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

Abstract

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 test for temporal responses of benthlc invertebrate populations to severe stress imposed by the algal mats. Algae corresponding to amounts recorded in the field were enclosed In netbags and attached to the bottom. Population abundance of the zoobenthic species under algae were compared w ~ t h control plots for 5 ivk with weekly sanipl~ng. Mass~ve die-offs of benthic populatlons 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, populat~on crashes were recorded for the sedentary po1ychaett.s Manayunkia aestuanna and Pygosplo elegans, ivh~le populations of the errant polychaete Nereis diverslcolor and tubificid oligochaetes remained stable under the algal mats. Initial short-term recovery after the algae were removed was rapid and do~nlnated 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. KEY MIORDSMacrofauna . Drifting macroalgae Disturbance . Baltic Sea . Macoma balthica . Hydrobia spp