MAR group of the Universitat Politècnica de Catalunya (UPC), which operates the OBSEA, anchored it in front of the Observatory’s video surveillance camera to study its effect on the benthic ecosystems and fish communities, including commercial species, in the area.

This information will be complemented with data on the temperature and salinity of the water, as well as the speed and direction of currents at this point, among other environmental variables.

"Over the last few years we have studied the presence and absence of species according to these variables, but now the presence of the dolphin may alter the results we have obtained up to now and attract species that we would not see under normal conditions", says Joaquín del Río, director of the SARTI-MAR research group and head of the OBSEA.

"All of this will help to interpret the environmental processes associated with the decomposition of a carcass in coastal environments with innovative technology", add the ICM-CSIC researchers Jordi Griñó and Jacopo Aguzzi.

This type of experiment had not yet been carried out in Mediterranean waters, so the main actors involved in the decomposition of marine mammal carcasses in the Mediterranean, which can range from bacteria to cephalopods and predators such as sharks, remain unknown.

In other areas of the world, however, similar studies have been carried out on deep-sea benthic ecosystems. In 2002, for example, scientists at the Monterey Bay Aquarium Research Institute (MBARI) in California (USA) were able to observe, thanks to an underwater robot, how a dead whale on the seabed caused major spatial and temporal changes in the local marine community. Also, in 2017, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out, together with the ICM-CSIC, the same observations with a whale carcass located at a depth of 500 metres.

"High-frequency, real-time monitoring of the ecosystem’s evolution is possible thanks to the current infrastructure of the OBSEA Observatory, where research projects such as RESBIO (UPC-ICM), which precisely enable remote monitoring of biological indicators, have been carried out in recent years", explains del Río.

The fall of a marine mammal onto the seabed can provide an amount of organic matter equivalent to that which falls over several thousand years through the water column in the form of marine snow, which is made up of all the plants and animals that die in the ocean and are not consumed by predators.

As has been observed on other occasions, when a marine mammal falls to the bottom, scavengers can gobble up the flesh that covers the bones of the carcass in a few days. From then on, different populations of polychaete worms, crustaceans, molluscs and other opportunistic invertebrates will colonise the surface of the bones in a few months.

Finally, when these communities have ingested all of the easily digestible organic material, the bacterial community will come into play, taking advantage of the fats and oils found inside the bones of the carcass. At the same time, this process will release compounds, in this case chemosynthetics, that will serve to feed other types of microorganisms.