Degradation of mussel (Mytilus edulis) fecal pellets released from hanging long-lines upon sinking and after settling at the sediment

Long-line mussel farming is well established in many coastal areas around the world, and the concurrent carbon enrichment below farms potentially can have local environmental impacts. The degradation of released mussel (Mytilus edulis) pellets was investigated by a number of complementary incubation approaches. After 140 h in suspension, most of the labile fraction of the pellet material had been degraded. However, because of the shallow water depth at our study site (8 m), only a minute fraction of carbon was released to the water column during sinking, of which similar to 50% was rapidly turned over. Pellets settling at the sediment in realistic concentrations immediately stimulated the benthic O-2 uptake. The elevated O-2 uptake gradually declined and reached the background level after 96 h, at which time similar to 25% of the added material had been degraded. The main fraction of pellets (75%) was more refractory and remained in the sediment where it was either retained or degraded on much longer time scales. The quantitative response of benthic pellet enrichment observed in the laboratory was confirmed by in situ trap measurements and incubations of sediment collected below and away from active farms.