Seasonal dynamics of geochemical gradients along with anammox and denitrification activities in a subterranean estuary located in the York river estuary

Subterranean estuaries (STEs) form at the interface between groundwater and seawater. This interaction of two distinct water bodies creates a highly reactive zone supporting various biogeochemical processes such as anammox and denitrification. Groundwater advection supplies dissolved inorganic nitrogen (DIN) and carbon (DIC) to the STE, whereas overlying seawater supplies fresh organic matter and oxygen. We have examined the geochemical depth profiles and measured anammox and denitrification rates in a STE located at the mouth of the York River estuary (YRE) in Virginia (USA). STE porewater was sampled seasonally for environmental parameters including; salinity, dissolved oxygen, DIN, DIC, sulfide, as well as other analytes. Anammox and denitrification rates were measured in sediment slurry incubations with ¹⁵N-nitrate as a tracer. The depth gradients of these analytes and rates were compared to examine seasonal changes in the STE profiles. We have shown that this system is not static with seasonal variation in geochemical gradients. Anammox and denitrification rates may be limited in the STE due to the dynamic nature of the system and a lack of labile organic matter respectively. Seasonal monitoring of geochemical and microbial profiles and groundwater discharge rates allow us to calculate the potential flux of DIN to the YRE, which may contribute to on-going eutrophication of the estuary. This study enhances our understanding of poorly constrained aspects of STE of seasonal dynamics including microbial processes occurring at various depths and nutrient fluxes to the overlying water.