Ecological consequences of ground water discharge to Delaware Bay, United States

Submarine ground water discharge to the ocean has the potential to create estuarine conditions near the point of discharge, thereby dramatically altering local benthic habitats and ecology. Aerial thermal infrared imaging along the southwestern margin of Delaware Bay indicated abundant discharge at Cape Henlopen, Delaware, adjacent to the Atlantic Ocean. On the sandflat there, we have documented low salinity in sedimentary pore waters within 20 m of the beachface that are associated with dense assemblages (in thousands per square meter) of a deep, burrow‐dwelling polychaete worm, Marenzelleria viridis, otherwise regarded as a species characteristic of fresher, oligohaline conditions. Where present, M. viridis is a numerical and biomass dominant in a benthic community strikingly different from that in nearby nonseep locations. At Cape Henlopen, the ecological role of the ground water discharge appears to be a multifaceted one. Seeps are localized regions of significantly reduced salinity, stabilized temperature, increased nutrient flux, high microalgal abundance, and enhanced sediment stability. M. viridis feeds on sediment diatoms and may provide an important trophic linkage between microalgal growth fueled by nutrients associated with the discharging ground water and worm‐feeding predators such as bottom fish or shorebirds common on the Cape Henlopen sandflat. Calculations based on our sampling suggest that nutrients supplied by the ground water substantially exceed what is needed to support benthic biomass and productivity estimated for this site.