ABSTRACT. The abundances of benthic naked amoebae in the sediments of the Clyde Sea Area, Scotland, were studied throughout 1991. Four sites differing in sediment grain size and organic carbon content were chosen for study. Amoebae were enumerated by enrichment cultivation methods and found to be numerically important, attaining densities of up to 14,883 amoebae/cm3 on one occasion. They were most abundant, and temporally stable, in finer sediments where they averaged 2,224 cells/cm3 and lower and more variable in the sandy sediments, averaging 874 cells/cm3 throughout the year. In general, amoebae were most abundant in the surface sediment layers. Around 70 different morphotypes were recorded, and 61% of all amoebae counted were less than 10 μm in length. This is the first detailed quantitative study of benthic marine gymanamoebae and shows that naked amoebae and flagellates are numerically the dominant interstitial fauna in sediments of this area. Moreover, the gymnamoebae comprised the largest proportion of total protozoan biomass (excluding foraminiferans) and clearly need to be considered in future models of benthic carbon flow.

Eukaryotic microorganisms (protists) are a very important component of microbial communities inhabiting groundwater aquifers. This is not unexpected when one considers that many protists feed heterotrophically, by means of either phagotrophy (bacterivory) or osmotrophy. Protistan numbers are usually low (< 10(2) per g dw of aquifer material) in pristine, uncontaminated aquifers but may increase by several orders of magnitude in aquifers subject to organic pollution. Small flagellates (typically 2-3(5) microns in size in situ) are by far the dominant protists in aquifers, although amoebae and occasionally ciliates may also be present in much lower numbers. Although a wealth of new taxonomic information is waiting to be brought to light, interest in the identity of aquifer protists is not exclusively academic. If verified, the following hypotheses may prove to be important towards our understanding of the functioning of microbial communities in aquifers: (1) Differences in swimming behavior between species of flagellates lead to feeding heterogeneity and niche differentiation, implying that bacterivorous flagellates graze on different subsets of the bacterial community, and therefore play different roles in controlling bacterial densities. (2) Bacterivorous flagellates grazing on bacteria capable of degrading organic compounds have an indirect effect on the overall rates of biodegradation.

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SUMMARY The population density, diversity and productivity of the microbial plankton in an oligotrophic maritime Antarctic lake were studied for a 15‐month period between December 1994 and February 1996. In the lake, concentrations of nutrients and dissolved organic carbon were uniformly low, temperature varied over a small annual range of 0.1–3 °C, and the surface was ice‐covered except during a period of approximately 6 weeks in summer. The total of 57 morphotypes of protozoa observed during the study is a higher taxonomic diversity than previously reported from continental Antarctic lakes, but lower than that found in more eutrophic maritime Antarctic lakes. Likewise, planktonic abundance and productivity were lower than has been reported in other lakes on Signy Island, but generally higher than those of lakes on the Antarctic continent. There were marked seasonal and interannual variations in planktonic population density. Chlorophyll a concentrations ranged from undetectable to 4.2 µg L‐1 and the greatest rate of primary productivity measured was 4.5 mg C m‐3 h‐1. The phytoplankton was dominated by small chlorophytes and chrysophytes, with phototrophic nanoflagellate abundance ranging from 1.1 × 103 to 1.2 × 107 L‐1. Bacterial densities of 3.6 × 108 to 1.9 × 1010 L‐1 were recorded and bacterial productivity reached a peak of 0.36 µg C L‐1 h‐1. Numbers of heterotrophic nanoflagellates between 5.0 × 104 and 1.8 × 107 L‐1, and of ciliates from undetectable to 1.1 × 104 L‐1 were observed. Naked amoebae were usually rare, but occasionally reached peaks of up to 1.5 × 103 L‐1.