Artificial reef biofouling community impacts on ecosystem metabolism and biogeochemical cycling in estuarine waters of the northern Gulf of Mexico

Artificial reefs increase hard bottom habitat and bolster shoreline protection in coastal waters. Reefs are rapidly colonized by biofouling communities which can potentially alter ecosystem metabolism. Despite the high biomass and ubiquitous nature of biofouling communities, little is known about their ecosystem metabolism or role in biogeochemical nutrient cycling. Using a combination of laboratory and field-based methods, this study focused on the metabolic activity of artificial reef biofouling communities and their potential to contribute to water column hypoxia in a shallow subtropical estuary. Study objectives aimed to (1) measure in situ dissolved oxygen (DO) concentrations in surface and bottom waters along reef transects during summer months when episodic hypoxic events typically occur, (2) estimate respiration rates in relation to nutrient regeneration in the absence versus presence of biofouling communities, and (3) examine seasonal shifts in trophic structure by relating elemental C:N ratios and stable nitrogen isotope (delta 15N) values of the biofouling community across eight sample periods. In situ DO in bottom waters was low near high profile reefs. Water column respiration and nutrient regeneration rates were comparable to other estuaries. Respiration and regeneration rates of ammonium and phosphate were all manifestly higher in the presence of a biofouling community. Biofouling metabolic-driven processes were higher at low profile reefs than at high profile reefs. Seasonal shifts in biofouling elemental composition indicated by higher C:N and lower delta 15N during cooler months and lower C:N and higher delta 15N during warmer months reflected a transition from a more autotrophic community in winter to a more heterotrophic community during the rest of the year. Our results demonstrate that artificial reef biofouling communities can potentially contribute to hypoxic conditions in coastal waters of the northern Gulf of Mexico.