Accumulation, release and turnover of nutrients (C-N-P-Si) by the blue mussel Mytilus edulis under oligotrophic conditions

To evaluate the potential role of mussels in nutrient cycling in oligotrophic fjord ecosystems, we applied a multiple-element (C-N-P-Si) approach considering several physiological processes (excretion, tissue composition, biodeposition) simultaneously. The study covered one annual cycle, reflecting the effect of endogenous and exogenous factors. Respiration (6–29 µmol C g- 1 h- 1), ammonia (0.3–2.2 µmol N g- 1 h- 1) and phosphate (<0.3 µmol P g- 1 h- 1) excretion were correlated to food (POC) and temperature. Tissue nitrogen (127–167 mg N g- 1), carbon (547–629 mg C g- 1) and phosphorus content (547–629 mg P g- 1) seemed related to reproductive processes. Nutrient turnover showed a seasonal pattern with fast turnover in summer and slow turnover in winter. On an annual basis, nitrogen turnover (643 days) was slower compared to phosphorus (290 days) and carbon (181 days). Fluctuations in biodeposition rates (11–72 mg g- 1) were correlated to food parameters. Although the food concentration in our study area was low (Chl a <2 µg l- 1; POC <546 µg l- 1) and food quality high (CN ~ 9), physiological rates, except biodeposition, were similar to rates reported in other cultivation areas. The relative importance of each of the processes determines how mussels contribute to nutrient cycling in coastal ecosystems. Respiration and excretion of dissolved metabolites (1639 mg C, 92 mg N, 23 mg P g- 1 y- 1) is regarded as a source of regenerated nutrients and may enhance nutrient availability in the euphotic zone with potential consequences for phytoplankton dynamics. Nutrient accumulation in tissue material (560 mg C, 168 mg N, 12 mg P g- 1 y- 1) is regarded as a sink of nutrients given that mussels will be harvested. Biodeposition (981 mg C, 113 mg N, 9 mg P g- 1 y- 1) is regarded as another nutrient sink in fjord ecosystems, although a fraction is regenerated in the euphotic zone. This shows a dissimilar allocation of elements to each of the physiological processes, resulting in relatively more nitrogen being accumulated in tissue material, and relatively more phosphorus being regenerated. The relatively high fraction of regenerated nutrients (25–53%) and the low fraction of nutrients allocated to biodeposition (20–31%) indicates that the potential role of mussels in nutrient cycling in oligotrophic fjord systems is different from shallow eutrophic areas.