Long‐Term Changes in the Benthos of the Northern Adriatic Sea: A Phototransect Approach

Abstract

Abstract. The Northern Adriatic Sea has been repeatedly affected over the last three decades by bottom anoxia and benthic mortalities coupled with marine snow development. Eutrophication, meteorological conditions and 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 macro‐epibenthic community.In this study, an underwater phototransect was used to evaluate percent coverage of macro‐epifauna in order to determine the long‐term recovery of the community after a mass mortality in 1983. The area covered by 72 categories of organisms and biogenic structures was measured between 1985 and 1994. The former keystone genera Ophiothrix, Reniera and Microcosmus contributed only a small percentage to the coverage by epibenthic fauna. Reniera spp. was only detected in 1994, Ophiothrix quinquemaculata covered minimal areas in only three years (1986, 1991 and 1994), and Microcosmus spp. was only positively identified in 1985. The percent coverage of living organisms increased from 1.4 % in 1985 to 6.2 % in 1994, although it collapsed in 1988 after a mortality and suffered a setback in 1990 (over a 19‐day period from 6.0 to 3.5 %) after commercial fishing activities. Non‐living, biogenic structures (shells, tests etc.) increased from 1.5 % in 1985 to 5.8 % coverage in 1994. Lebensspuren increased in the same period from 2.3 to 3.9 %.The highly variable percent coverage values along the same transect from year to year show that the system was unstable. The original suspension‐feeding brittle stars, sponges and ascidians did not recover, while omnivorous hermit crabs increased. On the other hand, clear trends toward increasing coverage, interrupted by repeated anoxia and fisheries damage, were recorded. Although the aggregated distribution of the organisms (multi‐species clumps with intervening sediment surface) was partly preserved, these disturbances hindered the recolonization progress, which was very slow and did not lead to a climax stage of the former, intact community during this investigation.The photo‐documentation method is a valuable supplement to diver‐taken samples – which yield absolute biomass values – and provides a considerable information gain, for example in detecting and quantifying lebensspuren.