Response of macrofauna to drifting tidal sediments

The effect of hydrodynamically-mobilized sediment on abundance and vertical distribution of macrobenthic fauna was studied in Königshafen, a sheltered tidal bay at the northern end of the Island of Sylt (North Sea). Sediment drift tended to increase from high towards low tide level, while abundance of nearly all species decreased (with the polychaeteSpio filicornis as a notable exception). To test whether this decrease could be attributed to water currents affecting sediment stability, experimental flumes with funnels at both ends were set up to enhance sediment mobility by increasing tidal current velocities. Abundance and vertical distribution of fauna inside and outside the flumes were compared. Responses of individual species depended on their vertical position in the sediment, and resembled those observed along the gradient of sediment drift between high and low tide levels. Mainly juveniles ofPygospio elegans, Scoloplos armiger, Hydrobia ulvae andMacoma balthica, and the small polychaeteMicrophthalmus sczelkowii were washed out of the sediment. No effect of increased erosion inside the flume was found on the numbers ofCapitella capitata and the oligochaetesTubificoides benedii andT. pseudogaster. These oligochaetes probably migrated downwards with increasing erosion in the flumes. Numbers decreased in the upper cm and tended to increase below. A storm had a similar effect on oligochaete vertical distribution, while under conditions of permanently high sediment mobility near low tide level, these species were rare or absent. It is concluded that even under sheltered conditions, differential degrees of sediment mobility may have effects on the zonation of the tidal flat macrofauna.