Factors regulating summer phytoplankton in a highly eutrophic Antarctic lake

Lakes from Maritime Antarctica are regarded as systems generally inhabited by metazoan plankton capable of imposing a top-down control on the phytoplankton during short periods, while lakes from Continental Antarctica lacking these communities would be typically controlled by scarcity of nutrients, following a bottom-up model. Otero Lake is a highly eutrophic small lake located on the NW of the Antarctic Peninsula, which has no metazoan plankton. During summer 1996, we studied the density, composition and vertical distribution of the phytoplankton community of this lake with respect to various abiotic variables, yet our results demonstrated neither light nor nutrient limitation of the phytoplankton biomass. Densities of heterotrophic nanoflagellates (HNAN) and ciliates from three different size categories were also studied. Extremely low densities of HNAN (0–155 ind. ml^–1) could be due to feeding competition by bacterivore nanociliates and/or predation by large ciliates. A summer bloom of the phytoflagellate Chlamydomonas aff. celerrima Pascher reached densities tenfold those of previous years (158.10³ ind. ml^–1), though apparently curtailed by a strong peak of large ciliates (107 ind. ml^–1) which would heavily graze on PNAN (phototrophic nanoflagellates). Top-down control can thus occur in this lake during short periods of long hydrologic residence time.     

Protistan bacterivory in an oligomesotrophic lake: Importance of attached ciliates and flagellates

Seasonal and depth variations of the abundance, biomass, and bacterivory of protozoa (heterotrophic and mixotrophic flagellates and ciliates) were determined during thermal stratification in an oligomesotrophic lake (Lake Pavin, France). Maximal densities of heterotrophic flagellates (1.9 × 10³ cells ml^–1) and ciliates (6.1 cells ml^–1) were found in the metalimnion. Pigmented flagellates dominated the flagellate biomass in the euphotic zone. Community composition of ciliated protists varied greatly with depth, and both the abundance and biomass of ciliates was dominated by oligotrichs. Heterotrophic flagellates dominated grazing, accounting for 84% of total protistan bacterivory. Maximal grazing impact of heterotrophic flagellates was 18.9 × 10⁶ bacteria 1^–1h^–1. On average, 62% of nonpigmented flagellates were found to ingest particles. Ciliates and mixotrophic flagellates averaged 13% and 3% of protistan bacterivory, respectively. Attached protozoa (ciliates and flagellates) were found to colonize the diatom Asterionella formosa. Attached bacterivores had higher ingestion rates than free bacterivorous protozoa and may account for 66% of total protozoa bacterivory. Our results indicated that even in low numbers, epibiotic protozoa may have a major grazing impact on free bacteria. Correspondence: C. Amblard.     

An approach to measure ciliate grazing on living heterotrophic nanoflagellates

The complicated routes by which organic material is channelled up to higher trophic levels via bacteria and protozoans is a major issue in aquatic microbial ecology. Because of the fragile nature of protists it is not straightforward to perform experimental studies of prey–predator interactions. Here we present an approach for the assessment of ciliate grazing on living heterotrophic nanoflagellates. Stationary phase cultures of a heterotrophic nanoflagellate (Cafeteria sp.) were live-stained by allowing them to take up fluorescently labelled macromolecules. Controls revealed that this label persisted for several hours. Fluorescently labelled living flagellates (FLLF) were added into enriched natural assemblages of marine oligotrich ciliates and uptake of FLLF was monitored over time. Oligotrich ciliates did not incorporate fluorescent-labelled macromolecules but a linear FLLF uptake over time was observed for 20–30 min at 20°C. Ingestion rates were 21–46 FLLF h^–1 at a concentration of about 2×10⁴ FLLF ml^–1, which corresponded to clearance rates of 0.7–0.8 l ciliate^–1 h^–1. These results are in the same order of reported ciliate grazing on phytoplankton of similar size. This method represents a direct approach to measure ciliate grazing specifically on living heterotrophic nanoflagellates.     

Vertical distribution of planktonic ciliates in strongly stratified temperate lakes

The vertical distribution of planktonic ciliates in eight strongly stratified temperate lakes was studied in summer 1998. Ciliate abundance and biomass were highest (mean 39.9 cells ml^–1 and 181.9 g C l^–1) in the epi-, and lowest (mean 8.2 cells ml^–1 and 97.6 g C l^–1) in the hypolimnion. The community of ciliates was dominated by five orders: Oligotrichida, Haptorida, Prostomatida, Scuticociliatida and Peritrichida. The community composition varied greatly with depth. In the epilimnion, the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and haptorids were also numerous. In the metalimnion, these groups were replaced by scuticociliates and mixotrophic prostomatids. In the hypolimnion species known as benthic migrants appeared. We found a positive significant correlation (p < 0.05) between ciliate numbers and Chl a and bacterial densities. Only in the hypolimnion, the correlation between ciliates numbers and Chl a was not significant.     

