Trophic interactions within the microbial food web in a tropical floodplain lake (Laguna Bufeos, Bolivia)

Whether the primary role of bacterioplankton is to act as "remineralizers" of nutrients or as direct nutritional source for higher trophic levels will depend on factors controlling their production and abundance. In tropical lakes, low nutrient concentration is probably the main factor limiting bacterial growth, while grazing by microzooplankton is generally assumed to be the main loss factor for bacteria. Bottom-up and top-down regulation of microbial abundance was studied in six nutrient limitation and dilution gradient-size fractionation in situ experiments. Bacteria, heterotrophic nanoflagellates (HNF), ciliates and rotifers showed relatively low densities. Predation losses of HNF and ciliates accounted for a major part of their daily production, suggesting a top-down regulation of protistan populations by rotifers. Phosphorus was found to be strongly limiting for bacterial growth, whereas no response to enrichment with Nitrogen or DOC was detected. HNF were the major grazers on bacteria (g-0.43 d(-1)), the grazing coefficient increased when ciliates were added (g- 0.80 d(-1)) but decreased when rotifers were added (g- 0.23 d(-1)) probably due to nutrient recycling or top-down control of HNF and ciliates by rotifers.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Contrasting ‘Top-Down’ Effects of Crustacean Zooplankton Grazing on Bacteria and Phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.     

Relationship between Bacterial Community Composition and Bottom-Up versus Top-Down Variables in Four Eutrophic Shallow Lakes

Bacterial community composition was monitored in four shallow eutrophic lakes during one year using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified prokaryotic rDNA genes. Of the four lakes investigated, two were of the clearwater type and had dense stands of submerged macrophytes while two others were of the turbid type characterized by the occurrence of phytoplankton blooms. One turbid and one clearwater lake had high nutrient levels (total phosphorus, >100 μg liter⁻¹) while the other lakes had relatively low nutrient levels (total phosphorus, <100 μg liter⁻¹). For each lake, seasonal changes in the bacterial community were related to bottom-up (resources) and top-down (grazers) variables by using canonical correspondence analysis (CCA). Using an artificial model dataset to which potential sources of error associated with the use of relative band intensities in DGGE analysis were added, we found that preferential amplification of certain rDNA genes over others does not obscure the relationship between bacterial community composition and explanatory variables. Besides, using this artificial dataset as well as our own data, we found a better correlation between bacterial community composition and explanatory variables by using relative band intensities compared to using presence/absence data. While bacterial community composition was related to phytoplankton biomass in the high-nutrient lakes no such relation was found in the low-nutrient lakes, where the bacterial community is probably dependent on other organic matter sources. We used variation partitioning to evaluate top-down regulation of bacterial community composition after bottom-up regulation has been accounted for. Using this approach, we found no evidence for top-down regulation of bacterial community composition in the turbid lakes, while grazing by ciliates and daphnids (Daphnia and Ceriodaphnia) was significantly related to changes in the bacterial community in the clearwater lakes. Our results suggest that in eutrophic shallow lakes, seasonality of bacterial community structure is dependent on the dominant substrate source as well as on the food web structure.     

Short communication. Direct and indirect effects of strong grazing by Daphnia galeata on bacterial production in an enclosure experiment

Bacterial production was stimulated in Daphnia enclosures at the beginning of the experiment due to the release of dissolved organic carbon during Daphnia grazing of algae (sloppy feeding). In contrast, bacterial production decreased in the treatments at the end of the experiment. The decline was not caused by reduction of bacterial biomass, but by decreasing biomass-specific production due to a decline of the resources for bacterial growth (phytoplankton biomass and production).      

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

The microbial loop in the planktonic communities in lakes with various trophic status

The structure of planktic trophic chains was studied in eight lakes of European Russia and five lakes in Central Asia. The lakes differed in the level of productivity, morphometric parameters, and the type of agitation and mineralization. It is found that the microbial loop of picophototrophic organisms, bacteria, heterotrophic flagellates, infusoria, and viruses constitutes 12.3-64.7% of the total plankton biomass. Positive correlation between the biomass of microbial community and the primary production of phytoplankton is observed, whereas no relation is revealed between the share of microorganisms in the plankton biomass and the trophic status of the water body. The presence of a great number of cladocerans decreased the role of the microbial loop in the structural organization of the planktic community. Heterotrophic flagellates consuming 3-81% of daily bacterial production were the principal cause of bacteria elimination only in some of the studied water bodies.     

