Oyster reef restoration in the northern Gulf of Mexico: effect of substrate and age on ecosystem services

Abstract

Oyster reefs provide estuaries with ecosystem services including carbon sequestration, shoreline stabilization, refugia for invertebrate biodiversity, and fisheries enhancement, but have been disappearing over the past century due to increasing overharvesting, disease, and eutrophication. The northern Gulf of Mexico has over 400 artificial oyster reefs constructed of various materials created through efforts to restore ecosystem services lost with declining oyster populations. The purpose of this study was to determine how two common artificial reef construction materials (rock vs. oyster shell) at various ages affect ecosystem services, specifically blue crab abundance, the commensal community, and oyster recruitment, as well as to determine the refuge value of oyster reefs. To determine whether the provision of ecosystem services varied over time, rock (n = 7) and oyster shell (n = 6) reefs of varying ages (new < 5 years since construction; old > 5 years since construction) were compared to natural oyster reefs (n = 7) and were sampled twice in the summer of 2011. Sampling devices included baited crab traps, spat settlement plates and commensal collectors. Results indicate no difference between reef category (reef by age) and blue crab abundances, most likely due to their mobility. Diversity, abundance and richness of the commensal community are lowest on old shell reefs. Old rock reefs were most similar to natural reefs in regards to commensal community assemblages, even surpassing them in commensal abundance seasonally. Spat densities and size varied across reef categories, and were more closely correlated with salinity than reef category. To determine refuge value of oyster structure, a mesocosm with a depth gradient and different oyster reef complexities in the deepest end was used. When high complexity shell areas were provided, juvenile crabs were more likely to use these deep water reef refugia when a predator was introduced to the tank; when no shell areas were available, juvenile crabs selected shallow water refuge. The refuge value of oyster reefs, based both on the field survey and laboratory experiment, seems to be the driver of higher commensal abundances, richness, and diversity on older rock reefs, which last longer than old shell reefs which seem to disperse over time in sediments.