Influence of allochthonous plant detritus on Gammarus insensibilis Stock (Amphipoda) occurence in the soft-bottom epifauna of the northern Adriatic sea.

Sediments and benthic epifauna were examined at a coastal station (ca.26 m water depth) located in the northwestern Adriatic Sea. Sediment cores were taken every 90 days from April 1995 to January 1996. Epifauna was further investigated by means of trophic traps deployed on the sea bottom. Sediments were characterized by the occurrence of coarse plant detritus derived from aquatic angiosperms (Zostera sp., Cymodocea nodosa) commonly distributed in lagoons and salt marshes of the Po delta area. The epifauna was dominated by the brackish amphipod Gammarus insensibilis Stock, whereas other taxa occurred in negligible densities (< 2% quantitative dominance). A positive relationship was found between coarse organic matter in sediments and G. insensibilis spatial abundance. Analyses of population parameters from core samples and of size-frequency distributions from trophic traps permitted to exclude passive dispersal as the causative factor for the constant, even though seasonally variable, presence of the amphipod in the benthic system. Our results provide clear evidence of an “outwelling” flux of particulate plant detritus linking brackish coastal areas to adjacent marine environments in the northwestern Adriatic Sea. Moreover, they give support to the hypothesis that advection of allochthonous particulate detritus in marine environments may determine an “estuarine” characterization of epifaunal soft-bottom assemblages.