Influence of seagrass beds (Zostera noltii) on the species composition of juvenile fishes temporarily visiting the intertidal zone of the Wadden Sea

It is generally accepted that the presence of seagrass habitats significantly contributes to faunal diversity and density in coastal ecosystems. Most scientific research has been conducted in subtidal seagrass beds. Especially for highly motile fauna such as fishes, the relevance of intertidal seagrass stocks has scarcely been documented, and the studies that exist are limited to tropic coastal areas and to the temperate zone of the southern hemisphere. Therefore, no information is available on the impact of intertidal Zostera noltii (Hornemann) beds on shallow-water fish communities in the Wadden Sea (North Sea), although such meadows survived the major seagrass loss in the 1930s. We compared species composition and abundances of juvenile tidally migrating fishes in vegetated and unvegetated parts of the intertidal zone of a shallow tidal basin in the northern Wadden Sea near the Island of Sylt (Germany). During the seagrass growth period, the occurrence of juvenile fishes varied considerably. Ten fish species visiting the intertidal zone showed no significant difference between numerical densities in Z. noltii beds and adjacent bare sand flats. However, based on their distribution in the intertidal zone at high tide, three groups of fishes could be identified by cluster analysis: (i) five species found exclusively in Z. noltii beds, (ii) two species found predominantly in Z. noltii beds but also on bare sand in lower numbers, and (iii) three species found exclusively on bare sand flats. On tidal flats, Groups (i) and (ii) appeared simultaneously with the main growth period of Z. noltii in August and September, whereas the seasonal distribution of Group (iii) did not correspond with the period of maximum seagrass coverage. The results indicate an – up to now unknown- qualitative impact of Z. noltii beds on the species composition of 0-group common North Sea fishes temporarily visiting the intertidal zone, asking for further research.