Blue mussels Mytilus edulis do not interfere with eelgrass Zostera marina but fertilize shoot growth through biodeposition

Blue mussels Mytilus eduljs L. and eelgrass Zostera marina L. commonly CO-occur in mlxed stands at sheltered sites of the Western Baltic.The effects of mussels on density, vegetative propagation and growth of eelgrass were tested experimentally.Mussels were either added to 2. marina patches or removed from existing Zostera/Mytilus associations.We found no effect of these experimental manipulations on the shoot density of Z. marina from April to October L~kewise, observatlons on a series of permanent plots over 1 growth period showed that adjacent mussel patches did not impede the vegetative propagat~on of eelgrass patches.Instead of damaging eelgrass by interference competition, mussels enhance eelgrass growth.At the end of August, plants in the M. edulis addition treatment had a 36% higher leaf area than the controls, whereas mussel removal led to an area decrease of 16 "10 compared to the controls Since, at the same time, the sedlment porewater concentrations of amrnonlum and phosphate doubled in presence of M. edulis, we infer that 2. marina is nutrient-limited in the sandy, organically poor sediments of the shallow subtidal zone.M. edulis facilitates Z. marina by the biodeposition of organic material via faeces and pseudofaeces.A correlation between porewater ammonium concentration and plant size supports the contention that nitrogen is growth limiting.In contrast, no relationship was found between porewater phosphate concentration and plant size.