Role of biodeposition by Mytilus edulis in the circulation of matter and nutrients in a Baltic coastal ecosystem

Abstract

Biodeposit production of Mjrtilus eduLis was studied in situ over an annual cycle in the northern Baltic proper. Faeces and pseudofaeces were collected in traps covered by a layer of mussels and placed on the bottom at 5 m depth. The summed deposition due to sedimentation and biodeposition was compared with deposition in mussel-free control traps on the bottom and suspended in the water column. Both quantity and quality of deposited matter increased through biodeposition. Relative contribution of matenal from M. edulis was hlghest in summer, whereas both sedimentation and biodeposition peaked in autumn. Annual biodeposition by M. edulis was 1.76 g dry weight, 0.33 g ash- free dry weight, 0.13 g carbon, l.? X IO-~ g nitrogen and 2.6 X 104 g phosphorus per g mussel (dry weight including shells). During the productive season from April to September the organic content of biodeposits was higher than that of naturally sedimenting material (30.4 vs 24.8%), despite having passed the mussels' alimentary tracts. Nitrogen content of biodeposited material was also higher than that in control traps (1.68 vs 1.20 % of dry weight), while phosphorus content was the same (0.19 %) in both. The higher contents of carbon and nitrogen in biodeposits may be explained by the fact that the mussels to a large extent remove and initiate the sedimentation of small particles of high organic content whch would otherwise stay in suspension. We argue that M. edulis occupies an important role as a connecting link between pelagic and benthic ecosystems by increasing total annual deposition of C, N and P by 10 % and by circulating and regenerating 12 and 22 %, respectively, of the annual N and P demands for pelaqc pnmary production in the area. INTRODUCTION