In estuaries worldwide, macroalgae blooms fueled by land-based nutrient loading have been associated with declining seagrass populations. In Coos Bay, Oregon, strong summer upwelling leads to estuarine systems dominated by marine nutrients. These marine-dominated nutrient conditions are associated with high seasonal productivity of both macroalgae and seagrass in the marine regions of the estuary. Seasonal monitoring of eelgrass (Zostera marina) and ulvoid macroalgae show that biomass of both producers is similar to other estuaries negatively affected by eutrophication. Our goal was to determine the nature of species interactions between macroalgae and eelgrass under the context of upwelling-influenced nutrient inputs. To determine the interaction strength between macroalgae and eelgrass, an intertidal manipulation experiment was conducted from summer 2007-fall 2008. A natural gradient in macroalgae production along the estuarine gradient was used to determine how eelgrass is affected under differential background levels of macroalgae productivity. Three sites were compared along the estuarine gradient including the marine zone (high macroalgae), mesohaline zone (medium macroalgae), and riverine zone (low macroalgae). Macroalgae was added and removed in pulse events to plots of seagrass beds during the summer months, and eelgrass dynamics were measured as a response to the manipulation throughout the experiment duration.
Contrary to many studies on macroalgae-seagrass interactions, our results do not show consistently negative interactions between macroalgae and seagrass. Instead, interaction strength was found to be context specific; changing along the estuarine gradient (marine to riverine zones), and with experiment duration. Interaction strength was ultimately positive in the marine zone, neutral, in the mesohaline zone, and negative in the riverine zone. Initial results after the first summer of macroalgae additions did not persist through the second summer of macroalgae additions. In the marine zone, the direction of the sign changed from negative to positive as the experiment progressed, whereas it remained close to zero through time in the mesohaline zone, and became increasingly negative in the riverine zone. A survey of Oregon estuaries shows similar patterns of macroalgae and eelgrass biomass in other estuaries where upwelling is strong. While biomass of macroalgae observed in these upwelling-influenced estuaries is similar to those observed in eutrophic estuaries, interactions with seagrass do not appear equivalent. In order to manage these important habitats under different environmental contexts, it is clear that increased understanding of the spatial distribution and directionality of nutrient inputs to producer communities is necessary.