Biodiversity and distribution of polychaetes and molluscs along the Dese estuary (Lagoon of Venice, Italy)

Mollusc and polychaete distribution in the mud flats along the Dese estuary (lagoon of Venice, Italy) was analysed in May, August and October to evaluate taxonomic biodiversity and zonation pattern of soft macrobenthos in an area of the lagoon hardly known from this point of view. Throughout the studied area most of the species were polychaetes, particularly Streblospio shrubsolii (Buchanan), which showed greater abundance from May to October. Molluscs and polychaetes showed a zonation of marine species which colonize brackish environments in the outer and intermediate areas of the estuary and of brackish water species in the inner one. In the outer area different assemblages were attributed to different sediment types. This distribution pattern, put in relation to the distance from the sea, was evident in May and partly blurred in August and October: the assemblages of the inner area of the estuary invaded the intermediated area in August and affected outer area in October because of spreading of the brackish water species S. shrubsolii, which became dominant. The distribution pattern of molluscs and polychaetes can be explained in terms of confinement as the leading factor influencing water quality and dynamics of the ecosystem. It is also needed to be taken into consideration the role played by competition due to different degree of opportunism of the most abundant species.