Spatial and Temporal variability of nutrient profiles with depth in a Subterranean Estuary

Abstract:

Subterranean estuaries (STEs) form at the interface between groundwater and seawater. The interaction of two distinct water bodies creates a reactive zone supporting a variety of biogeochemical reactions. STEs are often regarded as highly variable systems, but when calculating nutrient fluxes associated with submarine groundwater discharge (SGD) it is common for a single endmember to be chosen. This can lead to extensive error as groundwater nutrient concentrations may change with space and time. The objective of this study was to determine how geochemical gradients in STEs vary over different scales. We have examined the biogeochemical profiles with depth in a STE located at the mouth of the York River estuary in Virginia (USA). STE porewater was sampled spatially, with a network of groundwater wells, and temporally, over both tidal and seasonal cycles. We measured environmental parameters including; salinity, dissolved oxygen, DIN, DIC, sulfide, as well as other analytes. The gradients of these analytes were compared to one another across space and time to analyze changes in the profiles with depth. The study system exhibits greater variation in geochemical gradients across seasons as compared to variation with tidal stage or space. This has important implications for our understanding of STEs, the potential for different microbial processes at various depths, and fluxes to the overlying water. The flux of nutrients sourced from groundwater to estuarine ecosystems is one of the least known numbers. Monitoring of geochemical profiles and groundwater discharge rates allow us to calculate the potential flux of DIN to overlying water, which may contribute to on-going eutrophication of the estuary. This study enhances our understanding of poorly constrained aspects of STE dynamics including variability over different time scales and nutrient fluxes to the overlying water.