Interactions between dense seasonal macroalgal mats and oysters on natural and constructed shellfish reefs

Oysters are important coastal foundation species that provide valuable hard substrate for the recruitment of epibiotic organisms in environments otherwise dominated by soft sediment. Yet, their interactions with epibionts are relatively understudied. Despite the proliferation of macroalgal mats across the Southeastern United States in winter months, the relationship between oysters (Crassostrea virginica) and seasonal macroalgae is poorly understood. We conducted an observational field survey and two manipulative field experiments to document seasonal macroalgal dynamics on intertidal oyster reefs and to better understand the interaction between the oysters and algae. We found that algal mats in North Carolina were primarily composed of two genera, Ulva and Ectocarpus, which together reached extremely high cover (up to 100%) over large areas of reef. Macroalgae appeared in January and declined in May, with peak cover in February and March. Algal cover was significantly higher on constructed oyster reefs versus natural oyster reefs. Our field experiments showed that algal cover was significantly higher on dead oyster mimics versus live oysters, suggesting that the primary mechanism of algal facilitation is associated with the provisioning of hard substrate rather than fertilization. Reciprocally, we found no significant effects of macroalgae on oyster abundance or growth, likely due to relatively low algal cover in the experimental treatments. With a predicted proliferation of macroalgae under global change, our study highlights the important role that intertidal oyster reefs play in providing substrate for macroalgae, but more research on this key species interaction in intertidal areas of the Southeastern United States is needed.