Cascading predation effects of Daphnia and copepods on microbial food web components

1. We performed a mesocosm experiment to investigate the structuring and cascading effects of two predominant crustacean mesozooplankton groups on microbial food web components. The natural summer plankton community of a mesotrophic lake was exposed to density gradients of Daphnia and copepods. Regression analysis was used to reveal top–down impacts of mesozooplankton on protists and bacteria after days 9 and 15. 2. Selective grazing by copepods caused a clear trophic cascade via ciliates to nanoplankton. Medium‐sized (20–40 μm) ciliates (mainly Oligotrichida) were particularly negatively affected by copepods whereas nanociliates (mainly Prostomatida) became more abundant. Phototrophic and heterotrophic nanoflagellates increased significantly with increasing copepod biomass, which we interpret as an indirect response to reduced grazing pressure from the medium‐sized ciliates. 3. In Daphnia‐treatments, ciliates of all size classes as well as nanoflagellates were reduced directly but the overall predation effect became most strongly visible after 15 days at higher Daphnia biomass. 4. The response of bacterioplankton involved only modest changes in bacterial biomass and cell‐size distribution along the zooplankton gradients. Increasing zooplankton biomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) of bacterial biovolume, activity and production. Patterns of bacterial diversity, as measured by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE), showed no distinct grouping after 9 days, whereas a clear treatment‐coupled similarity clustering occurred after 15 days. 5. The experiment demonstrated that zooplankton‐mediated predatory interactions cascade down to the bacterial level, but also revealed that changes occurred rather slowly in this summer plankton community and were most pronounced with respect to bacterial activity and composition.     

Interactions between metazoans,autotrophs, mixotrophs and bacterioplankton in nutrient‐depleted high DOC environments: a long‐term experiment

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Microbial decomposition of dissolved organic matter in a hypertrophic pond

Planktonic heterotrophic bacteria in lakes utilize the labile fraction of dissolved organic carbon (DOC), although information about seasonal changes in labile DOC in hypertrophic lakes in terms of absolute amount and relative proportion of the total DOC is still limited. We conducted DOC decomposition experiments using GF/F filtrates in water samples from hypertrophic Furuike Pond, together with monitoring of DOC concentration and bacterial abundance in water samples from the pond, to examine seasonal changes in the amount of labile DOC and growth of bacteria on labile DOC. DOC concentrations fluctuated between 2.7 and 11 mg C l⁻¹, and bacterial abundance fluctuated between 1.5 × 10⁶ and 1.0 × 10⁸ cells ml⁻¹. In the DOC decomposition experiment when grazers of bacteria were removed, small portions of DOC (18% ± 12%) were labile for decomposition by bacteria, and the growth yield of bacteria on labile DOC ranged between 3.3% and 19%. Furthermore, addition of nitrogen to water samples enhanced bacterial growth. Thus, not only labile DOC but also nitrogen limited bacterial growth in the pond. Considering the results in the present study together with those of previous studies, bacterial abundance in Furuike Pond is subjected to bottom-up control, such as by limitation of DOC and nitrogen throughout the year, although top-down control of bacterial abundance such as by grazing is seasonally important. Received: May 1, 2001 / Accepted: July 22, 2001     

Structural and functional patterns of bacterial communities in response to protist predation along an experimental productivity gradient

Substrate supply and protist grazing are two of the most important forces that determine the composition and properties of bacterial assemblages. General ecological theory predicts that the relative importance of these factors is changing with the environmental productivity. In the present study, the interplay between bottom-up and top-down control was studied in a productivity gradient simulated in one-stage chemostats containing natural assemblages of freshwater bacteria and heterotrophic nanoflagellates. Bacterial assemblages in the chemostats differed strongly with respect to their morphological, physiological and compositional properties in the presence versus the absence of predators. However, theses differences were modified by the productivity gradient. Whereas in predator-free chemostats the mean abundance and biomass of bacteria increased proportionally with increasing substrate supply, in treatments that included flagellates bacterial production was largely channelled into predator biomass. The bacterial morphological diversity increased along the productivity gradient with increasing substrate input but even more so with predators. Proportional to the increasing substrate supply, predation shifted the remaining bacteria towards morphologically inedible forms. Predation also caused shifts in bacterial substrate-utilization profiles, and in bacterial community composition, as analysed by terminal restriction fragment length polymorphism of PCR-amplified 16S-rRNA genes. Without predators, bacterial richness increased along the productivity gradient whereas with predators bacterial richness was higher at intermediate substrate levels. In accordance with ecological theory, these results demonstrated that predators influence all of the major characteristics of bacterial assemblages but the magnitude of this effect is modulated by the productivity of the system.     

