Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river.

The polychaete fauna of muddy bottoms off the Rhone delta (NW Mediterranean) was seasonally sampled at two sites at 70 m depth, from 1993 to 1996. During this period, five severe flood events occurred. A clustering analysis (distance coefficient of Whittaker) and three way fixed factor ANOVAs (site x season x year) showed strong year-to-year changes in species density and community structure, changes that masked seasonal variations. Total density increased by a factor of 3 and density of most species significantly increased following the first flooding event. Changes in the community structure were due to the successive dominance of a few species. The opportunistic species, such as Cossura sp., Mediomastus sp. and Polycirrus sp., exhibited peaks in density 1-3 months after the flood. These peaks were followed by a drastic decline. For species with a long life span, such as Sternaspis scutata, a slower but continuous increase in density was observed which was maintained for several months. The density fluctuations of these species exhibited a good correlation with river flow with time lags of 1-2 years. The successional dynamics observed are explained according to the geographical origin of the floods and the biology and feeding ecology of species.

Fluctuations of macrobenthic populations: a link between climate-driven river run-off and sole fishery yields in the Gulf of Lions.

The Rhone river is the most important input to the Mediterranean Sea, responsible for 50% of the primary productivity of the Gulf of Lions. A highly variable amount of 1-23×106 t year-1 of terrestrial material is exported to the sea by the Rhone and stocked on the continental shelf for the most part. Soft-bottom communities off the Rhone delta were dominated by polychaetes both in species richness and abundance, and exhibited strong temporal fluctuations mainly related to flooding events. Floods caused pulses of organic matter followed, with different time lags, by peaks of polychaetes. Opportunistic, short-lived species, such as Mediomastus sp. and Aricidea claudiae, exhibited high short-term peaks in density and biomass a few months after flooding events. Conversely, long-lived species, such as Laonice cirrata and Sternaspis scutata, peaked in density and biomass with a time lag of 1-3 years, and their population increase lasted for a few years. The common sole, Solea solea, is a voracious predator of polychaetes which represent >80% of its prey. A positive correlation was found between the mean annual discharge of the Rhone river and the annual commercial landings of S. solea with a time lag of 5 years in the two fishing harbours (Sete and Martigues) located close to the Rhone delta. The long-term increase in food (i.e. polychaete density and biomass) after flooding events might favour the different stages of the sole life cycle, enhancing its population size for several years. Fluctuations of sole fishery yields in the Gulf of Lions could be influenced by climate, as the Rhone river flow is related to the North Atlantic Oscillation that drives precipitation over Western Europe.