Depth variation in terrestrial particulate organic matter exploitation by marine coastal benthic communities off the Rhone River delta (NW Mediterranean)

Carbon and nitrogen stable isotopes were used to study depth variation in terrestrial particulate organic matter (POM) sedimentation off the mouth of the Rhone River delta, and its use by the main benthic invertebrates (polychaetes, crustaceans, molluscs and echinoderms) and flatfishes (Arnoglossus laterna, Buglossidium luteum and Solea solea). Coastal water POM and surface sediment exhibited low delta(13)C values, denoting dominance of terrestrial material in their carbon pools (25 to 80% and >50%, respectively), but the importance of river input differed as a function of depth. Terrestrial organic matter contribution to the total POM was greatest at 30-50 m (72 to 99%), and least at 70-100 in depth (25 to 77%). Most of the invertebrate groups studied (delta(13)C = -24.5 to -17.5parts per thousand; delta(15)N = 4.3 to 9.7parts per thousand) mainly relied on marine primary production irrespective of depth. However, deposit-feeding polychaetes mainly exploited terrestrial POM, and carnivorous and suspension-feeding polychaetes, bivalves, brachyurans and shrimps (Caridea) showed a partial uptake of terrestrial POM, depending on its availability. Incorporation of terrestrial POM by flatfishes (delta(13)C = -22.4 to -16.8parts per thousand; delta(15)N = 9.7 to 11.2parts per thousand.) was species- and depth-dependent. It was related not only to the fishes' diets but also to the trophic adaptability of their prey. S. solea, the main consumer of deposit-feeding polychaetes, showed the lowest VC irrespective of depth, but flatfish exploitation of terrestrial POM peaked at 30-50 m, where both polychaete consumption by fishes and terrestrial POM use by the benthos were maximal. Unlike most estuarine systems, terrestrial inputs in deltaic areas can therefore peak at intermediate depths, where some of the marine macrobenthos, including flatfishes, efficiently exploit them. These results allow better assessment of the role of river input to marine coastal zones in increasing fish abundance, and will therefore be of major interest for demersal fisheries management off deltaic areas.