Despite differences in the species compositions and absolute abundances of planktonic microorganisms in fresh-and saltwater, there are broad similarities in microbial food webs across systems. Relative abundances of bacteria and nanoplanktonic protozoa (HNAN, primarily heterotrophic flagellates) are similar in marine and freshwater environments, which suggests analogous trophic relationships. Ranges of microbe abundances in marine and fresh waters overlap, and seasonal changes in abundances within an ecosystem are often as great as differences in abundances between freshwater and marine systems of similar productivities. Densities of bacteria and heterotrophic nanoplankton, therefore, are strongly related to the degree of eutrophication, and not salt per se. Data from the literature is compiled to demonstrate a remarkably consistent numerical relationship (ca 1000 bacteria: l HNAN) between bacterioplankton and HNAN from the euphotic zones of a variety of marine and freshwater systems. Based on the results of a simple food web model involving bacterial growth, bacterial removal by HNAN, predation on HNAN, and the observed relationships between bacterial and HNAN abundances in natural ecosystems, it is possible to demonstrate that bottom-up control (food supply) is more important in regulating bacteiial abundances in oligotrophic environments while top-down control (predation) is more important in eutrophic environments.

A detailed investigation of the benthic prokaryot~c community from 3 permanently cold stations near Svalbard (Arctic Ocean) and 1 site near Tromss (northern Norway) was conducted. Prokaryotic abundances, determined by DAPI-staining, were in the range of ca 2 X 108 to 4 X log cells cm-3 wet sediment. They showed little variation among sampling stations. Vertical profiles were characterized by a decrease of cell numbers with increasing sediment depth. The prokaryotic community composition was investigated employing rRNA (ribosomal RNA) slot-blot hybridization with domain-specific probes. Irrespective of station and vertical depth, Eubacteria always dominated the population, and the relative contribution of Archaea never exceeded 4 %. The measured total rRNA concentration and the prokaryotic cell counts in each sample were used to calculate per cell rRNA contents. Mean rRNA content (averaging all samples) was close to 3 fg rRNA cell-'. None of our data showed considerable differences to comparable results from temperate or warm habitats; therefore our findings do not allow conclusions on special adaptations of the prokaryotic community to their existence in permanently cold systems. In all samples, but most pronounced in the 3 coldest stations, per cell rRNA contents showed steep vertical gradients with maximum values at the sediment surface. Taking into account all stations, rRNA concentration and prokaryotic abundance were strongly positively correlated below ca 5.5 cm (r2 = 0.739), whereas in the upper sediment layers (0 to ca 5.5 cm) there was no significant correlation between these 2 parameters. This implies that there may be ddferent mechanisms involved in the control of prokaryotic rRNA contents in Wferent sedment horizons. Cellular rRNA concentrations can q v e an indication of growth rate and thereby the activity of prokaryotes. This is supported by the fact that we recorded the highest per cell rRNA contents in those stations and sediment depths where other studies conducted simultaneously with our investigation demonstrated the highest rates of metabolic processes.

The study of the ecological role played by benthic ciliates is hampered by the lack of a methodology to accurately enumerate benthic ciliates with good taxonomic resolution. As a result, a technique was developed that combined 2 previously published methods developed to identify and count pelagic ciliates and benthic flagellates, respectively. The new method utilizes centrifugation in a non-linear density gradient to separate ciliates from sediment, and the QPS (quantitative protargol stain) silver-staining technique to stain the cilia and nuclei of ciliates which, after centrifugation, are concentrated on cellulose nitrate filters. The wide applicability of the method was shown by utilizing it to count and identify ciliates in cores taken from intertidal sediment and sampled on a 2 mm depth interval. The intertidal cores had a total of 41 species or morphotypes present, but no more than 21 species or morphotypes in any single 2 mm layer. Total ciliate abundance was as high as 2500 cells ml -1 , with the upper layers having higher abundance than deeper layers. There was no obvious pattern with depth either for the number of species found in any one layer, or for ciliate diversity. The method was further utilized in an experiment that explored the interactions between benthic ciliates and the ostracod Cyrideis trosa. Ostracods reduced the final abundance of ciliates, but this effect was confined to the upper 5 mm of sediment. Two-thirds of the ostracods were found in the 5-10 mm layer at the end of the experiment, but while there was an effect on ciliate diversity in this layer, there was no effect on total ciliate abundance. We conclude that the method is suitable for studying both the natural distribution and diversity of benthic ciliates and their response to experimental manipulations. KEY WORDS: Benthic ciliate distribution · Benthic ciliate enumeration · Benthic ciliate identification · OstracodResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 22: 79-91, 2000 species or group. Ecologically important differences between morphospecies in the genus Euplotes have already been demonstrated (Dini & Nyberg 1999), emphasizing the importance of combining good taxonomic resolution with accurate enumeration of benthic ciliates. It has been hypothesized that benthic ciliates consume only a minor amount of benthic bacterial production (Kemp 1988, 1990, Epstein & Shiaris 1992, but without satisfactory methods for enumerating benthic ciliates and differentiating between species the hypothesis cannot be adequately tested.A major reason for the lack of understanding of benthic protists is the methodological difficulties encountered when working with benthic organisms. Gradients of light, oxygen and redox potential are orders of magnitude steeper in sediment than in the overlying water (Jørgensen & Revsbech 1985, Kühl et al. 1994, Berninger & Huettel 1997, requiring sampling on a very fine spatial scale. What is normally an even greater problem is the difficulty in ...

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The vertical distributions of two pond‐dwelling zoochlorellae‐bearing ciliates (Euplotes daidaleos Diller & Kounaris, 1966 and Frontonia vernalis Ehrenberg, 1838) were monitored over a 24‐h period. Both species maintained peak abundance at a low O2 level (usually < 1 mg/liter). They did not migrate in response to the changing light level. Experiments with laboratory cultures indicated that the characteristic distribution in an O2 gradient in the dark was largely controlled by the oxygen tension. The increased motility in anoxia and high pO2 was independent of large changes in pCO2 and pH. Ciliates living in anoxia or a very low pO2 would migrate out of the dark and into the dimly lit (10 μE m‐2 sec‐1) part of a glass cell because there they could photosynthesize, produce O2, and create a suitable oxygenated microenvironment; a further increase in the light level caused a slow migration out of the light. Similar migrations were observed when the light level remained low but the pO2 was artificially raised. Ciliates suspended in 1 μM DCMU (an inhibitor of photosynthetic O2 evolution) took longer to migrate into the light and they did not avoid high light levels (> 100, μE m‐2 sec‐1)‐ Frontonia suspended in water with a pO2 of 1% aggregated at a low light level (1 μE m‐2 sec‐1); peak daytime abundance in the pond occurred at about this light level. Frontonia vernalis tends to swim vertically upwards (anterior end up) when suspended in anoxic water. This apparent negative geotaxis compensates for the high sedimentation velocity (0.36 mm sec‐1) of this large ciliate and facilitates its aggregation at the metalimnion. The O2 tension appears to be the principal factor controlling the vertical distributions of both species. Occasional, enhanced convection within the metalimnion has a secondary influence. Light influences the vertical profile only if it promotes photosynthesis and increases the intracellular pO2.