Effects of red macroalgal (Gracilariopsis sp.) abundance on eelgrass Zostera marina in Tomales Bay, California, USA

Observations of increasing red macroalgal (Gracilariopsis sp.) abundance in Tomales Bay, California (USA) prompted a field survey and an in situ experiment testing algal mat effects on eelgrass (Zostera marina) growth. At the conclusion of a 3 mo macroalgal enclosure/exclosure study, experimental plots containing high macroalgal loads (1700 g m(-2) wet weight = mean maximum in a field survey) had significantly lower Z. marina shoot densities and growth rates than plots with no algae or with mean algal field abundance (325 g m(-2)). Eelgrass aboveground biomass followed a similar trend, decreasing under high macroalgal loads. We explored possible mechanisms of algal impacts on eelgrass, including changes to redox potential, sediment nitrogen, and light. Of these, light limitation is the most likely explanation for decreases in eelgrass shoot density. Light reaching the benthos where new shoots establish was only 2% of surface irradiance under high algal loads, while the canopy of adult eelgrass shoots received nearly 40% of surface irradiance regardless of macroalgal abundance. Reduced Z. marina growth rates may also be attributable to light limitation, as light levels neared the compensation irradiance for eelgrass under high macroalgal loads. Phase shifts from seagrass to macroalgae have been well documented in other estuaries, although studies are generally conducted well after the shift to macroalgal dominance has occurred. Our results indicate a similar transition is possible in Tomales Bay and suggest the need to monitor and manage this system prior to a future shift.