|Seizoenale en ruimtelijke patronen in de hyperbenthische gemeenschap van de Westerschelde|
Dewicke, A. (1991). Seizoenale en ruimtelijke patronen in de hyperbenthische gemeenschap van de Westerschelde. BSc Thesis. University of Gent, Zoology Institute, Marine Biology Section: Gent. 119 pp.
The Westerscheldt is an estuary characterised by an important salinity-gradient. The hyperbenthic fauna (organisms living in the water-column, near the bottom), was sampled along this gradient. Fourteen localities were investigated over three seasons (April, August and December). The hyperbenthic fauna consists of the temporary and the permanent hyperbenthos. Mysids are the most important representatives of the permanent hyperbenthos, this is exemplified by their density as well as their biomass. Other important groups are the Isopodes and the Amphipodes. The highest density and biomass is reached in summer. In December the hyperbenthos is still present. It is, however, far less abundant than in August. Compared to other months, the ratio biomass/density is strikingly bigger in April. The temporary hyperbenthos consists mainly of crab-larvae, fish-eggs and-larvae. It is mainly present during spring. The distribution of organisms along the estuary is largely dependent on their salinity-preference and -tolerance. For more planktonic creatures the water currents and the tidal activity also prove to be important. Along the estuary , one can clearly discriminate species-communities. To the East there is an estuarine community. Downstream, the typically estuarine species are gradually being replaced by marine animals, resulting in a typical marine community at the mouth of the river . The species-communities at the mouth are the most diverse, they are characterised by low densities and biomasses. The estuarine fauna, however, is less diverse, the present species have high densities and biomasses. The species-communities not only show a spatial sequence, there is also a temporal succession. There are always one or two dominant mysid species who replace one another in space and time. The CCA-analysis reveal that there is a strong correlation between the species- communities and the salinity/extinction gradient. Besides their own biology, characterizing the species in the community, a lot of species show a salinity preference. This variable is determining for the occurrence of a lot of biota in the estuarine ecosystem. Finally, the population structure of several mysids was studied. In April, a population of mainly large, overwintering adults was found. In August all life stadia were present in reasonable numbers. The adults become mature earlier and are therefore less big. In December the population largely consists of juveniles and subadults who mature slowly and will live through winter. The large body-sizes found in the April-population are an explanation for the large biomass/density ratio found in that month.
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