The Mediterranean is suffering
By Sylvie Gobert, Marc Binard et Pierre Lejeune
2015: exceptional biological observations and singular meteorological events were recorded at STARESO (Underwater and Oceanographic Research Station). The year 2015 was a particular year in more ways than one in Calvi Bay in Corsica: there was an exceptional flowering of the posidonia seagrass, a storm linked to a subtropical depression, massive grouper deaths due to a virus, a new invasive species…, these events, observed at STARESO are probably linked to the effects of climate change associated with the increasing anthropogenic pressure in the Mediterranean. Is the Mare Nostrum suffering?
1. A seagrass in full flowering
The seagrass (Posidonia oceanica (L.) Delile)is one of the rare flowering plants colonising the marine environment. This plant which makes the photosynthesis like other terrestrial plants, is essential for sustaining the ecological and economic services intoMediterranean sea because, in the marine realm, it plays the same role as forests on land (production of oxygen, stabilisation of sediments, it provides shelter, habitats and is a source of food for thousands of species). Posidonia forms seagrass meadows between the surface and depths of 40 metres.
In contrast to land plants, posidonia do not flower every year and the number of shoots which produce flowers (Flowering index) is very variable from one year to another. In addition, the presence of flowers does not necessarily lead to the development of seeds, fertilization does not always take place, the fruits can be aborted before reaching maturity, the seeds are subject to predation (by birds or fishes), the seeds before or after germination can be carried away by currents in areas that do not allow the plant to take root.
This seagrass, which has been the subject of study by the University of Liege and STARESO since 1975 in the Calvi Bay, seems to be showing the effects of climate change. While flowering was an extremely rare event during the eighties and was relatively sparse up to the nineties, for the last 20 years the rate and intensity of flowering has been increasing. Even more exceptional, since September 2015 (posidonia flowers in winter), an rare phenomenon has been observed where the seagrass is flowering from the surface to a depth of 25 metres with an intensity never before recorded (30 to 40% of the shoots flowered).
This change in the frequency and intensity of flowering by posidonia seagrass meadows is not without consequence, because, while sexual reproduction maintains genetic diversity and also increases the chances of recolonization of zones which have been altered by human activity, it also leads to morphological and physiological changes in the plant.
2. A storm linked to a subtropical depression