Freshwater springs on intertidal sand flats cause a switch in dominance among polychaete worms

Effects of freshwater seepage on benthic macrofauna were investigated on the sandy tidal flats near the island of Sylt (German Wadden Sea) in 2002. Several permanent seepage areas (50 to 200 m offshore; up to 200 m2 in area) were examined, in which salinity ranged from 22–29 outside to 0–16 psu inside seepage areas in the upper 20 cm of sediment during summer low tides. The freshwater seepage areas were characterised by an absence of lugworms (Arenicola marina) and a twelve-fold increase in nereid polychaetes (Nereis diversicolor and N. virens) relative to non-seepage areas. Lugworms and ragworms were scarce in a transition zone, which was colonised by juvenile lugworms. We suggest that nereid polychaetes avoid competition with bioturbating lugworms by adapting to areas of low salinity (freshwater seepage).