Nutrient dynamics and production in San Francisco Bay eelgrass (Zostera marina) beds

Seagrasses, marine angiosperms forming habitat in protected shallow waters worldwide, \nare at risk. Macroalgae or epiphytes (algae attached to seagrass leaves) are important in \nestuarine food webs but can compete with seagrasses for light and nutrients. In Chapter \n1 ,1 assessed the abundance and identity of algae in San Francisco Bay (SFB) eelgrass \n(Zostera marina) beds, documenting the range of producers and determining if algae \nreach abundances found in other regions to negatively affect eelgrass. I conducted \nquarterly surveys of primary producers in four SFB eelgrass beds and found highly \nvariable species composition and large seasonal fluctuations among beds (two to fourfold). \nAs the total and relative contributions of biomass among producers vary \nsubstantially across space and time, resources for consumers and interactions with \neelgrass are also likely to be highly variable. I found evidence that macroalgae \noccasionally reach abundances high enough to likely be detrimental to eelgrass. In \nChapter 2, I addressed the hypothesis that increased light availability, as expected in \nSFB with reductions in sediment supply, will enhance eelgrass growth as well as the \nability of both eelgrass and its epiphytes to utilize large nutrient pools in SFB. Using \nmesocosm experiments to manipulate light (three levels: from low to above average \nrelative to values measured in SFB eelgrass beds) and nutrients (ambient and enriched), \nI found that above-ground biomass, heights, and growth rates increased with higher \nlight levels. Ambient nutrient levels are high, and added nutrients increased N uptake \nbut not growth. These results indicate that SFB eelgrass is likely to respond positively \nto increasing light availability, but nutrients are not limiting even under higher light \nconditions.