Long-term changes in the benthos of the Northern Adriatic Sea: A phototransect approach

The Northern Adriatic Sea has been repeatedly affected over the last three decades by bottom anoxia and benthic mortalities coupled with marine snow d evelopment. Eutrophication, meteorological conditions acid climatic Changes are among the cited causes for the occurrence of these disturbances, which , along with benthic fisheries, have a major impact on the investigated mac ro-epibenthic community. In this study, an underwater phototransect was used to evaluate percent cov erage of macro-epifauna in order to determine the long-tron recovery of the community after a mass mortality in 1983. The area covered by 72 categorie s of organisms and biogenic structures was measured between 1985 and 1994. The former keystone genera Ophiothrix Reniera and Microcosmus contributed o nly a small percentage to the coverage by epibenthic fauna. Reniera spp. wa s only detected in 1994, Ophiothrix quinquemaculata covered minimal areas i n only three years (1986, 1991 and 1994), and Microcosmus spp, was only pos itively identified in 1985. The percent coverage of living organisms increa sed from 1.4 % in 1985 to 6.2 % in 1994, although it collapsed in 1988 afte r a mortality and suffered a setback in 1990 lover a 19-day period from 6.0 to 3.5 %) after commercial fishing activities. Non-living, biogenic struct ures (shells, tests crc.) increased from 1.5 % in 1985 to 5.8 % coverage in 1994. Lebensspuren increased in the same period from 3.3 to 3.9 %. The highly variable percent coverage values along the same transect from ye ar to year show that the system was unstable, The original suspension-feedi ng brittle stars, sponges and ascidians did not recover, while omnivorous h ermit crabs increased. On the other hand, clear trends toward increasing co verage, interrupted by repeated anoxia and fisheries damage, were recorded. Although the aggregated distribution of the organisms (multi-species clump s with intervening sediment surface) was partly preserved, these disturbanc es hindered the recolonization progress, which was very slow and did not le ad to a climax stage of the former, intact community during this investigat ion. The photo-documentation method is a valuable supplement to diver-taken samp les which yield absolute biomass values - and provides a considerable infor mation gain, for example in detecting and quantifying lebensspuren.