Sediment features, macrozoobenthic assemblages and trophic relationships (δ13C and δ15N analysis) following a dystrophic event with anoxia and sulphide development in the Santa Giusta lagoon (western Sardinia, Italy)

Macrozoobenthic assemblages and stable carbon (d13C) and nitrogen (d15N) isotope values of various primary producers (macroalgaeand angiosperms) and consumers (macroinvertebrate filter/suspension feeders, deposit feeders, detritivores/omnivores and carnivoresand fishes) were studied in the Santa Giusta lagoon (Sardinia, Italy) before (spring) and after (autumn) a dystrophic event which occurredin the summer of 2004. A few days after the dystrophy, the physico-chemical characteristics of sediments and macrozoobenthic assemblageswere also investigated. In the latter occasion, high total organic carbon (3.9%) and organic matter (15.9%) contents of surfacesediments went together with peaks in acid-volatile sulphide concentrations. Certain immediate effects were quite extreme, such as thedrastic reduction in macrozoobenthos and the massive fish kill in August 2004. Among the macrozoobenthos, there were few individualsof chironomid larvae and Capitella cf. capitata left. However, by October, chironomid larvae were numerous, indicating a lack of predators(e.g. fish) and competitors. In addition, some bivalve species and polychaetes which were absent, or present in small numbersbefore the event, became relatively numerous. The results are discussed based on a knowledge of the sulphide tolerance of these species.Stable isotope analysis clearly showed that the basal level of the food web for most consumers consisted mainly of macroalgae and sedimentaryorganic matter, and that the values before and after the dystrophic event were not significantly different from one another. Thisindicates that the relations among different trophic levels were quickly restored following the dystrophic event.