Population responses of coastal zoobenthos to stress induced by drifting algal mats

In the large archipelago area of the northern Baltic Sea, increasing occurrences of drifting benthic macroalgae have been recorded in the subtidal zone. Their role as a structuring factor on the zoobenthic community has been altered from inducing occasional small-scale disturbances to inducing large-scale mortality of macrobenthic populations. A controlled field experiment was conducted on a sandy bottom in order to lest for temporal responses of benthic invertebrate populations to severe stress imposed by the algal mats. Algae corresponding to amounts recorded in the field were enclosed in net-bags and attached to the bottom. Population abundance of the zoobenthic species under algae were compared with control plots for 5 wk with weekly sampling. Massive de-offs of benthic populations were recorded in both the experiment and under natural occurrences of drift algal mats. The community dominants in abundance (mudsnails Hydrobia spp.) and biomass (bivalve Macoma balthica) exhibited strong population reductions after 9 d of algal stress. Within 5 wk, population crashes were recorded for the sedentary polychaetes Manayunkia aestuarina and Pygospio elegans, while populations of the errant polychaete Nereis diversicolor and tubificid oligochaetes remained stable under the algal mats. Initial short-term recovery after the algae were removed was rapid and dominated by Hydrobia spp. The strong negative effects on key species such as M. balthica may have severe effects on entire food-web dynamics in Baltic coastal ecosystems.