Temporal and Spatial Variations in Subterranean Estuary Geochemical Gradients and Nutrient Cycling Rates: Impacts on Groundwater Nutrient Export to Estuaries

Abstract

Subterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m−2 d−1. The total groundwater discharge rate was 38 ± 11 L m−2 d−1; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m−2 d−1 for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.

Sea‐Level Rise Impacts on Tidal Marshes and Estuarine Biogeochemical Processes

“…Overall, this modeling system tends to encompass the dominant hydrodynamic and biogeochemical processes. For example, as reported by Wilson et al (2023), the nutrient fluxes via groundwater discharge are much lower than the fluvial inputs. This study neglects the groundwater coupling but is dynamically linked to the watershed loadings.…”

Nitrification in a Subterranean Estuary: An Ex Situ and In Situ Method Comparison Determines Nitrate Is Available for Discharge

“…It is a sandy sediment beach; approximately 30 m long with a rock jetty on either side and a tidal height of ∼0.8 m; a detailed site description can be found in (Beck et al., 2016). Annual average groundwater discharge at this site has been estimated as 38 ± 11 L m −2 d −1 by Wilson, Anderson, Song, and Tobias (2023) and 39–62 L m −2 d −1 by Beck et al. (2016).…”

“…This study was conducted at the Gloucester Point Beach STE (GP‐STE, 37.248,884°N, 76.505,324°W), which is located along the York River Estuary, a tributary of the Chesapeake Bay. The GP‐STE is a well‐studied system where groundwater discharge to overlying water has been observed (Beck et al., 2016; Luek & Beck, 2014; Wilson, Anderson, Song, & Tobias, 2023). It is a sandy sediment beach; approximately 30 m long with a rock jetty on either side and a tidal height of ∼0.8 m; a detailed site description can be found in (Beck et al., 2016).…”

“…Porewater samples were collected in four seasons (2018–2019) from dedicated piezometers and analyzed for sample salinity and concentrations of NH 4 + and NO 3 − as described by Wilson, Anderson, Song, and Tobias (2023). Profiles used here included samples taken every 10 cm from surface water (0) to 110 cm in the subsurface.…”

“…It was suggested by Schutte et al (2017) that oxygen penetration into the subsurface drove nitrification rates and likely played a role in organic matter remineralization. Nitrification in the surficial 50 cm of the GP-STE is not oxygen limited as concentrations are >3.5 mg L 1 throughout the year and NH 4 + could be supplied by both the nutrient-rich groundwater and remineralization of dissolved organic matter in recirculated surface water (Wilson, Anderson, Song, & Tobias, 2023), therefore, it is unlikely that oxygen scarcity is driving low rates compared to other sites. Alternatively, it is possible that the observed low rates may be the result of low NH 4 + supply.…”