A monopoly for Gammarella

‘The goal of my doctoral research was to make the first comprehensive description of the community of mobile invertebrates living in the litter exported from Calvi Bay, and to determine to what extent this P. oceanica litter is a source of food for these invertebrates. Which invertebrates feed off them, and at what time of the year? What variations occur from one season to the next, and from one year to the next? Then, what other invertebrates in higher trophic levels consume these primary consumers and, through them, also indirectly consume dead P. oceanica leaves? In other words, I attempted to offer the first description of the trophic network within which the litter’s organic matter circulates.’ So every year from 2010 to 2012, during each season, François Remy carried out several sampling campaigns at two spots of Calvi Bay in order to gain detailed insight into the organisms that live there as well as their seasonal dynamics. Various types of sieve were used during the 9 diving campaigns in order to separate the target macrofauna (larger than 500 µm) from the accumulated leaves. At the same time, water samples were collected throughout this period both inside and outside the litter; their analysis in the lab would reveal their concentrations in nutrients and oxygen. From the Autumn of 2012 until the Autumn of 2014, other samples were collected on a weekly basis with a view to better understanding not the inter-seasonal or inter-annual variations, but rather the short-term random variations of the fauna living in the meadow’s litter, especially when a storm occurs. The scale of François Remy's work was unprecedented both in terms of duration and comprehensiveness, and it completes the individual studies that had already been carried out on this topic. After over 230 days of fieldwork, François Remy’s research activities had identified some 115 species of invertebrates, far more than the 45 to 80 species that had been inventoried in the literature up until that point. ‘This number seems high, but we should point out that it is three times lower than the total number of species living in the P. oceanica meadow. The litter is therefore not a very diverse environment, but it does host a large number of invertebrates, who are almost twice as abundant as in the meadow.’ Out of the 115 species identified, 77% are arthropods (among which, to the researcher’s surprise, one species is predominant: Gammarella fucicola, a small crustacean), with annelids (12%) and molluscs (7%) trailing far behind.

Oxygen, an important factor among others

‘One observation that stands out is that only 19 species make up 90% of this abundance of organisms,’ which means that the accumulated dead leaves are home to a few organisms that are entirely adapted to a rather inhospitable environment. Food is abundant, but, as we have mentioned, difficult to assimilate. More importantly, the conditions existing inside the litter have proven to be highly variable, requiring constant adaptations from the macrofauna living there. ‘At a depth of only ten metres, the litter is influenced by the movement of the waves: even in the smallest storm, the litter is tossed around by the current and sees its oxygen level rise and fall quickly, sometimes almost to the point of anoxia – the absence of oxygen –, in only a few hours.’ An experiment conducted in October, 2014, demonstrates that the existence of layers with different oxygen concentrations has a direct impact on invertebrates: some species are only present in the litter’s upper layers, where oxygen concentration is higher than in the layers that are closer to the sediments. Conversely, small crustaceans such as Nebalia strausi are present only in the layers of the litter that have low oxygen concentration: this keeps them away from predators and competition, and they seem to tolerate the lower oxygen concentration very well. As for the species that dominates the P. oceanica litter, Gammarella fucicola, it does not appear to be influenced by these changes, as it can be found in all layers of the litter. ‘The influence of oxygen concentration should be put into perspective,’ urges François Remy. ‘My research suggests that the variations only affect a dozen species out of the 115 that have been identified. The vast majority of species are not affected. Generally speaking, the presence or absence of certain species at certain times of the year is related to other factors, which were not studied here.’