Oligochetes in the Southern basin of the Venetian Lagoon: community composition, species abundance and biomass.

The oligochaete community of the Chioggia Lagoon (southern basin of the Venetian Lagoon) has been studied from May 1992 to May 1994, at ten fixed sampling stations, distributed across the lagoon. Numerous data regarding composition, abundance, biomass and the breeding periods of the species in the community have been obtained. Nine tubificid species, recently reported for the Mediterranean and Ponto-Caspian areas have been collected. Among them only three were abundant everywhere, except in the innermost part of the lagoon, and throughout the year: Tubificoides vestibulatus, T. swirencowi and Limnodriloides maslinicensis. Only one enchytraeid genus, Grania, has been found restricted to an area near the connection with the sea. The values for the Shannon Diversity Index for the community never exceeded 0.8 and were mainly between 0.4 and 0.7. The most interesting result was the decrease of abundance and biomass, proceeding from the sea mouth (about 6000 ind m–2) towards the innermost part of the lagoon (less than 100 ind. m–2). Correlations are suggested with the different grain size of the sediment.