In the frame of the Fore-Delta-research (see introduction and fig. 1) a soil animal inventarisation has been done. Three trips were made: fall 1984, spring 1985(1985-1) and fall 1985 (1985-2). In this report the densities of the first and the second trip are discussed. In the (sublittoral) research area, that has a surface of ca. 1200 km², 125 species were found, which are mentioned in table 1.The 14 most general species (vgl. Tables 16 and 27) are: -the chaetopods Scoloplos armiger, Nephtys hombergii, Nephtys cirrosa, Magelona papillicornis, Spiophanes bombyx, Spio filicornis and Capitella capitata; -the freshwater shrimp Bathyporeia elegans and Urothoe poseidonis; -the bivalves Spisula subtruncata, Tellina fabula and Macoma balthica; -the sea star Ophiura texturata and -the Nemertinea group). The approach is summarised in figure 2: the strategy of sample taking was adjusted to the results of the previous trip.By TWINSPAN-classification of density data of 1984 it showed a special pattern could be distinguished, as far as soil animal composition and densities were concerned, on two similar stations. This made a strata-classification (6 strata) based on soil animal data possible. At the start of the project this was not yet possible based on available soil or soil animal data. At the second trip, a “stratified random” sampling was then done to be able to estimate accurately the densities per stratum. Reliability intervals for the geometric average density are calculated. The data are log-transformed, then the average of the observations is determined and this value is then transformed back for the individual strata and for the entire area (see table 27, figures 29-35 and the figures of appendix 4a). A strong correlation between the geometric average density and the frequency of the species was found (see 4 and 6). During the comparison of the TWINSPAN-classification of the data of the first two BOVO-trips, similar special patterns were discovered, stable zonation be it (see figures 25-28). The strata classification, deduced from the second trip and used for the third, is refined (10 strata), compared to the previous strata classification (6 strata) based on the first trip (see overlays 1 and 2). The northern strata (fine sand with low-average mud content) are relatively rich in species and individuals per sample (see figures 52 and 53). The Wester Scheldt mouth (with a high mud content in the mainly fine sand soils and the highly suppressive dust content in the water column) and parts of the Easter Scheldt mouth (where, in accordance to the high speed of streams, low mud contents are found in the middle grove sandy soil) showed few species and individuals per sample in 1984-1985 (see figures 23 and 24). Per stratum an abiotic description is given (see 3 and table 9). Per species correlations were sought in a graphic way with measured soil factors and the depth, and the ecologic distribution was researched, based on data from the second trip (see 5 and e.g. the figures 38, 41 and 45). On the entire density-dataset (1984 and 1985-1 apart) an ordination (DECORANA) technique has been applied, and more research for order-correlation with measured soil factors and the depth (see 2.5). These proved to be statistically significant correlated with one or more of the DECORANA-ordination-axes (see 5). Extremely high correlations were found with the mud content (and the group of correlated variables) (see table 26 and fig. 49). In relation to the application of different techniques for sample taking (Reineck-box corer and Van Veen-grab), these have been compared to each other on penetration-depth and density estimations for soil animals (see 2). The Box-Corer penetrated (mean 26 cm) noticeably deeper than the Van Veen-grab (mean 15 cm) (see table 4 en figures 3a en 3b). Only for the polychaetes Magelona papillicornis and Nephtys cirrosa and for the amphipode Urothoe poseidonis the density (p< 0.05) was significantly higher in the box. This also counts for the total density of all species, for the polychaetes and for the Amphipoda (cfr. table 5). In the discussion (see 6) a.o. a comparison is considered with data from Wolff from the period 1958-1969. From this we can conclude that sea species such as Tellina fabula have become more common in the Fore Delta. In the report a selection of the most remarkable figures is presented ; the others can be found in some appendix. (These can be consulted at the library of the NIOZ in Texel and at RWS-DGW in Middelburg). |
