POPULATION RESPONSES OF COASTAL ZOOBENTHOS TO STRESS-INDUCED BY DRIFTING ALGAL MATS

In the large archipelago area of the northern Baltic Sea, increasing o ccurrences of drifting benthic macroalgae have been recorded in the su btidal zone. Their role as a structuring factor on the zoobenthic comm unity has been altered from inducing occasional small-scale disturbanc es to inducing large-scale mortality of macrobenthic populations. A co ntrolled field experiment was conducted on a sandy bottom in order to lest for temporal responses of benthic invertebrate populations to sev ere stress imposed by the algal mats. Algae corresponding to amounts r ecorded in the field were enclosed in net-bags and attached to the bot tom. Population abundance of the zoobenthic species under algae were c ompared with control plots for 5 wk with weekly sampling. Massive de-o ffs of benthic populations were recorded in both the experiment and un der natural occurrences of drift algal mats. The community dominants i n abundance (mudsnails Hydrobia spp.) and biomass (bivalve Macoma balt hica) exhibited strong population reductions after 9 d of algal stress . Within 5 wk, population crashes were recorded for the sedentary poly chaetes Manayunkia aestuarina and Pygospio elegans, while populations of the errant polychaete Nereis diversicolor and tubificid oligochaete s remained stable under the algal mats. Initial short-term recovery af ter the algae were removed was rapid and dominated by Hydrobia spp. Th e strong negative effects on key species such as M. balthica may have severe effects on entire food-web dynamics in Baltic coastal ecosystem s.