ECOLOGICAL CONSEQUENCES OF ESTUARINE GROUNDWATER DISCHARGE AT CAPE HENLOPEN, DELAWARE BAY, USA

Submarine groundwater flux to the coastal ocean creates estuarine conditions at and near the point of discharge, thereby altering local benthic habitats and ecology. At a sandflat at Cape Henlopen, Delaware, adjacent to the Atlantic Ocean, we have documented low salinity in sedimentary pore waters within 10 m of the beachface. These low salinity patches are correlated with dense assemblages (in thousands per square meter) of a deep tube-dwelling polychaete worm, Marenzelleria viridis, otherwise regarded as an estuarine endemic species of fresher, oligohaline conditions and typically found farther up the Delaware Bay. Size-frequency analysis indicates that recruitment of juvenile M. viridis occurs in the spring of the year, coinciding with the time of greatest extent of low-salinity patches. Worm and low salinity patches persist for a year or longer. Where present, M. viridis is a numerical and biomass dominant, defining a benthic community strikingly different from that in nearby, non-seep locations. Because of its ecological role as a feeder on sediment diatoms, M. viridis may provide an important trophic linkage between microalgal growth fueled by high nutrient loads associated with the discharging groundwaters and worm-feeding predators such as bottom fish or shorebirds common on the Cape Henlopen sandflat.