THE ROLE OF THE BLUE MUSSEL MYTILUS-EDULIS IN THE CYCLING OF NUTRIENTS IN THE OOSTERSCHELDE ESTUARY (THE NETHERLANDS)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured i n situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant posi tive correlation, and the average C:N ratio of the fluxes was 9.4. The re was a high release of phosphate, nitrate, ammonium and silicate fro m the mussel bed into the water column. The effluxes of dissolved inor ganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements w ith individual nutrient excretion rates showed that excretion by the m ussels contributed 31-85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17-94% of the amm onium flux from the mussel bed. The mussels did not excrete silicate o r nitrate. Mineralization of biodeposition on the mussel bed was proba bly the main source of the regenerated nutrients. From the in situ obs ervations net budgets of N, P and Si for the mussel bed were calculate d. A comparison between the uptake of particulate organic N and the re lease of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification i s a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nut rient ratios in the water column. A comparison of N regeneration by mu ssels in the central part of the Oosterschelde estuary with model esti mates of total N remineralization showed that mussels play a major rol e in the recycling of nitrogen.