Submarine groundwater discharge as a potential driver of eastern oyster, Crassostrea virginica, populations in Georgia

Reef-building eastern oysters, Crassostrea virginica, provide many ecosystem services, including production of valuable commercial products, formation of complex habitats, improved water quality and shoreline protection. Despite this, oyster populations have experienced dramatic declines throughout their range, spawning massive investment in management and restoration. Restoration efforts typically consider several well-studied metrics that normally influence oyster success; however, one potential factor that has not received much prior attention is submarine groundwater discharge (SGD). We conducted a series of field surveys and field experiments to explore the relationship between SGD and oysters in a marsh-lined tidal creek in Georgia, USA. SGD was mapped across multiple time points using a natural radon tracer (radon-222), and fluxes were paired with discrete measurements of oyster density, condition, size, recruitment and growth at multiple locations along the creek. Variation in oyster metrics was best explained by a combination of SGD, pH, and DO, which displayed a high degree of multicollinearity. We found an overall negative, nonlinear relationship between oyster density and groundwater flux. Interestingly, juvenile and adult condition and growth were not negatively impacted by groundwater. Rather, our results suggest that the likely mechanism for the density-flux relationship was inter-ruption of the larval recruitment, which was also negatively related to flux. We hypothesize that larval inter-ruption is due to the low dissolved oxygen and pH conditions of the groundwater at high flux sites. Overall, the interaction between SGD and oysters appears complex, and may be affected by other variables. This study provides evidence of a potential negative effect of a previously understudied natural phenomena on oyster de-mographics, and we suggest that SGD be considered in future management efforts.

Automated summary

The study found a negative, nonlinear relationship between oyster density and groundwater flux, with high flux groundwater interrupting larval recruitment and leading to a decrease in oyster density. However, the condition and growth of juvenile and adult oysters were not affected by groundwater, suggesting that interruptions in larval recruitment may be due to low dissolved oxygen and pH conditions in high flux sites. The interaction between SGD and oysters appears complex and may be influenced by other variables, highlighting the need for future management efforts to consider SGD.