Platyhelminth species composition and abundance in sheltered coastal habitats of the eastern North Sea (1982-1983)

In 1982 and 1983, sheltered upper intertidal and supratidal habitats on Sylt island were quantitatively studied for meiofauna, with special emphasis on platyhelminths. This included (1) monthy sampling of two accretion sites, one vegetated and the other bare, and two salt marsh sites, one grazed and the other ungrazed; (2) seasonal sampling of 4 transects across salt marsh areas and accretion zones; and (3) some 50 sites sampled once only. Platyhelminth life cycles were analysed from the monthly sampled areas. In supratidal salt marshes, abundance of most meiofaunal taxa followed soil moisture with low abundance during the dry summer months and high abundance in winter and spring with high precipitation and frequent floodings by the sea. In the accretion zones seasonality was weak, presumably as a result of the addition of the winter/spring maxima in the landward salt marshes, and the summer/autum maximum in the seaward flats. A total of 9 Acoelomorpha and 144 platyhelminth species was recorded. In the accretion zone, the main factors stucturing platyhelminth assemblages were sediment type (sand or mud), the frequency of flooding as determined by tidal level (twice a day, during spring tides, or during storm floods only), and the density of vegetation. In supratidal salt marshes these factors were overruled by salinity and sediment humidity. Most Platyhelminth species had a life span of 1 year with a single reproductive period in late winter or early spring while Acoelomorpha commonly had a plurivoltine life cycle.