This study forms part of a global research project, which aims to evaluate the effect of the Delta works on the ecosystem of the Dutch Delta. The Dutch Delta is the sublittoral region along the coast of the south of the Netherlands, from the Belgium border in the south to the Hoek van Holland in the north. Off shore, the area is arbitrary limited by the 10m depth line. From north to south, the Nieuwe Waterweg, the Haringvliet, Lake Grevelingen, the Eastern Scheldt and the Western Scheldt debouch into this part of the Southern Bight of the North Sea. As a result of the tidal currents and the thereon coupled transport of the sediments, a heterogenous pattern of banks and channels has evolved. The hydrodynamic regime and the bottom morphology have changed drastically since the start of the Delta works in the beginning of the sixties. Correlated to the changes of the abiotic environment, alterations of the biotic characteristics are expected. To investigate this, in the first place an inventarisation of the region is required. The present study deals with an important component of the marine benthic ecosystem: the free-living nematodes. Twenty sampling stations were selected, of which 8 are located in the channels, while the others are situated on the banks. Data of the abiotic environment were available for these locations. In total, the stations were sampled three times (Autumn '84, Spring '85 and Autumn '85), with the help of a Van Veen grip or a boxcorer. Apart from the inventarisation, it is tried to gain insight in the relationship between the structure of the nematode communities and the present abiotic environmental gradients. The Dutch Delta Region is characterized by large physical and chemical gradients which might be important for the determination of the structure of the nematode communities. The studied factors can be grouped as follows: -Hydrodvnamics: The highest current velocities (up to 120 cm/s) and fluctuations are measured in the channel of the Western Scheldt. In the mouth of the Eastern Scheldt, near the storm surge barrier, high velocities (up to 91 cm/s) exist as well. On the banks, the current velocities are always low (67 cm/s). -Sediment composition: the sediment mostly consists of well sorted fine sand (0.14- 0.37 mm), with only low percentages of gravel and silt. The finest sediments are found in the north, at the level of and above the Lake Grevelingen. The highest silt fractions (5 to 20 %) are present in the channels, while on the banks the level of this fraction never surmounts 5 %. In the Western Scheldt stations, temporal silt accumulations occur. Due to the fluctuations of the currents, these silt fractions may vary between 1 and 60 %. -Water quality: the highest chlorophyll-a values are found on the shallow banks. Low values are also associated with some of the deeper stations, mainly those situated at the level of the seawards offshoots of the channel of the Eastern Scheldt. The freshwater input in the Delta region only influences the salinity to a low degree. Only at the level of the sluices of the Haringvliet, the percentage of freshwater may be higher than 20 %. The density, diversity, species composition, trophic structure, biomass and production of the nematode communities has been studied. Density always appears to be correlated with the sediment composition. Highest densities are found in the silty and fine sandy stations. In the mouth of the Lake Grevelingen, the density is exceptionally high (almost 12000 N/10 cm²), this in comparison with the other silty and fine sandy regions (as found in the north of the Dutch Delta and in a part of the channel of the Eastern Scheldt; 2000 to 6000 N/10 cm²). Most likely, it is the natural organic enrichment of the lake Grevelingen which causes this spectacular increase. Shallowest stations are characterized by relatively low densities (80 to 300 N/10 cm²). Deeper stations (with high velocities and fluctuations of the currents; e.g. in the mouth of |
