Abundance of seagrass (Zostera marina L.) and macroalgae in relation to the salinity-temperature gradient in Yaquina Bay, Oregon, USA

The relative abundances of the seagrass, Zostera marina L., and associated macroalgae were examined for Yaquina Bay, Oregon, U.S.A., to investigate variability in autotroph abundance along the salinity-temperature gradient and the potential for nuisance algal blooms. Possible explanations for the patterns in autotroph abundances were explored through examination of their correlations with the physicochemical characteristics of the water column. Study sites were established in each of three zones in the estuary defined by temperature and salinity and were sampled monthly June through September 1998 and in July 1999. Z. marina and macroalgal cover and Z. marina shoot density were measured in 0.25-m(2) plots at each site. After cover estimates and shoot counts were made, material was harvested for determination of Z. marina and macroalgal biomass. Water column variables were measured from stations near each study site and composited on a depth-averaged, monthly basis for each zone. Both Z. marina and green macroalgal abundance differed between sites, over the summer in 1998, and between years. Seasonal patterns were most obvious for Z. marina at the site closest to the ocean while the pattern in macroalgal abundance suggested a bloom moving up river as summer progressed. The physicochemical characteristics of the zones differed with the season and could be related to the patterns in Z. marina and macroalgal abundance. In particular, salinity was positively correlated with Z. marina abundance, while abundance of both autotrophs was related to light availability. Z. marina biomass ranged 19109 g dry weight m(-2); green macroalgae biomass ranged 5-234 g dry weight m(-2). The biomass of the green macroalgae at several sites and dates equaled or exceed that of the Z. marina suggesting the potential for nuisance algal blooms does exist in Yaquina Bay.