EFFECT OF OYSTER MARICULTURE ON SUBMERGED AQUATIC VEGETATION - AN EXPERIMENTAL TEST IN A PACIFIC-NORTHWEST ESTUARY

The effects of commercial culture of oysters, Crassostrea gigas, on su bmerged aquatic vegetation (SAV), Zostera marina, were examined with r eplicated field experiments in the South Slough estuary, Oregon, USA. Both stake and rack methods of oyster culture resulted in significant decreases in the abundance of SAV compared to undisturbed reference ar eas. SAV cover in both stake and rack treatments was less than 25 % of that in reference plots after 1 yr of culture, and was absent from ra ck treatments after 17 mo of culture. Field experiments using marked p lants revealed no difference in growth between plants in stake and ref erence plots. Comparisons of sediment surface topography demonstrated that oyster culture resulted in significantly greater sediment deposit ion in stake plots and greater erosion in rack plots. Silt-clay fracti ons and carbon content of sediments tended to increase with stake cult ure and decrease with rack culture, but only for carbon content at rac ks were the differences significant between culture and reference plot s. Stake culture likely affected SAV via increased sedimentation and d irect physical disturbance during placement and harvest, while increas ed erosion and perhaps shading resulted in the marked decrease in SAV coincident with rack culture. These results indicate the potential for significant loss of SAV from estuarine ecosystems where these methods of oyster culture and SAV coincide.