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.
(Read : Listen to posidonia seagrasses and Mediterranean Sea : sentinels that speaks the truth)

Posidonia is perfectly adapted to the marine environment but it has also kept characteristics that are typical of flowering plants on land. It can multiply by vegetative propagation (asexual reproduction: formation of young shoots on a rhizome  forming clones) and it can also multiply by sexual reproduction in the water (from a flower fertilized by pollen and production of fruit and then seeds). The multiplication of clones and sexual reproduction allow the species to increase the colonised surface and maintain the population by replacing old shoot.

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.

In fact, the studies conducted in Calvi Bay by the  Oceanology Laboratory on previous flowering events have shown that, not only does the flowering posidonia “concentrate” on its flowering to the detriment of its leaf production but it also needs more nutrients (these plants live in a very oligotrophic environment, that is to say, which is extremely poor in nutrients), which means that it has to draw on its stored reserves to initiate the springtime growth.

Samples are being taken in order to ascertain whether these repeated flowerings will result in exhaustion of the seagrass meadow.

See : http://hdl.handle.net/2268/11517 -  http://hdl.handle.net/2268/11516

2. A storm linked to a subtropical depression

At the beginning of October 2015, STARESO was into the the centre of a depression with winds reaching up to 110 kilometres per hour (Force 8 to 9 on the Beaufort scale). During the night of September 30th on the 1st of October 2015, a subtropical depression (with a hot-core) developed on the coasts of North Africa. This depression then passed over Eastern Corsica and crossed the island before reaching the sea between Corsica and the continent on the afternoon of the 2nd of October (see map). Gusts of wind reaching 100 to 130 km/h on the coast and in the mountains and 150 to 160 km/h at Cap Corse were recorded.

Impressive waves crashed into the STARESO station, our equipment measured violent winds.

See: http://hdl.handle.net/2268/181058

3. Stranding of groupers

During this same period at the beginning of October, 6 Epinephelus marginatus groupers beached on Calvi Bay, these individuals had glazed eyes, damaged skin and a swollen swim bladder (fish older than 10 years).

The results of anatomopathological analyses showed the presence of a nodavirus responsible for a viral encephalopathy and retinopathy (affecting the brain and the eyes). A month later, after an analysis of one of the groupers beached at Calvi in October confirmed an infection by the nodavirus (Viral Pathology Unit for fish, Plouzané-France), 16 other individuals presenting the same symptoms also died.

4. Invasive species

On the 20^th of July 2015, a small strange crab was found in shallow water around STARESO. We determined that it was a Percnon gibbesi. Introduced into the Mediterranean fifteen years ago, it is currently listed as an invasive species we collected it for the first time on the West coast of Corsica.

Conclusions 

Considered separately, these events may seem to be mere scientific curiosities, “general news items”, but:

-Posidonia flowering is linked to a greater quantity of light and/or an increase in temperature,
- The nodavirus, suspected of being particularly active during hot periods, it can be transmitted to other species of fish, furthermore fishes that survive remain carriers of the infection,
- The violence of the subtropical depression was fuelled by abnormally high surface water temperatures,
- The Mediterranean, which is a biodiversity hotspot, is being subjected to growing pressure from newly introduced species (carried by boats, aquaculture, the Suez Canal…) which can become invasive…

These extreme events are certainly linked to climate change. Aware of these changes, the oceanographers of  the University of Liege and of  STARESO station launched a interdisciplinary programme. STARESO is a marine reference site, an exceptional  area, well preserved, studying coastal marine environment since 1970.This STARE-CAPMED project (Station of Reference and research on Change of local and global Anthropogenic Pressures on Mediterranean Ecosystems Drifts )  aims to maintain and develop time series of the physical, chemical and biological properties of the Calvi Bay to understand the Mediterranean changes, climate variability, and ecosystem responses.

The  responses to current local and global changes due to natural and anthropic pressure.It should make it possible either to implement or verify relevant evaluation indicators, new research methodologies and the establishment of a reference basis with an operational orientation in real time, in order to supply data and scientific expertise making it possible to help manage coastal waters. (Coordinator: Pierre Lejeune; Director, STARESO President: Gérard Bonifacio)

See : http://hdl.handle.net/2268/58948 - http://hdl.handle.net/2268/187710