The intertidal hyperbenthos of sandy beaches was sampled with a handsledge in winter and summer. 10 stations along the Belgian coast were sampled on each occasion. Two additional localities in the north of France were sampled during summer. Mysidacea dominated the hyperbenthos, both in terms of density and in terms of biomass. Other important taxa were Decapoda, Pisces, Amphipoda, Annelida, Coelenterata and Copepoda. Several multivariate statistical analyses revealed clear differences between winter and Summer hyperbenthic communities. The species composition of samples taken in the north of France deviated from that of the other samples. The remaining samples of each season could be divided in a rich and a poor community. The densities and the diversity of the so-called rich communities and the French community were much higher than those in the so-called poor communities. Mesopodopsis slabberi Was often very abundant during SUmmer while Schistomysis kervillei and S. spiritus reached highest densities in winter. It was remarkable that the structure of the population for one species could be very different according to the place of sampling. For every species a positive correlation was found between the length of the gravid females and the number of embryo's in their marsupium. The nauplius-stage always spent a longer time in the marsupium, the embryo stage a smaller period and the postnauplius-stage an even shorter period. For both species of the genus Schistomysis, the number of postnauplius-stages in the marsupium was significantly lower than the amount of embryo stages. The intramarsupial mortality for these species was a lot higher in the intertidal zone than the values that were calculated by Mauchline (1980). This could be caused by the extremely energetic conditions in the intertidal zone. Diel patterns: A day cycle was carried out in Zeebrugge to study the diel pattems in the intertidal hyperbenthos. For this purpose every hour samples were taken with a handsledge. Throughout the 24 hour period Mysidacea Were the most abundant group, but Decapoda and Pisces Contributed most to total biomass. Coelenterata, Isopoda and Cumacea also reached high densities. Four communities could be distingUished by classification and ordination. A first division separated the day samples from the night samples. Light-intensity was an important variable in the Canonical Correspondence Analysis. Both communities Were subsequently divided into an ebb-tide and a flood-tide situation. Tidal height was an important variable in the Canonical Correspondence Analysis. The species-richness was always lower at high tide than at low tide, while the night-samples had a higher diversity than the day-samples. |
