We have quantified the bacteria, heterotrophic nanoplankton (HNAN), and other microorganisms in 108 lakes, ponds, rivers, and bogs worldwide. These water bodies span the range of biological productivities in freshwater. Numbers of HNAN and bacteria are correlated over four orders of magnitude in each (3 × 102 to 4 × 106 ml−1 and 3 × 105 to 1 × 109 ml−1, respectively) and both increase with the productivity of the water body. Most HNAN are small (2– 5 µm), colorless, flagellated protists. They grow at about the same rate as bacteria (µ = 0.01–0.02 h−1) and are capable of consuming the entire bacterial production. We suggest that bacterial abundances are regulated by substrate supply and HNAN grazing pressure. Ciliates and other grazing microzooplankton probably limit HNAN abundance, especially in the more productive water bodies. The structure and function of microbial food webs in freshwater environments may thus be similar to those suggested for marine systems.

During the spring bloom in 1988, the dynamics of planktonic carbon flow were studied weekly in the euphotic layer in the northern Baltic. The spring bloom developed after the formation of a s l~g h t vertical salinity gradient near the surface at the end of April, and a peak in phytoplankton primary productivity and biomass (dominated by the dinoflagellate Peridiniella catenata) was reached about 1 wk later. The biomass of all heterotrophic compartments, especially that of bactena and copepods, increased strongly durlng the peak and declining phases of the algal bloom, showing that their success was closely linked with the bloom. During the whole bloonl period, the integral pnmary production (I4C incorporation) was 45.5 g C m-', and 'new' (NO<-N-based) production contributed about 80% of this value. The rotifers-copepods grazing chain and the bacteria-heterotrophic nanoflagellates-ciliates 'microbial loop' consumed directly about the same amount (3.5 g C m-') of phytoplankton carbon. Algae accounted for 64% of the total carbon consumption of zooplankton. Sedimentation corresponded to 72% of the primary production. The sum of algal biomass increase and loss factors (exudation, grazing and sedimentation) was 94% of the integral primary production, which supports our conclusion that there is a strong imbalance between primary and secondary production in the vernal planktonic food web off the SW coast of Finland.

Grazing by heterotrophic nanoflagellates and < 100 Fm microzooplankton on planktonic bacteria was followed during a mesocosm experiment in the Baltic Sea between 23 July and 12 August 1988 on the SW coast of Finland. During the succession of the planktonic community in one mesocosm, 4 grazing experiments were run with a size-fractionation technique. The size fractions used were: < 1 pm (bacterioplankton), < 5 pm (bacteria and heterotrophic nanoflagellates), and < 100 pm (bacteria, heterotrophic nanoflagellates and ciliates). Clearance rate of < 5 pm flagellates was 0.6 to 5.3 nl flag.-' h-' Grazing in the < 5 pm size fraction was 34 to 134 % of grazing in the < 100 pm fraction. The < S pm and < 100 pm protozoa harvested hourly on average 75 and 90 % of bacterial production, respectively. Nonetheless, heterotrophic nanoflagellates could not satisfy their carbon demand from bacteria. Grazing by protozoa altered bacterial size distribution, and reduced bacterial cell number and production. When the water temperature was ca 20°C, < 100 pm microzooplankton could consume about 50 to 100 % of flagellate standing stock daily. During the 3 wk mesocosm experiment, protozoan clearance rate fluctuated in the < 5 and < 100 pm filtrates by a factor of 9 and 3, respectively. A fall of water temperature from 18 to 14°C was the main factor affecting the activity of the microbial community in the mesocosm.

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Experimental enclosures were used to follow responses of the plankton~c microbial food web to varying short-tern~ (5 d) perturbations induced by adding inorganic nutrients ( N and P) and a top predator (fish) during a 21 d period in late summer, on the coastal area of the Baltic Sea. Biomass, production, growth and grazing of pico-and nanoplankton assemblages were estimated, and a carbon budget for the microbial loop during the experiment was constructed. The microbial food web was a highly dynamic system. Varying perturbations due to nutrient loading and the top predator provoked eutrophication in the enclosures, but they affected the microbial loop only slightly. The amplitudes of oscillation in abundance of coupled communities were amplified, but the frequencies of oscillations in the microbial loop were not affected by the perturbations. Changes in the route of carbon flow through the microbial food web occurred In relatively short time scales. These changes seemed to be dependent on the phasing of the coupled oscillations between the communities, and the structure within the different communities. Ciliates were only loosely connected to the microbial loop: although ciliates and heterotrophic nanoflagellates (HNF) showed predator-prey-like coupled oscillations, the ciliates gained most of their carbon from other sources, and most of the HNF carbon loss was due to factors other than ciliates. HNF were the most important consumers of picoplankton during the HNF maximum, but they were also dependent on other sources of nutrition.

Bacterioplankton biomass and dark fixation of inorganic carbon were measured in the highly humic (water colour up to 550 mg Pt 1-') and acidic lake, Mekkojarvi. Strong thermal and chemical stratification developed in the water column early in spring and led rapidly to anoxia in the hypolimnion, which extended to less than 1.0 m from the surface. In the epilimnion only small bacteria were abundant. In the anoxic zone both the abundance and the mean size of bacteria were considerably higher than in the epilimnion. These differences are thought to be the result of different grazing pressure from zooplankton in the two zones. In late summer a high concentration of bacteriochlorophyll d in the upper hypolimnion indicated a high density of photosynthetic bacteria. Bacterial biomass was similar to that of phytoplankton in the epilimnion, but 23 times higher in the whole water column. In August, dark fixation of inorganic radiocarbon in the anaerobic zone was 51% of the total 4 C-incorporation and the contribution of light fixation was only 5.4%. In the polyhumic Mekkojarvi, bacterioplankton was evidently a potentially significant carbon source for higher trophic levels, but bacterioplankton production could not be supported by phytoplankton alone. Allochthonous inputs of dissolved organic matter probably support most of the bacterial production.

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