Life cycle, secondary production and nutrient stock in Heleobia australis (dʹOrbigny 1835) (Gastropoda: Hydrobiidae) in a tropical coastal lagoon

The aim of this research was to evaluate density, biomass, life cycle, secondary production and nutrient stock of Heleobia australis population (d’Orbigny 1835) in a coastal tropical lagoon (Imboassica lagoon). Carbon (C), nitrogen (N), phosphorus (P) and organic matter (OM) concentrations in the sediment were determined in order to evaluate their influence on the density, biomass and concentrations of C, N, P and OM in the biomass of H. australis population. Two sampling stations were established: one in the central part of the lagoon (station 1, not influenced directly by sewage release) and the second near the littoral region (station 2, close to the outlet of a canal discharging domestic sewage). Triplicate samples were collected monthly for one year (from May 1999 to April 2000) with a ‘‘core’’ sampler for determination of density, biomass, life cycle and secondary production of H. australis. For determination of C, N, P and OM in both sediment and H. australis, the samples were carried out in May and November 1999, and in April 2000. Density was significantly lower at station 1, whereas biomass did not differ significantly between the stations. Secondary production at station 1 was 28.33 g ash-free dry weight (AFDW) m 2 year 1 and at station 2 it was 49.36 g (AFDW) m 2 year 1. The concentrations of OM, C and P in the sediment and N and P in the biomass of the organisms were higher at station 2. The release of domestic effluents into this lagoon results in an increase in OM, C and P concentrations in the sediment which are reflected in H. australis chemical composition. Considering the high densities, biomass and N and P content of these snails in Imboassica lagoon, their importance for the nutrient cycling in this ecosystem must be accounted.