Seasonal distribution of free-living nematodes in the Korsch, a coarse-grained submountain carbonate stream in southwest Germany

The temporal distribution of the nematode fauna in the upper 5 cm sediment of the streambed of a small submountain carbonate stream (Korsch, Germany) was investigated over a 1-year period. The total nematode density was low during summer, autumn and winter and reached a maximum value of 2238 individuals/10 cm(2) in spring. The fresh weight biomass of nematodes ranged from 13.5 to 1313 mug/10 cm(2). Deposit-feeders (76% of total fauna) and suction-feeders (15%) were the two dominant feeding-types. Of the 12 042 nematodes examined, 9% were gravid females, 29% females, 3% males and 59% juveniles. At least 113 species were present. The relatively low value of the Shannon-Wiener index of diversity (3.28) is not the consequence of a low number of species, but of the presence of the Eumonhystera filiformis Bastian, 1865/vulgaris-group (de Man, 1880) and the E. pseudobulbosa von Daday, 1896/sp.group at a much higher frequency (50% of total fauna) than other taxa. The density of E. filiformis/vulgaris and E. pseudobulbosa/sp. and both deposit- and suction-feeders was low during summer, autumn and winter and showed highest values in spring, similar to the variation in abundance of total nematode density. Throughout the year, nematodes were found in both depth layers analysed (0-2 and 2-5 cm), but higher mean annual densities occurred in the upper layer (77% of all nematodes). The higher population densities in the upper sediment were independent of feeding-type, age structure or species. The calculation of the Maturity Index and the percentage of Secernentea indicates 'good status' water quality at the analysed sampling site.