Passive settlement of Macoma balthica spat on tidal flats of the Wadden Sea and subsequent migration of juveniles

Spatfall of the tellinid bivalve Macoma balthica may occur in fine-grained sediments at different tidal elevations in the Wadden Sea, but it is not clear which mechanism (active habitat selection or passive sinking of larvae, or both) can explain the observed distribution pattern. Spatfall and the subsequent development of juvenile abundances and size distribution were studied along a transect in the northern Wadden Sea near the Island of Sylt. Initial spatfall was highest in the lower intertidal and correlated significantly with hydrographic parameters. The enhanced larval settlement observed in dense aggregates of the tube-building polychaete Lanice conchilega confirmed the prominent role of hydrographic factors in initial spatfall. Subsequently, abundance strongly decreased in the lower intertidal but increased in the upper intertidal due to immigration of newly settled spat from lower sites. By the end of October, abundance was higher in the upper intertidal. Growth of the juveniles was higher in the upper intertidal. It is suggested that (1) initial spatfall is mainly ruled by hydrographic factors without active sediment selection; (2) young Macoma remain highly mobile during the first weeks of their benthic life and by byssus drift may achieve a net transport into areas with weak initial spatfall; (3) at the end of their first winter a second period of high floating activity occurs, bringing them back towards the lower intertidal and shallow subtidal. Thus, by successive postlarval migrations Macoma may several times change the intertidal site occupied during their first year of life. The comparatively low interannual variability of recruitment success noted in this species in the Wadden Sea may be a result of these migrations

Synoptic patterns of meiofaunal and macrofaunal abundances and specific composition in littoral sediments

During recent years, many investigations on small zoobenthos have been performed at the island of Sylt. As these studies were carried out sporadically over many years and as different extraction methods were used, comparisons of the results have been hampered. Therefore, in August/September 1986, 24 sites were sampled and evaluated using one quantitative method throughout. Sites range from mud to exposed sand and from the sublittoral to the supralittoral. Macrofauna and the taxa Plathelminthes, Polychaeta, and Oligochaeta are determined to species level. Macrofaunal (>0.5 mm) abundance is highest in mud and continuously decreases with increasing exposure to wave action. Meiofaunal (<0.5 mm) abundance is less variable. Nematoda dominate in mud and muddy sand, Copepoda in sheltered and exposed sand, other taxa only intermittently. Related to surface area, no correlation between macro-and meiofaunal abundance is apparent. Plathelminthes and Copepoda reach highest abundance per surface area in sand but their per volume density is higher in mud and muddy sand. Related to sediment volume instead of surface area, the meiofaunal abundance pattern is very similar to the macrofaunal pattern. The faunal composition changes gradually along the tidal gradient without general faunal boundaries. On an averange, the faunal similarity of neighbouring sites is highest in Oligochaeta and lowest in Plathelminthes. Presumably, Oligochaeta tolerate wider ranges of environmental factors. This may explain the low number of oligochaete species. On the other hand, Plathelminthes seem to adapt to relatively narrow ranges of factors and their species richness is highest. Because of macrofaunameiofauna interaction it is suggested that the meiofaunal assemblage will be least stable in mud and muddy sand, and most stable in exposed sand

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

Meiobenthic gradients with special reference to Plathelminthes and Polychaeta in an estuarine salt marsh creek—a small-scale model for boreal tidal coasts?

Environmental conditions in salt marsh creeks are intermediate between the open tidal coast and estuaries. A large salt marsh creek at the island of Sylt (North Sea) was studied in order to test whether its fauna is more similar to that of the open tidal coast or to that of estuaries. Because of a sandy bar at the seaward opening, the tidal range is only 10 cm in the creek, and the water level never drops below the level of the sand bar. Zoobenthos in the sandy bottom and on the sandy shores was studied at both ends and in the middle of the creek. Polychaeta and Plathelminthes were determined to species level. On an average, 2115 metazoans were found below 10 cm2 of surface area. At the seaward end of the creek, abundance and taxonomic composition are similar to that of the adjoining Wadden area. Nematoda are the dominant taxon, followed by Copepoda, Plathelminthes and Oligochaeta. Taxonomic composition is different at the landward end. Plathelminthes and Nematoda are most abundant followed by Copepoda. Both Oligochaeta and Polychaeta are scarce at these newly eroded sites. Plathelminth abundance at the landward end of the creek is exceptionally high (770–935·10 cm−2). Contrary to what is generally found in estuaries, the species density of Plathelminthes shows a significant increase toward the land. The species composition of Polychaeta and Plathelminthes indicates that the sites below mean high tide level of the creek correspond to the adjacent eulittoral Wadden area while the fauna of the supralittoral sites of the creek is similar to the fauna of supralittoral tidal coasts. Typical sublittoral species did not occur in the salt marsh creek. Thus, salt marsh creeks may be regarded as a small-scale model for the tidal coast. In context with the results obtained, the definition of estuaries is discussed

Species composition and abundance of macrozoobenthos in the SE North Se

Exposed sandy coasts are predominantly physically controlled environments where benthic communities are structured by the independent response of species to the physical environment, with minimal biological interactions (swash exclusion hypothesis). This prevalence of physical control may be regarded as a typical property of exposed coastal areas. In an offshore direction, the importance of wave effects on the benthos will diminish until a depth is reached where they are no longer significant [wave exclusion hypothesis (WEH)]. This loss of a coastal property may be used to define an offshore depth limit of the coastal zone. We used a large set of benthos data from the SE North Sea to test whether an offshore limit of the coast can be clearly recognised despite strong small-scale variability and how this limit would vary seasonally and from year to year. In accordance with WEH, both species density and total abundance of macrobenthos were low in the surf zone, strongly increased with depth, and averaged over all sampling dates became relatively constant below 30 m depth. Seasonally, these gradients were weaker during summer recruitment than during autumn. Species richness, by contrast, showed no significant difference with depth. In single years, the depth of the turning point from increasing abundances to constant abundances varied between 20 and 31 m (equivalent to 40-80 km off the coastline) depending on wave height. We conclude that this zone can be derived from benthic community gradients