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.     

Channeling of bacterioplanktonic production toward phagotrophic flagellates and ciliates under different seasonal conditions in a river

The objective of this study was to analyze the flux of biomass through the communities of bacteria and phagotrophic protists in the cold and warm conditions occurring seasonally in Butrón River. Bacterial and heterotrophic protistan (flagellate and ciliate) abundance was determined by epifluorescence direct counts; protistan grazing on planktonic bacteria was measured from fluorescently labeled bacteria uptake rates; and the estimate of bacterial secondary production was obtained from [³H]thymidine incorporation rates. The abundance of bacterial, flagellate, and ciliate communities was similar during cold and warm situations. However, we observed that estimates of dynamic parameters, i.e., secondary bacterial production and protistan grazing, in both situations were noticeably different. In the warm situation, grazing rates of flagellates and ciliates (bacteria per protist per hour) were, respectively, 7 times and 18 times higher than those determined in the cold situation, and the grazing rates of the protistan communities (bacteria per protists present in 1 ml of water per hour) increased up to 5 times in the case of flagellates and 42 times in the case of ciliates. Estimates of bacterial secondary production were also higher during the warm situation, showing a ninefold increase. The percentage of bacterial production preyed upon by flagellates or ciliates was not significantly different between the two conditions. These results showed that in the different conditions of a system, the flux of biomass between the trophic levels may be quite different although this process may not be reflected in the abundance of each community of bacteria, flagellates, and ciliates. Offprint requests to: J. Iriberri.     

Complex interactions and different possible pathways among functional components of the aquatic microbial world in Farasan Archipelago,Southern Red Sea,Saudi Arabia

This work aims to outline the dynamics of trophic links between the three main microbial components (bacteria, nanoflagellates, and ciliates) of the Farasan Archipelago in order to establish a baseline for future research in this area. The Farasan Archipelago lies along the southwestern coast of the Saudi Arabia, southern Red Sea between 16°20′–17°10′N and 41°30′–42°30′E and had been declared as marine and terrestrial reserve by the year 1996. Three different sites were chosen for this study, with each site visited bimonthly for 18 months from September 2016 to February 2018. Bacteria, nanoflagellates and ciliates were enumerated in order to explore the complex interactions between the main microbial categories in sea waters of the Farasan Archipelago. High abundances were recorded during the present study for bacteria (8.7 × 10⁶ bacteria ml⁻¹), nanoflagellates (3.7 × 10⁴ TNAN ml⁻¹) and ciliates (40.4 ciliates ml⁻¹). The paper discusses the various potential pathways controlling the complex interactions between these microbial groups in this part of the southern Red Sea. It is concluded that a linear trophic chain consisting of bacteria; heterotrophic nanoflagellates; filter feeding ciliates is a major route by which the production of bacteria is transferred to the higher consuming levels, thereby confirming the high importance of t bottom-up control (food supply), alongside top-down control (predation) in regulating bacterial abundances in the Farasan Archipelago. During the present investigation, each nanoflagellate ingested between 11 and 87 bacteria in one hour, while each ciliate consumed between 20 and 185 nanoflagellates every hour. These calculated grazing rates of protistan eukaryotes confirmed the role of heterotrophic nanoflagellates as the main consumers of bacteria, and the role of ciliates as the major control for the heterotrophic nanoflagellate population dynamics, and thus the top predators within the microbial plankton assemblage in the Farasan Archipelago.     

Vertical and Seasonal Dynamics of Planktonic Ciliates in a Strongly Stratified Hypertrophic Lake

Seasonal population dynamics and the vertical distribution of planktonic ciliates in a hypertrophic and strongly stratified temperate lake were studied from April to October in 2000 and from April to June in 2001. In the epi- and metalimnion the ciliate abundance peaked in spring and late summer, reaching maximum values in the metalimnion (86 cells ml⁻¹) on 7th August 2000. In the epilimnion, the highest biomass content (414 μg C l⁻¹) was observed on 8th May 2000. In the hypolimnion only a late summer peak occurred and the ciliate numbers were always lower than in the epi- and metalimnion. Five groups dominated the community of ciliates: Oligotrichida, Gymnostomatea, Prostomatida, Hymenostomata and Peritrichia, and the community composition varied greatly with depth. In the epilimnion the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and gymnostomes were also numerous. In the metalimnion these groups were gradually replaced by scuticociliates and mixotrophic Coleps spp. In the hypolimnion scuticociliates and species known as benthic migrants dominated. In the epilimnion and upper metalimnion in spring large herbivores and in summer small bacterivores were more numerous.     