Resource Availability and Spatial Heterogeneity Control Bacterial Community Response to Nutrient Enrichment in Lakes

The diversity and composition of ecological communities often co-vary with ecosystem productivity. However, the relative importance of productivity, or resource abundance, versus the spatial distribution of resources in shaping those ecological patterns is not well understood, particularly for the bacterial communities that underlie most important ecosystem functions. Increasing ecosystem productivity in lakes has been shown to influence the composition and ecology of bacterial communities, but existing work has only evaluated the effect of increasing resource supply and not heterogeneity in how those resources are distributed. We quantified how bacterial communities varied with the trophic status of lakes and whether community responses differed in surface and deep habitats in response to heterogeneity in nutrient resources. Using ARISA fingerprinting, we found that bacterial communities were more abundant, richer, and more distinct among habitats as lake trophic state and vertical heterogeneity in nutrients increased, and that spatial resource variation produced habitat specific responses of bacteria in response to increased productivity. Furthermore, changes in communities in high nutrient lakes were not produced by turnover in community composition but from additional taxa augmenting core bacterial communities found in lower productivity lakes. These data suggests that bacterial community responses to nutrient enrichment in lakes vary spatially and are likely influenced disproportionately by rare taxa.     

Zooplankton-mediated changes of bacterial community structure

Enclosure experiments in the mesotrophic Schöhsee in northern Germany were designed to study the impact of metazooplankton on components of the microbial food web (bacteria, flagellates, ciliates). Zooplankton was manipulated in 500-liter epilimnetic mesocosms so that either Daphnia or copepods were dominating, or metazooplankton was virtually absent. The bacterial community responded immediately to changes in zooplankton composition. Biomass, productivity, and especially the morphology of the bacteria changed drastically in the different treatments. Cascading predation effects on the bacterioplankton were transmitted mainly by phagotrophic protozoans which had changed in species composition and biomass. When Daphnia dominated, protozoans were largely suppressed and the original morphological structure of the bacteria (mainly small rods and cocci) remained throughout the experiment. Dominance of copepods or the absence of metazoan predators resulted in a mass appearance of bacterivorous protists (flagellates and ciliates). They promoted a fast decline of bacterial abundance and a shift to the predominance of morphologically inedible forms, mainly long filaments. After 3 days they formed 80–90% of the bacterial biomass. The results indicate that metazooplankton predation on phagotrophic protozoans is a key mechanism for the regulation of bacterioplankton density and community structure.Correspondence to: K. Jürgens.     

Elevated methylmercury in High Arctic Daphnia and the role of productivity in controlling their distribution

Mercury is a contaminant of concern in polar regions due to long‐range atmospheric transport of this metal from southern latitudes followed by intense deposition on snow. We surveyed zooplankton in 16 lakes and ponds in the Canadian Arctic Archipelago (74–76°N) to determine methylmercury (MeHg) content and the role of environmental characteristics and taxonomic composition on accumulation processes. Zooplankton communities containing Daphnia (mainly D. middendorffiana) had on average five times the MeHg content of copepod‐dominated communities. The percent biomass of Daphnia best explained MeHg variation in bulk zooplankton compared with water chemistry and morphometric variables. Water‐column concentrations of MeHg were low at most study sites (mainly ≤0.07 ng L⁻¹), and Daphnia strongly bioaccumulated mercury through species‐specific processes. As we observed Daphnia in more productive water bodies (i.e., ponds, a eutrophied lake), we then tested the role of productivity in determining the distribution of this keystone herbivore using a broad‐scale literature dataset of 47 High Arctic lakes (65–77°N). Daphnia density was positively related to the amount of organic carbon in the water column in both dissolved and particulate fractions [dissolved organic carbon (DOC) partial , P < 0.001; particulate organic carbon (POC) partial , P=0.032]. The strong influence of DOC suggests that bacterial production is an important energy source for Arctic Daphnia. Our findings indicate that productivity influences the MeHg content of zooplankton communities through its control of species composition; specifically, low productivity limits the presence of mercury‐rich Daphnia in many copepod‐dominated lakes of the High Arctic. Aquatic productivity is expected to increase with climate warming, and we present a conceptual model that predicts how environmental drivers could extend the distribution of Daphnia in lakes and alter the movement of mercury in food webs of the Canadian High Arctic.     

