Annual Weather Patterns Drive Large Pulses of Saline Submarine Groundwater Discharge and Major Summer Inputs of Nutrients to the South Atlantic Bight

Abstract Submarine groundwater discharge (SGD) is a significant source of nutrients to continental shelf waters. Increasing evidence suggests that most of this flow is saline (∼seawater salinity) and occurs across broad continental shelves, making it challenging to observe. Using new, cost‐effective heat‐tracer methods and geochemical analyses of subseafloor saline groundwater, we found that offshore saline SGD delivers repeated pulses of nutrients to the overlying South Atlantic Bight (SAB) water column in the summer. Thermal time‐series measurements were collected below the sandy seafloor 10–15 km offshore of Charleston, SC, during three consecutive summers. Up to seven pulses of saline SGD occurred each summer. Discharge velocities ranged from 1 to 8 cm day −1 . Pulses of SGD coincided with upwelling‐favorable winds related to annual shifts of the North Atlantic Subtropical High and with winds from storm activity. We observed spatial and temporal variability between SGD pulses from analogous sites 5–10 km apart. Cumulative summertime SGD in 2019 corresponded to 65%–70% of the radium‐based estimates of SGD offshore of South Carolina and was similar in magnitude to the input from regional river discharge. Thermal and geochemical analyses indicate two‐way seawater‐groundwater exchange across the seafloor, reflecting infiltration of seawater into the seafloor and SGD to overlying seawater. Geochemical analyses confirmed high nutrient concentrations in SGD compared to the overlying water column and river water. The δ 15 N of total dissolved nitrogen and δ 13 C of dissolved inorganic carbon in groundwater suggest significant contributions from mineralized organic matter with a geochemical composition distinct from overlying seawater.