MEPS prepress abstract  -  doi: 10.3354/meps07401

ABSTRACT: We performed a detailed analysis of the food web structure of Laguna de Rocha, a temporally open coastal lagoon in the Southwest Atlantic characterised by spatial gradients in salinity, nutrient levels and trophic status. Carbon and nitrogen stable isotopes of main producers, invertebrate and vertebrate consumers, and stomach contents of ichthyofauna were analysed seasonally at freshwater (North, N) and marine influenced (South, S) sites to determine whether environmental differences induced changes in food web structure. Contribution of primary and secondary food sources and trophic linkages were assessed with the multisource-partitioning model IsoSource and an index that quantifies the contribution of each organism as food for the ecosystem (CFE). Isotopic analyses were performed for 10 primary organic matter sources (OM), 13 invertebrates and 9 fish species. Suspended and sediment OM constituted most important primary sources, while direct consumption of macrophytes and macroalgae was marginal. Small invertebrates Neomysis americana, Nephtys fluviatilis, Pseudodiaptomus richardii and an amphipod where the most important intermediate consumers at both sites (except P. richardii present only in the N). Top predators were fish Micropogonias furnieri, Paralichthys orbygnianus and Hoplias malabaricus, and maximum estimated trophic level (between 3.4 and 4.8) varied seasonally but not between sites. Fish stomach contents analyses largely confirmed results from the IsoSource mixing model. Overall results indicated that despite environmental differences between sites the structure of the biological assemblages and general trophic patterns were similar at both sites.