Microcrustacean community structure and biomass in marsh and lake habitats of the Okefenokee Swamp: seasonal dynamics and responses to resource manipulations

Microcrustacean community and biomass dynamics were studied for two years in a Nymphaea-Eriocaulon macrophyte marsh and a nearby shallow lake which lacked macrophytes in the Okefenokee Swamp. In this blackwater, acidic wetland, microcrustacean diversity and biomass were similar to other circumneutral lakes and littoral areas, contributing to a productive fish assemblage. In the lake, the annual biomass pattern (15–1627 μg 1⁻¹) was unimodal and was dominated by the crustaceans Diaptomus sinuatus and Eubosmina tubicen. Rotifers were occasionally important, constituting up to 55% of total biomass. Over the long term, mean annual biomass in this post-drought study are higher than in pre-drought years. In the marsh, biomass (11–777 μg 1⁻¹) fluctuated biomodally with late winter depressions corresponding to low temperatures and midsummer declines indicative of increasing fish predation. Summer dominance shifted between years from Macrothricidae in 1982 to Sididae in 1983. Variation in biomass correlated most strongly with algal chlorophyll in the marsh and with bacterial density in the lake. In field enclosure experiments in which primary production was reduced by shading, microcrustacean responses varied between lake and marsh habitats and with season. Lake zooplankton were consistently suppressed by reduced algal resources in winter, spring and summer experiments, with greatest responses to shading in the summer. Marsh microcrustacea were most affected in the winter experiment and became less sensitive to manipulated resource levels in spring and summer. Decoupling of these consumers from autotrophic resources in the marsh, but not in the lake, coincides with times of high macrophyte turnover and warming temperatures which promote the conversion of detritus into heterotrophic resources such as bacteria. The conflict between interactions implied by the experimental approach vs statistical criteria emphasizes a need to interpret resource dependence from seasonal dynamics of field populations with caution.     

Patterns of resource limitation of bacteria along a trophic gradient in Mediterranean inland waters

The nature of the resource that limits heterotrophic bacteria, i.e. mineral nutrients or carbon (C), has consequences for biogeochemical cycles in aquatic ecosystems. Our aim was to identify the resource [C or phosphorus (P)] that mainly limits bacteria in a set of 31 Mediterranean inland water ecosystems spanning a wide trophic range. We followed an intersystem observational approach with three complementary perspectives, comparing the bacterial demand with the resource supply in terms of both the quantity (demand : supply ratio for C and P) and quality (C : P ratio of demand and supply), and assessing the relative strength of each resource in controlling bacterial production. The trophic gradient revealed a shift in the main limiting resource for bacteria, from C at the oligotrophic end (typically high-mountain, low-productivity lakes) to mainly P at the eutrophic end (typically nonmountain, high-productivity lakes). The patterns of resource limitation of bacteria found here may be related to the autotrophic nature of most of the studied ecosystems linked to a Mediterranean climate regime as representative of lakes with low inputs of allocthonous C. These patterns are consistent with the theoretical approaches and may potentially shape the contribution of this type of ecosystems to biogeochemical cycles.     

Occurrence of mixotrophic flagellates in relation to bacterioplankton production, light regime and availability of inorganic nutrients in unproductive lakes with differing humic contents

1. Field data from five unproductive Swedish lakes were used to investigate the occurrence of mixotrophic flagellates in relation to bacterioplankton, autotrophic phytoplankton, heterotrophic flagellates and abiotic environmental factors. Three different sources of data were used: (i) a 3‐year study (1995–97) of the humic Lake Örträsket, (ii) seasonal measurements from five lakes with widely varying dissolved organic carbon (DOC) concentrations, and (iii) whole lake enrichment experiments with inorganic nutrients and organic carbon. 2. Mixotrophic flagellates usually dominated over autotrophic phytoplankton in Lake Örträsket in early summer, when both bacterial production and light levels were high. Comparative data from the five lakes demonstrated that the ratio between the biomasses of mixotrophic flagellates and autotrophic phytoplankton (the M/A‐ratio) was positively correlated to bacterioplankton production, but not to the light regime. Whole lake carbon addition (white sugar) increased bacterial biomass, and production, reduced the biomass of autotrophs by a factor of 16, and increased the M/A‐ratio from 0.03 to 3.4. Collectively, the results indicate that the dominance of mixotrophs among phytoplankton was positively related to bacterioplankton production. 3. Whole lake fertilisation with nitrogen (N) and phosphorus (P) demonstrated that the obligate autotrophic phytoplankton was limited by N. N‐addition increased the biomass of the autotrophic phytoplankton but had no effect on mixotrophic flagellates or bacteria, and the M/A‐ratio decreased from 1.2 to 0.6 after N‐enrichment. Therefore, we suggest that bacteria under natural conditions, by utilising allochthonous DOC as an energy and carbon source, are able to outcompete autotrophs for available inorganic nutrients. Consequently, mixotrophic flagellates can become the dominant phytoplankters when phagotrophy permits them to use nutrients stored in bacterial biomass. 4. In Lake Örträsket, the biomass of mixotrophs was usually higher than the biomass of heterotrophs during the summer. This dominance could not be explained by higher grazing rates among the mixotrophs. Instead, ratios between mixotrophic and heterotrophic biomass (the M/H‐ratio) were positively related to light availability. Therefore, we suggest that photosynthesis can enable mixotrophic flagellates to outcompete heterotrophic flagellates.     