Effects of micrometazoa on the protistan assemblage of a littoral food web

SUMMARY. 1. Effects of two size-classes of micrometazoa (assemblages of rotifers and of rotifers/cladocerans/copepods) on the abundance and composition of a co-occurring, substrate-associated protistan assemblage were assessed by selectively transferring these groups from a small pond to laboratory microcosms.
2. Addition of micrometazoa to the microcosms resulted in significant changes in the dominance of different protistan groups within 7 days, including a substantial decrease in the abundance of larger omnivorous and bactivorous Protozoa, mostly ciliates, and dominant algal taxa, and an increase in the abundance of heterotrophic microflagellates.
3. Mechanisms whereby micrometazoa reduce the abundance of larger heterotrophic protists may include competition for overlapping resources, but probably involves interference and predation as well. Positive effects of Metazoa on the abundance of heterotrophic microflagellates may result from the elimination of ciliates that prey on and compete with smaller Protozoa.
4. Interactions indicated in this study may alter both quantitative and qualitative aspects of energy flow and mineral cycling in benthic food-webs and be part of a larger trophic cascade involving other Metazoa such as fish.     

Microbial Food Webs in an Artificially Divided Acidic Bog Lake

The microbial loop of a naturally acidic bog lake, Große Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top-down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (-1.5-3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (>95% of total ciliates), with an estimated grazing rate equal to 46–80% of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto- and bacterioplankton dynamics seemed to be top-down controlled by abundant populations of small fine-filter feeding cladocerans, Ceriodaphnia quadrangula and Diaphanosoma brachyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.     

Reproductive success of the rotifer Brachionus calyciflorus feeding on ciliates and flagellates of different trophic modes

SUMMARY 1. The nutritional value of the bacterivorous ciliate Tetrahymena pyriformis and the algivorous ciliate Coleps sp., as well as the heterotrophic flagellate Chilomonas paramecium and the autotrophic flagellate Cryptomonas ovata , were investigated in population growth experiments using the rotifer B. calyciflorus . The two ciliates, both flagellates, which were of similar size, shape and mobility, were each offered as a sole diet and as a supplement to the alga Monoraphidium minutum , known to support reproduction of B. calyciflorus .
2. To further test nutritional differences between the prey organisms, prey selection experiments were conducted in which B. calyciflorus was able to select between the bacterivorous and algivorous ciliate, and between the heterotrophic and autotrophic flagellate.
3. The results demonstrated that both ciliates and the heterotrophic flagellate were not sufficient to support reproduction of B. calyciflorus when offered as a sole diet. They were, however, a good supplement to algal prey (except for the bacterivorous ciliate T. pyriformis ). In the prey selection experiments, B. calyciflorus positively selected for the algivorous Coleps sp. and the autotrophic C. ovata.
4. Overall, ciliates and heterotrophic flagellates may enhance survival of B. calyciflorus , but reproduction of the rotifer is likely to rely on algal prey. Both higher population growth of B. calyciflorus when fed the algivorous Coleps and the autotrophic Cryptomonas, along with their positive selection, give evidence for prey specific differences in nutrition, with algivorous or autotrophic prey species tending to be of higher nutritional value.     

Trophic interactions among heterotrophic microplankton,nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.Clearance rates varied between 0.2 and 122.8 nl HNF^–1 h^–1 and between 0.2 and 53.6 µl ciliate^–1 h^–1, respectively.Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories — bacteria, HNF, and ciliates — and the grazing pressure within the microbial loop show pronounced seasonal variations.     

Succession of Protists on Estuarine Aggregates

Abstract Colonization by and succession of bacteria and bacterivorous protists on laboratory-made aggregates were determined over a period of 14 days during winter and spring in 1997. Aggregates were generated from natural water from the limnetic zone of the Elbe Estuary using a tilting tube roller system. Within 1 h after the beginning of the experiments, macroaggregates started to form. Aggregates reached a maximum size of 1 mm with a tendency toward large sizes at the end of the experiment after the 10th day. On the first day, high bacterial densities of more than 10⁹ cells ml⁻¹ were detected within the aggregates. The abundances of flagellates and ciliates within aggregates were also two or three orders of magnitude higher than in the surrounding water. Densities of aggregate associated organisms are comparable to those occuring in sediments. The first protistan colonizers on the aggregates were small heterotrophic flagellates, such as choanoflagellates and small euglenids. Later, beginning on the 4th day, small sarcodines and ciliates became abundant. The most abundant ciliates associated with aggregates were small species of the Hypotrichia, Cyrtophorida, and Hymenostomata. After 9 days, large omnivorous and carnivorous ciliates, such as large members of the Hypotrichia and the Pleurostomatida, occurred. In spring, large heterotrophic flagellates and amebae also appeared at this time. These findings indicated the existence of a succession of protists on newly formed aggregates and a microbial food net within the aggregates based on bacterial production. Additionally, most of the species observed during this study were adapted for living on surfaces. In natural environments they are more common in benthic than in pelagic environments. For them, aggregates are havens in the water column comparable to sediment communities. Received: 7 January 2000; Accepted: 15 May 2000; Online Publication: 28 August 2000     

