Reef building oysters, Crassostrea virginica, provide numerous ecosystem services, including provision of commercially valuable product, improving water quality and clarity, buffering shorelines, and creating complex habitats, among other things. Unfortunately, populations have declined throughout their geographic range, and considerable effort and investment is directed toward managing and restoring reefs. While several well-studied metrics are considered prior to restoration efforts (i.e., substrate, elevation, water quality), one potential factor that has not received much prior attention is submarine groundwater discharge (SGD). SGD may be beneficial for oysters by helping maintain favorable salinity and/or delivering nutrients that stimulate phytoplankton production, although groundwater flux may also deliver contaminants and decrease water quality.
Initial surveys in Oyster Creek, Georgia, suggested a negative, non-linear relationship between SGD and oysters – as SGD increased, oyster density decreased, although there were no patterns with oyster condition index or growth rates in caging experiments. To see if this pattern was consistent in other Georgia estuaries, we sampled 3 additional creeks along the Georgia coast and found similar patterns with density (Figure 1) and condition. Although the SGD in Georgia’s coastal creeks is recirculated seawater, it reenters surface waters with low pH (7.34 ± 0.25 mean ± SD), and we hypothesize that this results in recruitment disruption at high flux sites, as our results also suggest negative relationship between recruitment and SGD. However, surveys across the coast suggest that there are additional complexities in this relationship that should be investigated further. Since we observed potentially negative effects of SGD on oyster density and recruitment, we suggest that SGD be considered and potentially incorporated into management plans and restoration siting efforts.