RAPID ZOOBENTHIC COMMUNITY RESPONSES TO ACCUMULATIONS OF DRIFTING ALGAE

In the northern Baltic Sea, occurrences of benthic drifting macroalgal mats have become an increasing problem. Accumulations of algae induce hypoxia and anoxia in zoobenthos above the halocline on shallow sandy bottoms. The immediate and temporal responses of a macrozoobenthic co mmunity to accumulations of drifting algal mats were studied in a fiel d experiment on an exposed shallow sandy bottom (7.5 m depth) in the n orthern Baltic Sea. Experimental algal plots (50 x 50 cm) correspondin g to amounts recorded in the field (440 g dwt m(-2)) were enclosed in net-bags and attached to the bottom. Changes in zoobenthic community s tructure under the algae were compared with ambient control plots for a period of 5 wk with sampling every seventh day. Community parameters (species, abundance and biomass) in the control community remained st able. Structural differences were recorded after 9 d, and community br eakdown after 16 to 21 d of algal cover. Only opportunists and species tolerant to hypoxia remained under the algae. High particulate organi c C/N ratios in the sediment under algal plots were recorded after 4 w k, indicating deposition of organic material from the algae to the sed iment. Initial recovery (5 d) after terminated algal stress was rapid and dominated by a mass invasion of hydrobiid snails, possibly attract ed by enrichment of the sediment. Laboratory tests on algal degradatio n showed a 10-fold increase in phosphorus in the water due to nutrient leakage during hypoxia (20% O-2). Algal cover, and the induction of h ypoxia through degradation of the algae, exhibited severe effects on z oobenthic community structure and a potential to accelerate local eutr ophication. We therefore feel that the escalating amounts of drifting algae recorded in the field are a significant threat to the coastal bi ota.