In situ determination of PCB biodeposition by Mytilus edulis in a Baltic coastal ecosystem

Biodeposits of the blue mussel Mytilus edulis and pelagic and near-bed sett ling particulate matter were collected in situ over a 1 yr period in a coas tal area of the Northern Baltic proper. The amounts of carbon and PCBs (pol ychlorinated biphenyls) in the collected biodeposits were compared to those in pelagic and near-bed settling material and rates of carbon and PCB biod eposition by mussels were estimated. The filter-feeding activity and subseq uent release of faecal matter by the mussels increased gross sedimentation of carbon to benthos by 45 % if compared to areas with no mussels. By selec tively feeding on particles rich in organic carbon the mussels also concent rated associated contaminants and thereby increased gross sedimentation of PCBs by 50 %. This suggests that mussel biodeposition will enhance the avai lability of PCBs to benthic deposit feeders living in or in the vicinity of mussel beds. Extrapolation of the experimental results to the total Swedis h coastal zone of the Baltic proper indicates that mussel biodeposition is responsible for a significant part of PCB net sedimentation, i.e. 17 % or 9 6 kg yr(-1). Consequently, even when seen from a large geographical scale, mussels are important modifiers of PCB cycling by directing considerable am ounts of PCBs towards the benthic food web and thereby influencing the rete ntion time of these and probably many other contaminants in the coastal zon e. It is also Likely that changes in mussel biomass, for example owing to s hifts in primary production or salinity, will markedly affect the transport and fate of contaminants in the Baltic Sea.