VERTICAL DISTRIBUTION OF SOFT BOTTOM MACROZOOBENTHOS IN THE GULF OF TRIESTE (NORTHERN ADRIATIC SEA)

The macrozoobenthos is commonly considered to be the main agent of bioturbation in shallow water, and responsible for the enhancement of the interchange of both dissolved and gaseous species between the pore waters and the upper water column. As reported by several authors, this basic phenomenon is usually investigated by describing the vertical distribution of macrobenthic organisms. For this purpose, a site located in the central part of the Gulf of Trieste (depth = 21 m) was selected and, during 1999/2000, 9 core samples were collected in triplicate to a maximum depth of 20 cm. The vertical distribution of the macrozoobenthic communities was described by applying the bioturbation activity index (BA). The vertical distribution of macrofauna showed a clear decrease in both number of taxa and abundance when moving downcore. The BA suggested that surficial, detritus-feeding polychaetes have a pivotal role in the bioturbation of muddy sediments, whereas suspension-feeding molluscs are also important if the only top layer is considered. Taking into account the overall results, it can be stated that the BA index application based on the core sampling technique, could be a useful tool in describing bioturbation in the northern Adriatic soft bottom sediments.