DRIFTING ALGAE AND ZOOBENTHOS - EFFECTS ON SETTLING AND COMMUNITY STRUCTURE

Shallow (5 to 10 m) sandy bottoms in the Baltic Sea are important areas for zoobenthic production. The infaunal communities are generally governed by the hydrographical conditions and transport of the sediment through wind effects. With increasing eutrophication in the Baltic Sea, drifting mats of annual algae (Cladophora, Stictyosiphon, Polysiphonia, Rhodomela, Sphacelaria, Pilayella, Furcellaria, Ceramium, etc) have become increasingly common, adding to the structuring and regulating factors for the infauna. In 1990 and 91, a field-study (SCUBA diving; zoobenthos and algae sampling) was carried out in the Aland archipelago, in the northern Baltic Sea, to quantity the amount of drift-algae and their structuring effect on the zoobenthos. Algal biomass increased from 150 +/- 19 g DW.m-2 in 1990 to 832 +/- 60 g DW.m-2 in 1991, having no effect on oxygen saturation in 1990, but showing signs of reduced oxygen saturation in 1991. Organic content of the sediment remained stable (0.60 to 0.74%) during the entire study period. The zoobenthic community showed significant responses to the drifting algae at population level and in terms of community structure (by 1991: significantly reduced species number; low similarity values (40 to 65%) between bare sand and under the algae). The main species affected were the dominating bivalve Macoma balthica, the polychaetes Pygospio elegans and Manayunkia aestuarina, and the amphipod Corophium volutator The settlement of M. balthica spat was significantly reduced by the algae (>70% in 1990/91), and no individuals of the dominating polychaetes were recorded under the mat. C. volutator, however, benefited from the algae, and greatly increased in numbers. The results clearly demonstrate the types of physical effects drift-algae will have on sandy-bottom benthos, and show that significant changes in the communities over large areas can be expected with increasing eutrophication.