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Investigations into the protistan fauna of groundwater habitats are still underrepresented. We investigated 14 aquifers comprising studies in lowlands around Cologne, the Eifel Mountains and the Alps (depths ranging from 6 to 400 metres) with different abiotic conditions. 21 heterotrophic fl agellate taxa, fi ve ciliate, four amoeba taxa and one centrohelid heliozoan were recorded. There was no clear relationship between protozoan communities and abiotic parameters at the different sampling sites with the exception of bicosoecids and euglenids that were more abundant at higher nitrate/chloride concentrations. The abundance of cultivable protists in free aquifer water ranged between 0.1 and 3 ind/ml. At one sampling site, abundances in free and sediment-associated water were compared revealing a ten-times higher abundance in the latter. The diversity of genotypes was investigated for cercomonads. Studies of Cercomonas and Paracercomonas D3-D5 LSU rRNA sequences indicated that groundwater may be a habitat with a relatively large diversity of species. A high number of species is probably not yet discovered.

Planktonic ciliate abundance and taxonomic composition were investigated from November 1998 to January 2000 in the River Rhine at Cologne, Germany. The composition of the ciliate fauna revealed the occurrence of both typical benthic (drifting peritrichs) as well as pelagic ciliates (oligotrichs) in the potamoplankton. Oligotrich ciliates prevailed in the potamoplankton community. Peritrichs, represented almost exclusively by sessile species, appeared to be washed out from the substrate at increased water discharge during flood events. Highest abundances of ciliates were registered during the phytoplankton spring bloom, indicating a bottom-up control of ciliates during this time, an observation which was supported by laboratory experiments. In summary, the ciliate potamoplankton community of the River Rhine is controlled by a seasonally varying set of factors: 1) Flood events in early spring and autumn cause a short-term succession from drifting peritrichs to pelagic oligotrichs, 2) the phytoplankton spring bloom supports oligotrich population growth, and 3) the low water level and subsequently high grazing pressure by benthic predators (mainly the mussel Corbicula) reduce the abundance of ciliates. The latter was probably the major reason for the extremely low ciliate abundances observed during summer.

The taxonomic composition, abundance and biomass of heterotrophic flagellates (HF) in the sediment of Lake Fuhlinger See, a complex of gravel pit lakes, were studied at three sampling periods along a transect from littoral to profundal. One lake treated with hypolimnetic aeration was compared with an untreated lake. Abundances of heterotrophic flagellates ranged from 150 to 6370 ind./cm 3 and were significantly higher in the upper zone of the lake (0-4 in depth) than in the lower zone (7-14 m). Only minor variations were observed between the sampling dates in winter, spring and summer regarding both total abundance and structure of the benthic community. Bodonids and euglenids formed the major part of HF in numbers and biomass. Other groups such as chrysomonads, thaumatomastigids and other representatives of Protista incertae sedis were of minor importance. Diplomonads including anaerobic species were mainly observed in the profundal. Among the abiotic parameters, we assume oxygen concentration as the driving factor in determining the total flagellate abundances, but strong interactions between other sediment parameters inhibit a clear analysis of single effects. In a short-term experiment with artificial aeration of profundal sediments, the flagellate densities strongly increased for 15 days subsequent to growing bacteria numbers, but returned to the initial values after three weeks.

The enumeration of ciliates from riverine samples is often confounded by the problems of relatively low ciliate abundances combined with high turbidity. We successfully tested a method to determine abundance and taxonomic composition of planktonic ciliates in highly turbid running waters. The concentration of samples by sedimentation in Utermohl-chambers was combined with density gradient centrifugation (to exclude inorganic particles) and quantitative protargol staining (for taxonomic determination of species). The newly developed technique was compared with established methods (live counting, counting in Utermohl chambers and quantitative protargol staining without prior exclusion of inorganic particles). We could demonstrate that the newly developed technique gave, in addition to the quantification of ciliates, a detailed overview of taxonomic composition in the investigated samples.

A method based on the live-staining (DAPI, FITC, DTAF) of heterotrophic flagellates was developed. These fluorescently labelled flagellates (FLF, different species) were offered as living prey in grazing experiments. The ingested differentially labelled flagellates could easily be identified in food vacuoles of protists by using the epifluorescence microscopy. We tested the method by using a filter-feeding ciliate (Euplotes vannus) and a heterotrophic dinoflagellate (Oxyrrhis marina) as predators of FLF. The food selectivity regarding different species of heterotrophic (Cafeteria, Bodo, Spumella, Oxyrrhis) and autotrophic (Chlamydomonas) flagellates was estimated from food vacuole contents of protists. Both protistan predators showed a preference for large flagellates. Grazing rates of Euplotes and Oxyrrhis on heterotrophic flagellates ranged between 2-53 flagellates per ciliate and hour and 1-31 flagellates and flagellate and hour, respectively. The method was proofed to be very useful for the quantification of trophic interactions among protists using epifluorescence microscopy, it can also be applied for studies by laser scanning microscopy and should be applicable for investigation by flow cytometry.

The seasonal changes of the abundance and biomass of protozooplankton were investigated by a live-counting technique in the mesoeutrophic Lake Neumuhler See. In addition, the biomass of metazoans and phytoplankton was studied. During the annual cycle, the percentage contribution of the different protozoan groups changed significantly. Ciliates dominated the biomass in late spring, summer and autumn, heterotrophic nanoflagellates (