Variations in the sulfide regime and the distribution of macrofauna in an intertidal flat in the North Sea

Sulfide concentrations were measured in the inner and outer Königshafen (Sylt, Wadden Sea) from November 1990 to December 1991 to assess the fluctuations of sulfide levels in natural tidal habitats. Three different areas were compared: (1) muddy sediment (2) fine-medium sand, and (3) a mussel bed. Other abiotic factors such as Eh, pH, temperature, grain size and organic content were measured. After assessment of the macrofaunal distribution, an attempt was made to relate the distribution to the sulfide concentrations in the benthic habitat. Sulfide concentrations varied between sites throughout the year with considerable differences ranging from below 5 μM (limit of detection) to about 1 mM (Oct. 1991). The faunal composition (Table 2) at the different sites hardly varied; it was always dominated by annelids: The cirratulidTharyx marioni was the most abundant species in the upper layers of all sites, where it occurred at low sulfide concentrations (<50 μM).Heteromastus filiformis was commonly found in the deeper sediment layers of the muddy site where it was regularly exposed to sulfide levels around 75 μM and peak concentrations in autumn up to 1 mM.Capitella capitata, Tubificoides pseudogaster andTubificoides benedii were very common in the upper sediment layers where sulfide levels were about 150 μM in autumn. These species also occurred, however, in the deeper layers with higher sulfide concentrations. These results document not only the wide annual range of sulfide concentrations at different depths in a tidal flat, they also emphasize that under natural conditions tidal flat annelids are exposed to considerable concentrations of hydrogen sulfide.