Kill of benthic organisms as a reponse to an anoxic state in the northern Adriatic: (a critical review)

Abstract

        Hypoxic and anoxic states, occurring temporarily, count among the problems of the north­ern Adriatic Sea. In the course of the continental shelf long-term studies (PIPETA programme) of the Adriatic Sea, a drastic deficiency of oxygen concentrations in the bottom layer of the offshore part of the northern Adriatic was recorded in October 1988. Material for this paper was collected during PIPETA cruises (RV, 300 HP). It includes hydrographic and biological data (obtained by trawl and grab). The first signs of newly dead individuals of benthic organisms were recorded at oxygen concentration as low as 1 ml l^-1. At the stations where the total quantity of dissolved oxygen exceeded 1 ml l^-1 no signs of dead organisms were found. Fish and cephalopods accumulated in well-aerated area. In 1988 a kill of organisms was recorded within current eddies on the sea bot­tom rich in organic matter (muddy component). Varying rhythm of sedimentation in the northern Adriatic is affected by the Po River water regime and the velocity of general Adriatic current. Results obtained by grab are illustrative of the varying distribution of animal groups on ” mosaic­-like" distributed sediment in a very small area. Biological data obtained for different oxygen con­centrations at the different profiles and stations in 1988 were compared to the biological data from 1987 and 1991.