Importance of Submarine Groundwater Discharge (SGWD) and Seawater Cycling to Material Flux across Sediment Water Interfaces in Marine Environments

The movement of water across sediment/water interfaces is very important to the ecology of aquatic habitats. Submarine groundwater discharge (SGWD) occurs primarily by advective flow and may be due to a variety of factors. In nearshore marine environments, the major factor is probably groundwater flow from upland regions. In oceanic environments, SGWD is probably influenced more by tides and surge action. SGWD, therefore, represents various mixtures of fresh groundwater and seawater. This paper summarizes the first regional study of SGWD and its attendant solutes in shallow estuarine, continental shelf and coral reef habitats, and calls attention to the magnitude of water volumes which appear to be circulating through marine sediments. Data on SGWD from sites in the Florida Keys and on the southeastern continental shelf of the U.S. indicate that water movement across sediment/water interfaces is a common occurrence at least to water depths of 30 to 35 m. Discharge values from the Florida Keys were 8.9 l m-2 d-1 (N congruent-to 344) for depths < 27 m and 5.4 l m-2 d-1 (N congruent-to 261) for depths of ca 27 to 39 m. On the southeastern continental shelf, discharge ranged between ca 6 and 20 l m-2 d-1. One site was found in 20 m depth where there was a persistent negative hydraulic head and a mean influx of seawater to the sediments (ca 10.8 l m-2 d-1). Even though geohydrological models would predict coupling of SGWD with landbased hydraulic heads, definitive lower salinity SGWD could not be detected. The driving force seems to be subtidal pumping and much of the discharge measured was probably recycled seawater. This research also demonstrated that SGWD serves to move dissolved solutes into the water column, and could be an important link in benthic-pelagic coupling in continental shelf ecosystems.