Coastal lagoon ecosystems are spatially complex environments often characterized by strong gradients in physical and chemical conditions. Such gradients are reflected by differences in primary and secondary consumers with unique tolerances and adaptations, resulting in the occurrence of multiple zones within the larger lagoon ecosystem. While much is known about factors that affect the spatial and temporal distributions of organisms in lagoon ecosystems, very little information is available about food web connectivity across scales and the magnitude and direction of such linkages over time. We used stable isotopes of carbon and nitrogen to examine food web structure in three zones (freshwater, estuarine, and lagoon mouth) of Lagoa do Peixe National Park, Brazil. Monthly samples were collected over a period of one year to represent the aquatic food web at each location and dynamics over time. 684 samples of primary producers, macroinvertebrates, crustaceans, and fishes were analyzed for C and N isotope ratios.
Differences in baseline d13C, reflecting dominant production sources, resulted in consumers with depleted d13C values in the freshwater wetland compared with d13C-enriched values in the estuarine and lagoon mouth sites (P<0.001). Consumer functional guilds (i.e. estuarine resident, estuarine dependent, marine vagrant, primary freshwater, secondary freshwater) sorted strongly across this landscape, but a one-way movement of primary and secondary freshwater species from the freshwater zone into the estuarine zone occurred during a period of widespread flooding. Estuarine resident species were still collected in the estuarine zone during this flooding event, but did not move into the freshwater zone. Carbon isotope signatures of freshwater species collected in the estuarine zone gradually shifted from depleted to more enriched “estuarine” signatures over a period of approximately 4 months, indicating that these species were integrated into the estuarine food web. Estuarine species exhibited a slight shift in d13C in the opposite direction during the same period, likely reflecting consumption of material derived from the wetland site. Our results indicate that the freshwater, estuarine and lagoon mouth food webs are fairly isolated and dependent on local production sources during part of the year, essentially serving as compartments of the larger lagoon food web. However, flooding led to high connectivity of these compartments, primarily with organisms and energy moving from the freshwater zone to the estuarine zone. Human activities that reduce flood duration and intensity in this system may negatively affect estuarine communities by disrupting subsidies from other compartments.