Long-term Annual and Seasonal Changes of Meta- and Protozooplankton in Lake Müggelsee (Berlin): Effects of Eutrophication,Grazing Activities,and the Impact of Predation

Annual changes of rotifers, copepods, cladocerans, the ciliate Epistylis rotans, and larvae of Dreissena polymorpha were analysed for the period 1908–1990. Though food resources increased 6–10 fold in the course of eutrophication, only rotifers and Epistylis increased accordingly. Probably as a result of increased predation pressure crustaceans increased only twice. The seasonal pattern of metazoans and protozoans (flagellates, sarcodines, ciliates) were analysed for 12 and 3 years, resp. During winter and spring, large heterotrophic flagellates and ciliates dominated the zooplankton and were responsible for a pronounced - formerly underestimated - grazing pressure on phytoplankton. In early summer, metazoan filter-feeders were often able to cause a significant reduction of phyto- and protozooplankton. However, during some years, phytoplankton declined in the absence of a pronounced grazing pressure. Field data and experiments revealed that predators were able to regulate the density of cladocerans in early summer (mainly cyclopoids) and summer (mainly Leptodora, smelt and fish juveniles).     

Estuarine Microbial Food Web Patterns in a Lake Erie Coastal Wetland

Composition and distribution of planktonic protists were examined relative to microbial food web dynamics (growth, grazing, and nitrogen cycling rates) at the Old Woman Creek (OWC) National Estuarine Research Reserve during an episodic storm event in July 2003. More than 150 protistan taxa were identified based on morphology. Species richness and microbial biomass measured via microscopy and flow cytometry increased along a stream–lake (Lake Erie) transect and peaked at the confluence. Water column ammonium (NH₄⁺) uptake (0.06 to 1.82 M N h^–1) and regeneration (0.04 to 0.55 M N h^–1) rates, measured using ¹⁵NH₄⁺ isotope dilution, followed the same pattern. Large light/dark NH₄⁺ uptake differences were observed in the hypereutrophic OWC interior, but not at the phosphorus-limited Lake Erie site, reflecting the microbial community structural shift from net autotrophic to net heterotrophic. Despite this shift, microbial grazers (mostly choreotrich ciliates, taxon-specific growth rates up to 2.9 d^–1) controlled nanophytoplankton and bacteria at all sites by consuming 76 to 110% and 56 to 97% of their daily production, respectively, in dilution experiments. Overall, distribution patterns and dynamics of microbial communities in OWC resemble those in marine estuaries, where plankton productivity increases along the river–sea gradient and reaches its maximum at the confluence.     

Benthic Bacterial Production and Protozoan Predation in a Silty Freshwater Environment

The interrelation of heterotrophic bacteria with bacterivorous protists has been widely studied in pelagic environments, but data on benthic habitats, especially in freshwater systems, are still scarce. We present a seasonal study focusing on bacterivory by heterotrophic nanoflagellates (HNF) and ciliates in the silty sediment of a temperate macrophyte-dominated oxbow lake. From January 2001 to February 2002 we monitored the standing stock of bacteria and protozoa, bacterial secondary production (BSP, ³H-thymidine, and ¹⁴C-leucine incorporation), and grazing rates of HNF and ciliates on bacteria (FLB uptake) in the oxic sediment of the investigated system. BSP ranged from 470 to 4050 µg C L^–1 wet sediment h^–1. The bacterial compartment turned out to be highly dynamic, indicated by population doubling times (0.6–10.0 d), which were comparable to those in the water column of the investigated system. Yet, the control mechanisms acting upon the bacterial population led to a relative constancy of bacterial standing stock during a year. Ingestion rates of protozoan grazers were 0–20.0 bacteria HNF^–1 h^–1 and 0–97.6 bacteria ciliate^–1 h^–1. HNF and ciliates together cropped 0–14 (mean 4)% of BSP, indicating that they did not significantly contribute to benthic bacterial mortality during any period of the year. The low impact of protozoan grazing was due to the low numbers of HNF and ciliates in relation to bacteria (1.8–3.5 × 10⁴ bacteria HNF^–1, 0.9–3.1 × 10⁶ bacteria ciliate^–1). Thus, grazing by HNF and ciliates could be ruled out as a parameter regulating bacterial standing stock or production in the sediment of the investigated system, but the factors responsible for the limitation of benthic protistan densities and the fate of benthic BSP remained unclear.     

