Seasonal and Geologic Controls on Submarine Groundwater Discharge‐Derived Nutrient Fluxes to Two Coastal Embayments

Abstract Submarine groundwater discharge and porewater exchange are critical but often overlooked sources of nutrients to coastal systems. This study investigates how sediment permeability and summer‐winter seasonal dynamics influence the magnitude and biogeochemistry of nutrient fluxes from submarine groundwater discharge and porewater exchange. Reaction rates in the subterranean estuary were used to identify the transformations and drivers of nutrient loading in two geologically distinct embayments in the U.S. East Coast: the highly permeable Peconic Bay estuary (NY), and the surficially fine‐grained coastal embayments of the southern Delmarva Peninsula (VA). We applied radium isotope mass balances to distinguish total submarine groundwater discharge and porewater exchange contributions to identify the dominant nutrient pathways. Seasonal comparisons of Peconic Bay revealed that increased surficial aquifer recharge in winter led to a seaward shift of the freshwater‐saltwater interface and elevated submarine groundwater discharge, whereas porewater exchange remained seasonally stable. In contrast, the Eastern Shore showed higher rates of submarine groundwater discharge and nutrient production despite the lower permeability at the sediment surface, highlighting the influence of deeper aquifer permeability. Despite similar subsurface residence times across sites, nutrient transformations within the subterranean estuary were largely governed by oxygen availability, with submarine groundwater discharge dominating nutrient delivery in winter and fine‐grained sediments enhancing ammonium, phosphate, and silicate production. These findings highlight the importance of hydrogeologic setting and seasonal forcing in controlling nutrient cycling in coastal aquifers and adjacent embayments.