Die Nematoden des Breitenbachs – Struktur, Dynamik und mögliche Funktion im Ökosystem eines Mittelgebirgsbachs

Free-living nematodes inhabiting the sediments of the Breitenbach stream were investigated from 1992 until 1995. The Breitenbach is a submontaneous 1st-order stream in Eastern Hesse, Germany. The study aimed to generate an inventory of free-living nematode species, determining their abundance, the composition of the communities and their fluctuation in space and time in relation to selected biotic and abiotic environmental variables. Data on the number of generations per year, biomass and production were required to allow a comparison other organism groups in the Breitenbach as well as with data from different aquatic communities. The investigated sediment ranged between sand and fine gravel, the organic matter content ranged between 0.27 and 6.7% of fresh sediment weight, and the discharge ranged between 2 and 386 L/sec in the period of the study. With all tested extraction methods the presence of fine detritus in the extracts proved to impede recovery of the nematodes. For the Breitenbach sediment, the most suitable method was a flotation technique using diluted Ludox® as the extraction medium. An aqueous magnesium sulphate solution proved to be less efficient. Using the available identification keys, the majority of adult Monhysterida proved to be indeterminable. The dominant species was new to science, Eumonhystera breitenbachi n. sp. Also with known species, the identification process was cumbersome and prone to errors. There are inconsistencies within the keys and disagreements between different keys. To obtain more reliable and reproducible identification results, a computerized polytomous identification system was developed as an Excel macro that was linked to the digitising tablet and the microscope, allowing instant comparison of all measured morphometric parameters with literature data. Extended by an Index-System (assuming unimodal distribution of morphometric characters) the system also allows examining complete data bases automatically. The Breitenbach sediments were inhabited by an abundant and diverse nematode community. 241 taxa were recorded; of these 53 were generally aquatic species, another 98 species are considered to be amphibious and the rest of 90 species are classified as terrestrial. Steady and dominant species generally were aquatic. The average abundance was determined to be 51 Individuals per millilitre sediment (3 – 787 ind./mL), corresponding to a density of ca 2.55*106/m². Abundances could vary between replicates by an order of magnitude. No clear seasonality could be detected, but consistent differences between sampling areas along the stream. Abundances were affected by a winter flood event in the lower reach, less distinctly in the central and upper reach. The abundance and contribution of Monhysterida decreased after the flood, whereas the populations of large Triplonchida and Enoplida remained more stable; their relative contribution increased. These changes partly coincided with distinct changes in the abiotic sediment parameters. Ordination analysis (CANOCO) revealed patterns in the community and allowed both the effects to be visualized and the influence of measured environmental variables to be quantified. 34 to 60% of the total variance was captured in the first four axes (indirect methods, e.g. PCA). With constrained analysis (direct method e.g. RDA) 17 to 30% of the total variance was captured as species-environment interaction (and used in the model). 29 to 41% of the variance was allotted to the covariable sampling date. The results indicated links between abiotic parameters and the composition of the community, but also that further environmental variables must be relevant but were not measured. From the environmental variables that were measured the sediment grain size was best correlated with the latent variables. R-strategic bacterial feeders dominated in the Breitenbach in terms of abundance, whereas in terms of biomass larger and K-strategic species were dominant. Generation times ranged between 23 and 365 days, corresponding to 1 – 17 generations per year. Nematode biomass (standing stock) was approximately 0.45 g fresh weight per square meter and the production was estimated to be 0.6 g carbon per square-meter and year. These results indicate that quantitatively freeliving nematodes play only a minor role in the trophic net of these sediments: Bacterial and macrozoobenthic biomass and production was more than an order of magnitude higher compared to those of the nematodes. Further functions the nematodes might have in the benthic community such as generating hot spots of increased bacterial production or producing rare exoenzymes are discussed. Observations from this study indicate that links between nematodes and other biota tend to be bidirectional, i.e. nematodes can predate on ciliates, but at the same time become prey of ciliates.