Ecological functions of intertidal seagrass beds for fishes and mobile epibenthos in the northern Wadden Sea

Seagrass beds are worldwide known to represent important habitats for fishes and invertebrates. Most investigations of the role of seagrass beds as habitats for animal assemblages were carried out in subtidal zones, whereas only little information is available on intertidal seagrass beds. However, different ecological functions are to be expected between both systems.This study was to investigate whether intertidal seagrass beds fulfil vital ecological functions for common North Sea fishes and -crustaceans although they are extensively exposed during the tidal cycle. Quantitative sampling strategies were combined with field experiments on vegetated and unvegetated tidal flats in the Sylt-Rømø Bight (German-Danish Wadden Sea, North Sea). The results showed that abundances as well as secondary production of dominant mobile epibenthos were significantly higher on vegetated tidal flats. Furthermore mobile epibenthos was found abundant within seagrass beds during ebb tide when a thin layer of residual water remains within the canopies. The faunal density was consistently higher in the canopy water layer than in experimentally installed tide pools although the content of dissolved oxygen dropped drastically at night. This study gave empirical evidence that Z. noltii beds were important spawning areas for fish species such as the herring (Clupea harengus L.) and the garfish (Belone belone L.) that periodically enter Wadden Sea waters for reproduction. Juveniles of migrating species showed low densities in the intertidal zone and the species composition was found strictly subjected to season. However, during their temporary residence on inundated tidal flats fish species showed distinct preferences for either vegetated or unvegetated habitats.