Rapid zoobenthic community responses to accumulations of drifting algae

In the northern Baltic Sea, occurrences of benthic drifting macroalgal mats have become an ~ncreasing 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 accumulat~ons 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-') 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 under algal plots were recorded after 4 wk, indicating deposition of organic material from the algae to the sediment. Init~al recovery 15 d ) after termmated algal stress was rapid and dominated by a mass invasion ot hydrobud snatls, poss~bly attracted by enl-~chment of the sediment. Laboratory tests on algal degradation showed a 10-fold increase In phosphorus In the water due to nutrient leakdye during hypoxia (20'% Oz). Algal cover, and the ~nduction of hypoxla through degradation of the algae, exhibited severe effects on zoobenthlc community structure and a potential to accelerate local eutrophication. We therefore [eel that the escalating amounts of drifting algae recorded in the f~e ld are a significant threat to the coastal biota.

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