Chlorella-bearing ciliates dominate in an oligotrophic North Patagonian lake (Lake Pirehueico, Chile): abundance, biomass and symbiotic photosynthesis

1. Large mixotrophic ciliates ( Stentor araucanus , S. amethystinus and Ophrydium naumanni ) were a characteristic component of a temperate, oligotrophic lake in North Patagonia. During a 1-year study, the abundance, biomass and primary production of these large Chlorella -bearing ciliates were compared with those of the total plankton community.
2. Mixotrophic ciliates peaked in spring and from late summer to autumn, accounting for 1.6–43% (annual average: 16.3%) and 67–99% (annual average: 92%) of total ciliate abundance and biomass, respectively. Their contribution to total zooplankton biomass, including flagellates, rotifers, ciliates and crustaceans, was 14–76%, or 47% as an annual average. Endosymbiotic algae accounted for up to 25% of total autotrophic biomass (annual mean: 3.9%).
3. Maximum cell-specific photosynthetic rates of S. araucanus and S. amethystinus at light saturation varied between 80 and 4400 pg C ciliate^–1 h^–1 with high values during autumn and winter, and low values during summer. The depth-integrated rates of photosynthesis (0–40 m) of algal endosymbionts contributed 1–25% to total photosynthesis (annual mean: 6.5%).
4. A comparison of calculated ingestion rates with photosynthetic rates of Stentor indicates that photosynthate produced by endosymbionts generally exceeded heterotrophic food supply of Stentor during autumn and winter, but was much lower during summer, when food supply was high.
5. The mixotrophic ciliates represent an important 'link' between nanoplankton and higher trophic levels within the plankton community because of their high heterotrophic biomass and considerable contribution to total photosynthesis.     

Predation on ciliates by freshwater calanoid copepods: rates of predation and relative vulnerabilities of prey

• 1 The susceptibility of ciliates in a mesotrophic lake to predation by Epischura lacustris, Diaptomus minutus and D. pygmaeus was assessed during summer. Oligotrichs, particularly Strobilidium velox (c. 43 μm), were removed efficiently by adult copepods and contributed substantially to the diet of female D. minutus. The presence of approximately 1,6 adult Epischura 1-^?1, or sixteen adult female Diaptomus 1-^?1, could halt the growth of S. vechx populations in summer.
• 2 Factors affecting the rate at which copepods prey on ciliates were examined in experiments with D. pygmaeus and cultured ciliates. Rate of predation on S. velox, the preferred species, became saturated at 5 S. velox ml^?1 (45ngCml^?1) and did not change with a 10-fold increase in alternative algal food.
• 3 Behavioural differences among ciliates, and the presence of other ciliates, contributed to differences in ciliate susceptibility to predation and suggest reasons why rates of removal of ciliates are not related to ciliate size.
• 4 By feeding selectively, at high rates, calanoids may suppress populations of some ciliates and thereby influence microzooplankton community structure.

     

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.     

Benthic flagellates and ciliates in fine freshwater sediments: Calibration of a live counting procedure and estimation of their abundances

Despite the recognized importance of protozoans (flagellates and ciliates) as predators of bacteria, there are very few estimates of their abundance in fine sediments of freshwater lakes. This is due, in part, to the lack of a standard methodology. Because of the low concentration of protists in relation to particles, epifluorescence counts can not always be used. Instead, dilution followed by live counting was used to solve the masking by sediment particles. One to twenty 1 sample aliquots were diluted with filtered lake water in a Palmer-Maloney counting slide. Four to eight replicates were sufficient to minimize the counting error, while minimizing effort. The method is highly replicable and could potentially be calibrated for different sediment types because sediment masking depends on the mean particle size of the sediment. When this method was applied in a survey of benthic sites in Quebec lakes, flagellate abundances were found to range from 100 to 180,000 cells ml^–1, while ciliate numbers ranged from 26 to 11,000 cells ml^–1. Bacteria are 105 to 10⁷ times more abundant than protists and, thus, the impact of these protists on sediment bacterial dynamics is likely to be minimal.