Rapid zoobenthic community responses to accumulations of drifting algae

Abstract

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 community to accumulations of drifting algal mats were studied in a field experiment on an exposed shallow sandy bottom (7.5 m depth) in the northern Baltic Sea Experimental algal plots (50 x 50 cm) corresponding 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 structure 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 stable. Structural differences were recorded after 9 d, and community breakdown after 16 to 21 d of algal cover. Only opportunists and species tolerant to hypoxia remained under the algae. High particulate organic C/N ratios in the sediment trader algal plots were recorded after 4 wk, indicating deposition of organic material from the algae to the sediment. Initial recovery (5 d) after terminated algal stress was rapid and dominated by a mass invasion of hydrobiid snails, possibly attracted by enrichment of the sediment. Laboratory tests on algal degradation showed a 10-fold increase in phosphorus in the water due to nutrient leakage during hypoxia (20% O2). Algal cover, and the induction oF hypoxia through degradation of the algae, exhibited severe effects on zoobenthic community structure and a potential to accelerate local eutrophication. We therefore feel that the escalating amounts of drifting algae recorded in the field are a significant threat to the coastal biota.