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

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