Planting density, hydrodynamic exposure and mussel beds affect survival of transplanted intertidal eelgrass

Transplantation of eelgrass Zostera marina has become a promising restoration tool since natural recolonisation during the last century failed after massive mortality, due to a combination of a wasting disease outbreak and a sequence of human impacts. We studied the interactive effects of planting density and hydrodynamic exposure on the survival of transplants of an annual population of intertidal eelgrass. Accordingly, eelgrass seedlings were planted in high density (HD: 14 plants m–2) and low density (LD: 5 plants m–2) units at 3 locations with varied wave and current exposures. We also tested the potential of blue mussel beds (Mytilus edulis) to facilitate eelgrass survival. Transplant survival decreased as hydrodynamic exposure increased. Survival was high (75% after 7 wk) at the low exposure location. The intermediate exposure location had slightly lower overall survival (60% after 7 wk), and lowest overall survival rate was at the most exposed location (20% after 7 wk). Facilitation existed among eelgrass plants. Survival was significantly higher in the HD units than in the LD units at both high and intermediate exposure locations. Planting density had no effect on survival at the low exposure location. Hence, there was an interactive effect of planting density, hydrodynamic exposure and shelter. Eelgrass planted in open spaces within a mussel bed survived significantly better than transplants situated 60 m seaward of the mussel bed. Thus, mussel beds facilitate eelgrass survival. The insights into the processes affecting transplantation success will be of use in eelgrass restoration around the world.