A seasonal survey of the food web in the Lapalme Lagoon (northwestern Mediterranean) assessed by carbon and nitrogen stable isotope analysis

We used carbon and nitrogen stable isotope analysis to describe the food web of the Lapalme Lagoon, one of the best preserved coastal lagoons along the French Mediterranean coast. Three surveys, corresponding to contrasting situations both in terms of continental inputs and of connection between the lagoon and the open sea, were conducted in June and September 2004 and in February 2005. There were significant spatio-temporal changes in the isotopic ratios of both primary producers and consumers. Temporal changes were mostly linked to important 13C-depleted continental inputs caused by the long period of heavy rainfall before the June survey. Conversely, isotopic ratios were rather similar in September and February despite the opening of the connection of the lagoon with the sea between these two surveys. The interpretation of the results in terms of the structure of the trophic network differed between the June period and the two other ones. In September 2004 and February 2005, the food web was mostly based on SOM and lagoon POM pools with only a few consumers departing from this general trend. In June 2004, a significant proportion of consumers were conversely 13C-depleted probably due to the assimilation of significant amounts of continental inputs by grazers and filter-feeders. This stresses the necessity of carrying out seasonal surveys to get a sound idea of the structure of the food web in highly variable ecosystems such as coastal lagoons. Spatial changes in isotopic ratios were likely linked to: (1) the salinity/confinement gradient with a trend toward lower δ13C and δ15N values (i.e., higher continental influence) in the inner part of the main lagoon; and (2) the high δ15N values of primary producers and discretely motile consumers in a semi-confined site located downstream a vineyard receiving large amounts of fertilizers. We hypothesised that these inputs are quickly 15N-enriched through denitrification and ammonia volatilisation processes and then contribute to the 15N-enrichment of salt marsh plants and seagrass at this site. This interpretation is supported by the fact that such a 15N-enrichment of primary producers only occurred in February (i.e., after the dispersion of the fertilizers). Conversely, discretely motile consumers were 15N-enriched all year round, which suggests that they were mostly exploiting the detritic pool derived from 15N-enriched salt marsh plants.