FAUNAL INTERRELATIONSHIPS BETWEEN LAGOONAL AND MARINE AMPHI· POD CRUSTACEAN COMMUNITIES OF THE PO RIVER DELTA (NORTHERN ADRIATIC)

Abstract

Extensive investigations on the benthic communities of the brackish lagoon «Sacca del Canarin» (Po river delta) and the adjacent marine area allowed the identification of four different biocenoses. The soft bottoms of the lagoon are inhabited by an euryhaline and eurythermic lagoonal biocenosis: amphipods are represented mainly by Corophium orientale and Echinogammarus pungentoides. The latter occurs also on the hard substrates of the lagoon, together with Leptocheirus pilosus, Corophium insidiosum, Gammarus aequicauda and others. On the soft bottoms of the adjacent marine area a typical biocenosis of fine sands is found, with Urothoe poseidonis, Atylus massiliensis and others. On the marine hard substrates there is a somewhat impoverished Mytilus galloprovincialis community in which amphipods are represented by Caprella equilibra and others. Amphipod species interchanges among these biocenoses were detected by multivariate techniques (correspondence analysis and hierarchical cluster analysis), based on a set of 16 species per 16 sampling stations. The largest number of faunal exchanges (Jassa marmorata, Echinogammarus olivii, etc.) turned out to occur between the two hard substrate communities, which therefore exhibit similar species composition. Almost no marine species enter the lagoon; on the contrary the shallowest marine areas undergo a large intrusion of lagoonal species when hydrological or sedimentological stresses occur. In the first case some euryoecious species (Gammarus crinicornis, Echinogammarus pungentoides, Melita palmata, etc.), coming from hard or muddy bottoms, tend to lose substrate specifity and colonize sands. In the second case the limicolous species Corophium orientale can extend its distribution.