VARIABLE RESPONSES OF NATIVE EELGRASS ZOSTERA-MARINA TO A NON-INDIGENOUS BIVALVE MUSCULISTA-SENHOUSIA

The transport and establishment of non-indigenous species in coastal m arine environments are increasing worldwide, yet few studies have expe rimentally addressed the interactions between potentially dominant non -native species and native organisms. We studied the effects of the in troduced mussel Musculista senhousia on leaf and rhizome growth and sh oot density of eelgrass Zostera marina in San Diego Bay, California. W e added M. senhousia over a natural ranee in biomass (0-1200 g dry mas s/m(2)) to eelgrass in transplanted and established beds. The effects of the non-indigenous mussel varied from facilitation to interference depending on time, the abundance of M. senhousia, and the response var iable considered. Consistent results were that mussel additions linear ly inhibited eelgrass rhizome elongation rates. With 800 g dry mass/m( 2) of M. senhousia eelgrass rhizomes grew 40% less than controls in tw o eelgrass transplantations and in one established eelgrass bed. These results indicate that M. senhousia, could both impair the success of transplantations of eelgrass, which spread vegetatively by rhizomes, a nd the spread of established Z. marina beds to areas inhabited by M. s enhousia. Although effects on leaf growth were not always significant, in August in both eelgrass transplantations and established meadows l eaf growth was fertilized by mussels, and showed a saturation-type rel ationship to sediment ammonium concentrations. Ammonium concentrations and sediment organic content were linear functions of mussel biomass. We found only small, non-consistent effects of M, senhousia on shoot density of eelgrass over 6-month periods. In established eelgrass beds , but not in transplanted eelgrass patches (approximate to 0.8 m in di ameter), added mussels suffered large declines. Hence, eelgrass is lik ely to be affected by M. senhousia primarily where Z. marina beds are patchy and sparse. Our study has management and conservation implicati ons for eelgrass because many beds are already seriously degraded and limited in southern California where the mussel is very abundant.