Pico- and Nanoplankton in Aquatic Ecosystems in the Valley of the Lakes and Great Lakes Depression (Mongolia)

The abundance and biomass of bacterioplankton, phototrophic picoplankton, and heterotrophic nanoflagellates has been determined in lakes, rivers, and reservoirs located in the Valley of the Lakes and Great Lakes Depression (Mongolia). The species richness of the heterotrophic flagellates and their consumption of bacteria are estimated. Pico- and nanoplankton are the most abundant in shallow mineral lakes Orog and Tatsyn and in the freshwater Durgun Reservoir. Heterotrophic nanoflagellates consume 26–92% (on average 66%) of the daily bacterioplankton production. Thus, flagellates are important in the transfer of bacterial carbon to the higher levels of planktonic trophic webs. A total of 30 species and their forms of heterotrophic flagellates from 14 large taxa are identified. The highest species diversity of these protists are found in the Durgun and Taishyr reservoirs.     

Bacterial Production in the Recently Flooded Sep Reservoir: Diel Changes in Relation to Dissolved Carbohydrates and Combined Amino Acids

The spatial distribution of bacterial abundance and production were measured every 4 h in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-De-Dôme, France), in relation to concentrations of dissolved carbohydrates and combined amino acids. The concentration of dissolved organic matter (DOM) components in the recently flooded Sep Reservoir were higher than those measured in other lakes of similar trophic status. Short-term variations in the bacterial production in this new reservoir appeared cyclical and endogenous to bacterial communities. These results highlight the need for the evaluation of diel changes in bacterial production, if estimation of the daily production rate of bacteria is to be done accurately for a reliable model of carbon flow through bacterioplankton and ultimately through aquatic microbial food webs. Bacterial growth, measured over time and space, did not appear exclusively governed by DOM components from phytoplankton primary production.     

Relationship between bacterial and primary production in a newly filled reservoir: temporal variability over 2 consecutive years

Seasonal and spatial variations in bacterial abundance, biomass and production in a recently flooded reservoir were followed for 2 consecutive years, in conjunction with phytoplankton biomass (chlorophyll a) and activity (primary production). Between the 2 years of the study, the mean value of primary production remained constant, while those of the chlorophyll a concentration, bacterial abundance (BA), bacterial biomass (BB) and bacterial production (BP) decreased. The observed trends of the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter. Moreover, this factor would explain discrepancies observed between the slope of the model II regression equations established from results of the present study and those of the predictive models from the literature, relating to bacterial and phytoplankton variables. An estimate of the carbon budget indicated that 22 and 5% of the ambient primary production in the Sep Reservoir might be channeled through the microbial loop via BP during the 1st and 2nd year of the study, respectively. We conclude that heterotrophic BP in the Sep Reservoir may, on occasion, represent a significant source of carbon for higher order consumers.     

Abundance and Biomass of Heterotrophic Flagellates, and Factors Controlling Their Abundance and Distribution in Sediments of Botany Bay

The abundance and biomass of heterotrophic flagellates were estimated monthly in sediments of Botany Bay during March 1999-February 2000. The annual abundance and biomass were in the ranges of 0.46-4.70 x 10(5) cells/cm(3) and of 0.30-8.61 micro g C/cm(3), respectively. The majority of heterotrophic flagellates (93-100%) were less than 10 mm in length and few flagellates were larger than 10 mm. Of the total microbial carbon biomass, heterotrophic flagellates made up about 5% (but at times up to 35%). The contribution of heterotrophic flagellates varied from month to month, and among the sites. The abundance of heterotrophic flagellates was negatively correlated with sediment grain size and positively correlated with the abundance of bacteria, algae (autotrophic flagellates and diatoms), and their probable grazers. A best subsets regression analysis showed that bacterial and algal abundance are the most important factors controlling the abundance of heterotrophic flagellates. When the previously reported grazing rates on bacteria were applied, heterotrophic flagellates would consume a maximum of 64% of bacterial standing stock daily in Botany Bay, suggesting that heterotrophic flagellates are important as bacterivores. However, the importance of heterotrophic flagellate grazing probably varies significantly among the sites and from month to month.     

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.     

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.     

Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance: HNF abundance ratio (B: HNF) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chla than in the eutrophic system, suggesting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, showed that the importance of bacteriovory increased in richer systems.     

Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria,and zooplankton (including heterotrophic flagellates, ciliates,rotifers and crustaceans) were studied in 55 lakes in NorthernGermany with different trophic status, ranging from mesotrophicto hypertrophic. Mean abundance and biomass of all groups increasedsignificantly with trophic level of the lake, but bacteria andmetazooplankton showed only a weak correlation and a slightincrease with chlorophyll concentration. Composition of phytoplanktonshowed a dominance of cyanobacteria in hypertrophic lakes, whereasthe importance of chrysophytes and dinophytes decreased withan increase in trophic status. Protozoans (heterotrophic flagellatesand ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophiclakes) of total zooplankton biomass on average, and were dominatedby ciliates (62–80% of protozoan biomass). Seasonally,protozoans can build up to 60% of zooplankton biomass in spring,when heterotrophic flagellates can contribute     

Effect of grazers and viruses on bacterial community structure and production in two contrasting trophic lakes

Background

Over the last 30 years, extensive studies have revealed the crucial roles played by microbes in aquatic ecosystems. It has been shown that bacteria, viruses and protozoan grazers are dominant in terms of abundance and biomass. The frequent interactions between these microbiological compartments are responsible for strong trophic links from dissolved organic matter to higher trophic levels, via heterotrophic bacteria, which form the basis for the important biogeochemical roles of microbial food webs in aquatic ecosystems. To gain a better understanding of the interactions between bacteria, viruses and flagellates in lacustrine ecosystems, we investigated the effect of protistan bacterivory on bacterial abundance, production and structure [determined by 16S rRNA PCR-DGGE], and viral abundance and activity of two lakes of contrasting trophic status. Four experiments were conducted in the oligotrophic Lake Annecy and the mesotrophic Lake Bourget over two seasons (early spring vs. summer) using a fractionation approach. In situ dark vs. light incubations were performed to consider the effects of the different treatments in the presence and absence of phototrophic activity.

Results

The presence of grazers (i.e. < 5-μm small eukaryotes) affected viral production positively in all experiments, and the stimulation of viral production (compared to the treatment with no eukaryotic predators) was more variable between lakes than between seasons, with the highest value having been recorded in the mesotrophic lake (+30%). Viral lysis and grazing activities acted additively to sustain high bacterial production in all experiments. Nevertheless, the stimulation of bacterial production was more variable between seasons than between lakes, with the highest values obtained in summer (+33.5% and +37.5% in Lakes Bourget and Annecy, respectively). The presence of both predators (nanoflagellates and viruses) did not seem to have a clear influence upon bacterial community structure according to the four experiments.

Conclusions

Our results highlight the importance of a synergistic effect, i.e. the positive influence of grazers on viral activities in sustaining (directly and indirectly) bacterial production and affecting composition, in both oligotrophic and mesotrophic lakes.     

The structure of the planktic microbial community in the lower reaches of the Ob River near Salekhard

We studied the distribution of the main components of planktic food web: bacteria, heterotrophic flagellates, and ciliates in the lower reaches of the Ob River near the town of Salekhard. In the early summer, the abundance, biomass, and diversity of protozoans were low; the plankton community was dominated by heterotrophic bacteria whose biomass was higher than that of phyto- and zooplankton. Downstream of Salekhard, biomass and abundance of bacterioplankton were increased due to household and industrial wastewater. The data obtained showed the importance of heterotrophic organisms in trophic dynamics and self-purification of the lower reaches of the Ob River.     

Phage Bacteriolysis, Protistan Bacterivory Potential, and Bacterial Production in a Freshwater Reservoir: Coupling with Temperature

Phage abundance and infection of bacterioplankton were studied from March to November 2003 in the Sep Reservoir (Massif Central, France), together with temperature, chlorophyll, bacteria (abundance and production), and heterotrophic nanoflagellates (abundance and potential bacterivory). Virus abundance (VA) ranged from 0.6 to 13 × 10¹⁰ viruses l⁻¹, exceeding bacterial abundance (BA) approximately sixfold on average. In terms of carbon, viruses corresponded to up to 25% of bacterial biomass. A multiple regression model indicated that BA was the best predictor for VA (R² = 0.75). The frequency of infected bacteria (estimated from the percentage of visibly infected cells) varied from 1% to 32% and was best explained by a combination of temperature (R² = 0.20) and bacterial production (R² = 0.25). Viruses and flagellates contributed about equally to bacterial mortality. Both factors destroyed 55% of bacterial production, with a shift from phage bacteriolysis in early spring to protistan bacterivory in late summer. The vertical differences in most of the biological variables were not significant, contrasting with the seasonal differences (i.e., spring vs. summer-autumn). All biological variables under study were indeed significantly coupled to temperature. We regarded this to be the consequence of the enhanced discharge of the reservoir in 2003 (compared to previous years). This substantially weakened the stability and the thermal inertia of the water column, thereby establishing temperature as a stronger forcing factor in setting the conditions for optimal metabolic activity of microbial communities.     

Heterotrophic organisms and viruses in the Oka River and Cheboksary Reservoir during the abnormally hot summer of 2010

In July 2010, abnormally high water temperature (25–29°C), as well as increased biomass and phytoplankton production caused intensive development of heterotrophic bacteria and heterotrophic nanoflagellates. It was found that the abundance, biomass, and production of heterotrophic bacterioplankton, as well as the abundance and biomass of heterotrophic nanoflagellates, and the number of planktonic viruses, which were calculated on average for the reservoir under study, turned out to be higher in the years with lower water temperature (20–23°C). The virus-induced mortality of bacterioplankton in the Oka River and the Cheboksary Reservoir averaged 25.4 ± 3.4 and 22.4 ± 2.7% of the daily bacterioplankton production.     

