MEPS 363:205-215 (2008)  -  DOI: https://doi.org/10.3354/meps07455

ABSTRACT: Groundwater discharge is a common phenomenon along sandy coasts. At Cape Henlopen, Delaware, USA, it creates low porewater salinity in spatially heterogeneous patterns over 1 to 10 m horizontally. In the present study, porewater salinity and macrofaunal communities were sampled along intertidal transects in summer and spring. We consistently observed high spatial variability in porewater salinity at 10 cm sediment depth, with changes up to 15.8 m^–1. Community composition and species abundance differed between the seasons, but, in each season, communities associated with low porewater salinity (<15) differed from those associated with higher porewater salinity (15 to 24 or >24). In general, species from a diverse pool of sand flat species gradually became absent as salinity decreased. These local community changes correlated with porewater salinity could compromise the interpretation of various benthic indices that are based on water-column salinity, which, at this site, remains around 28. In most samples, the polychaete Marenzelleria viridis was numerically dominant (up to 10000 m^–2) in burrows up to 50 cm deep. Both in the field and in cores constructed in the laboratory, we found that the number of worm burrows was highly correlated with hydraulic conductivity of the sediment. We propose that burrowing worms (especially M. viridis) at this groundwater discharge site act as hydraulic ecosystem engineers by means of their burrowing and tube building. In general, there appear to be strong physical–biological interactions among the distribution of organisms, their infaunal behavior, the hydrological properties of the sediment and the magnitude of groundwater discharge that may contribute to infaunal zonation on intertidal sand flats.

KEY WORDS: Benthic communities · Groundwater · Salinity · Species distribution · Intertidal · Hydraulic conductivity