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Colonisation by macrobenthos after defaunation due to anoxia and sulphide accumulation was studied in 2 field experiments, performed at shallow sites in the southwestern Baltic Sea. In the first experiment (Wismar Bight) an area was initially covered with dark foil which caused azoic anoxic sediments with high sulphide concentrations (up to 3.4 mm01 I-' a t 2 cm depth). After foil removal sulphide concentrations in the upper sediment layer decreased, but a t 10 cm depth high concentrations of 1.65 mm01 I-' sulphide remained even after 69 d . Within 3 d , adults of species living in the upper sediment layer, e .g . the ostracod Cyprideis torosa, appeared as first immigrants. After 1 mo the density, species composition, diversity and evenness of the macrobenthlc community in the experunental area were not significantly different from the reference area. In the second experiment (Hiddensee Island) the colonisation of an area defaunated by a thick cover of decaying algae Fucus vesiculosus was investigated. After removal of the algal mat, recovery of the experimental area (decrease of sulphlde concentrations, colonisation pattern of macrofauna, development of community parameters, recovery tune) was similar to that described for the first colonisation expenment. Laboratory tolerance expennlents revealed that species such a s C. torosa that returned first to the experimental areas also showed highest survival rates under hypoxia and sulphide. In contrast, the amphipod Corophium volutator, the most sensitive species to hypoxia and sulphide, was one of the last animals to reach both experimental areas. The macrobenthic community consists mainly of deposit feeding species with a high tolerance for temporary hypoxic conditions and frequent exposure to sulphide. The few less tolerant species have the capacity to produce large populations within short time periods (e.g. the oligochaete Paranais litoralis) andlor have a high mobility (e.g C. volutator). At these shallow sites in the Baltic Sea a combined set of abiotic and biotic factors seems to favour rapid recovery of a stress-preconditioned macrobenthos from recurring small-scale hypoxic and sulphidic periods which dominate the ecosystem as structuring factors.

Hypoxia and sulphide as structuring factors in a macrozoobenthic community on the Baltic Sea shore:colonisation studies and tolerance experiments