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.     

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.     

Carbon flow dynamics in the pelagic community of the Sau Reservoir (Catalonia,NE Spain)

Changes in the pelagic community structure and activity along the longitudinal axis of the eutrophic Sau Reservoir (Catalonia, NE Spain) were studied between 1996 and 1999. Samples were taken from several transects from river to dam, measuring dissolved organic carbon (DOC), bacterial abundance and production, chlorophyll a concentration, heterotrophic nanoflagelate (HNF) and ciliate abundances and their grazing rates, and zooplankton density. The role of microbial and classical food chains (i.e., based directly on phytoplankon) were compared in the Sau Reservoir by analysing river-to-dam gradients in biomass and carbon and their temporal changes. The detritic metabolic pathway was more important near to the inflow, due to high allochthonous organic matter loads allowing the rapid development of the microbial food web. Protozoans (HNF and ciliates) consumed most of the bacterial production (i.e., >50%) in the reservoir. As opposed to the systems of lower trophic status ciliate carbon biomass and bacterivory contributions were larger than those of the HNF. We estimated species-specific ciliate growing rates on bacteria and distinguished several periods with high importance of distinct ciliate communities.     

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.     

Patterns of the quantitative distribution of plankton heterotrophic flagellates in the high-mountain Lake Sevan (Armenia)

Patterns of the quantitative development and spatial distribution of heterotrophic flagellates have been studied in Lake Sevan (Armenia). The abundance and biomass of flagellates is lower in summer than in autumn, but on the whole these parameters correspond to the quantitative parameters in oligotrophic lakes. Differences in the vertical and horizontal distribution of flagellates have been found during summer stratification and autumn homothermy.     

Dynamics and estimates of growth and loss rates of bacterioplankton in a temperate freshwater system

The growth rate and losses of bacterioplankton in the epilimnion of an oligo-mesotrophic reservoir were simultaneously estimated using three different methods for each process. Bacterial production was determined by means of the tritiated thymidine incorporation method, the dialysis bag method and the dilution method, while bacterial mortality was assessed with the dilution method, the disappearance of thymidine-labeled natural cells and ingestion of fluorescent bacterial tracers by heterotrophic flagellates. The different methods used to estimate bacterial growth rates yielded similar results. On the other hand, the mortality rates obtained with the dilution method were significantly lower than those obtained with the use of thymidine-labeled natural cells. The bacterial ingestion rate by flagellates accounted on average for 39% of total bacterial mortality estimated by the dilution method, but this value fell to 5% when the total mortality was measured by the thymidine-labeling method. Bacterial abundance and production varied in opposite phase to flagellate abundance and the various bacterial mortality rates. All this points to the critical importance of methodological aspects in the elaboration of quantitative models of matter and energy flows over the time through microbial trophic networks in aquatic systems, and highlights the role of bacterioplankton as a source of carbon for higher trophic levels in the studied system.     

Trophic relationships between planktonic bacteria,heterotrophic nanoflagellates and viruses in a mesoeutrophic reservoir

The abundance, biomass, and production (Р _В) of bacrerioplankton; the taxonomic composition, abundance, biomass of heterotrophic nanoflagellates (HNF) and the rate of consumption of bacteria by HNFs; and the abundance of virioplankton, frequency of visibly infected bacterial cells, virus-induced mortality of bacterioplankton, and viral production were estimated in the mesoeutrophic Rybinsk Reservoir. The rate of bacterial mortality due to viral lysis (7.8–34.1%, on average 17.2 ± 2.0% of daily Р _В) was lower than the consumption of bacteria by the HNF community (15.4–61.3%, on average 32.0 ± 4.2% of daily Р _В). While consuming bacteria, HNFs simultaneously absorbed a significant number of viruses residing on the surface and inside the bacterial cells.     

Bacterial Activity in a Reservoir Determined by Autoradiography and its Relationships to Phyto- and Zooplankton

In the drinking water reservoir Římov (Southern Bohemia) bacterioplankton was studied during 1983. Special attention was given to the relationships between parameters of bacterial abundance, total and individual activity. Bacterial counts and biomass was assessed and autoradiographic determinations of the proportion of active bacteria incorporating thymidine (Th) and mixture of amino acids (AA) and total uptake rate of AA were made over a year in the surface layer and during summer stratification from the thermocline and 15 m depth. Specific activity of metabolically active bacteria (SAMAB) and specific activity per unit of biomass (SAUB) were negatively correlated with counts of metabolizing cells and with bacterial biomass, respectively. Total and individual heterotrophic activity and counts of bacteria coincided with the changes of phytoplankton biomass, whereas bacteria incorporating Th were more tightly correlated with primary production. The most significant relation of metabolically active bacteria was found to cladoceran biomass. Thus, this part of heterotrophic bacterial activity seems to be stimulated by leakage of dissolved organic matter from phytoplankton being disrupted and incompletely digested by cladocerans rather than from healthy photosynthetizing cells.