Plankton photosynthesis, extracellular release and bacterial utilization of released dissolved organic carbon (RDOC) in lakes of different trophy

The major objective of these studies was to assess the extent of the flow of released dissolved organic carbon (RDOC) from phytoplankton to heterotrophic bacteria. The second aim was to test Nalewajko and Schindler's (1976) hypothesis that the percentage of extracellular release (PER) of RDOC by phytoplankton is higher in oligotrophic than eutrophic waters. The studies on the relationship between the bacterial assimilation of RDOC and productivity of phytoplankton in the lakes of different trophy have shown a direct correlation. It was observed that higher utilization rate of phytoplankton RDOC occurred in lakes with higher primary production. The inverse relationship between productivity of lakes, i.e. trophy of waters, and amount of RDOC or PER is closely dependent on the heterotrophic activity of bacteria in waters.     

Purple Sulfur Bacteria Control the Growth of Aerobic Heterotrophic Bacterioplankton in a Meromictic Salt Lake

In meromictic Mahoney Lake, British Columbia, Canada, the heterotrophic bacterial production in the mixolimnion exceeded concomitant primary production by a factor of 7. Bacterial growth rates were correlated neither to primary production nor to the amount of chlorophyll a. Both results indicate an uncoupling of bacteria and phytoplankton. In the chemocline of the lake, an extremely dense population of the purple sulfur bacterium Amoebobacter purpureus is present year round. We investigated whether anoxygenic phototrophs are significant for the growth of aerobic bacterioplankton in the overlaying water. Bacterial growth rates in the mixolimnion were limited by inorganic phosphorus or nitrogen most of the time, and the biomass of heterotrophic bacteria did not increase until, in autumn, 86% of the cells of A. purpureus appeared in the mixolimnion because of their reduced buoyant density. The increase in heterotrophic bacterial biomass, soluble phosphorus concentrations below the detection limit, and an extraordinarily high activity of alkaline phosphatase in the mixolimnion indicate a rapid liberation of organically bound phosphorus from A. purpureus cells accompanied by a simultaneous incorporation into heterotrophic bacterioplankton. High concentrations of allochthonously derived dissolved organic carbon (mean, 60 mg of C(middot)liter(sup-1)) were measured in the lake water. In Mahoney Lake, liberation of phosphorus from upwelling purple sulfur bacteria and degradation of allochthonous dissolved organic carbon as an additional carbon source render heterotrophic bacterial production largely independent of the photosynthesis of phytoplankton. A recycling of inorganic nutrients via phototrophic bacteria also appears to be relevant in other lakes with anoxic bottom waters.     

Weak Coupling between Heterotrophic Nanoflagellates and Bacteria in a Eutrophic Freshwater Environment

In a study on the dynamics and trophic role of the heterotrophic nanoflagellate (HNAN) assemblage in the microbial food web of a eutrophic oxbow lake abundances, biomass, and production rates of HNAN and their potential prey organisms, namely heterotrophic bacteria and autotrophic picoplankton, were monitored for a period of 2 years. No coupling between HNAN abundance and biomass and the abundance and biomass of their picoplanktonic prey was observed for the investigation period. The ratio of heterotrophic bacterial to HNAN abundance ranged from 2.2 x 103 to 8.6 x 103 (mean: 4.2 x 103 +/- 1.8 x 103). HNAN carbon consumption could account for only 10% to 40% of bacterial secondary production. The lack of coupling between HNAN and their potential prey and the low HNAN abundance relative to bacterial abundance suggested (a) that HNAN grazing was an insignificant factor in the regulation of bacterial abundance and (b) that HNAN abundance was regulated by predation rather than by prey abundance. This hypothesis was supported by the fact that HNAN growth rates were high (in the range of 0.45 d-1 to 1.00 d-1 during spring and summer, yearly mean: 0.52 d-1), and only weakly correlated with prey abundance and biomass. The results indicated strong top-down control of HNAN and consequently a weak coupling of HNAN and picoplankton in the investigated eutrophic freshwater environment.