Native eelgrass (Zostera marina L.) survival and growth adjacent to non-native oysters (Crassostrea gigas Thunberg) in the Strait of Georgia, British Columbia

We investigated effects of the introduced Pacific oyster (Crassostrea gigas) on native eelgrass (Zostera marina) health on Cortes Island, British Columbia, Canada. Oysters physically alter their environment by increasing habitat complexity and altering water flow, and possibly by causing sulphide to accumulate in the sediment. Sulphide is toxic to eelgrass, and the current decline of eelgrass around Cortes Island may be a consequence of oyster population growth. While oysters and eelgrass coexist at a regional scale, eelgrass is typically absent directly seaward of oyster beds (the “below-oyster zone”) on Cortes Island. In a controlled experiment, we transplanted eelgrass plugs to below-oyster plots to determine whether this habitat is suitable for eelgrass growth. Shoot and leaf number were significantly greater over time in eelgrass-bed transplants than in below-oyster transplants. These results indicate that the below-oyster zone is unsuitable for eelgrass growth; if a causal link exists between oyster presence in the high intertidal zone and eelgrass absence directly seaward, then expansion of feral and farmed oyster beds will result in further eelgrass loss on Cortes Island.