Sep Reservoir Research Articles

Impact of environmental factors on couplings between bacterial community composition and ectoenzymatic activities in a lacustrine ecosystem

This study examined the effects of temporal changes in bacterial community composition (BCC) and environmental factors on potential ectoenzymatic activities (alpha-glucosidase, beta-glucosidase, alkaline phosphatase and leucine aminopeptidase) in a lacustrine ecosystem (Sep reservoir, France). BCC was assessed by terminal restriction fragment length polymorphism. Physical parameters, and inorganic and organic nutrient concentrations (dissolved carbohydrates and proteins) were measured in lakes and tributaries. According to the multivariate statistics (redundancy analysis), physical and chemical factors explained the largest part of leucine aminopeptidase activity, whereas the temporal changes of other ectoenzymatic activities were partly dependent on the variations in the BCC. In particular, the occurrence of occasional bacterial populations seemed to explain a lot of the variation in rates and patterns of polymer hydrolysis. The relation observed in this study between the bacterial structure and activity is discussed within the framework of biodiversity-ecosystem functioning.

A physical-microbial food web coupled model to study the evolution of the ecological functioning of a new reservoir after its flooding (Sep, Puy de Dôme)

The aim of this paper is to understand the temporal changes in planktonic populations and to highlight some important factors that control the biological functioning and evolution of a recently flooded reservoir, the Sep reservoir (Massif Central, France). To achieve this objective, a 1D vertical coupled physical–biological model was used and developed. It couples relevant physical processes (dispersion, advection and convection) with biological processes taking into account the complexity of interactions within the microbial food web, including the conventional grazing food chain with a microbial loop. Based on numerous data collected in the field over three summers (1996, 1997, and 1998) since the flooding of the reservoir, the model was applied over this period to investigate the evolution of the biological structure over these 3 years. Two main assumptions were tested: (1) biodegradability of organic matter (OM) increases over time leading to easier assimilation by heterotrophic and mixotrophic organisms; and (2) bioavailability of phosphorus increases over time leading to easier uptake by organisms. Based on an accurate description of the physical behavior of the water column and on numerous physical and biological available data, the evolution of the microbial food web structure was correctly simulated. During the first year of the study, the plankton community was dominated by mixotrophic organisms and bacteria followed by strictly autotrophic plankton. Then, during the second and third years, autotrophic organism biomass became dominant and reached around 64% of the total biomass in 1998. The results of this study support the hypothesis that the initial allochthonous dissolved organic matter (DOM) strongly influenced the structure and the functioning of the microbial food web in the reservoir.

Functional responses of prokaryotes and viruses to grazer effects and nutrient additions in freshwater microcosms

For aquatic systems, there is little data on the interactions between viruses, prokaryotes, grazers and the availability of resources. We conducted a microcosm experiment using a size fractionation approach to manipulate grazers, with a purpose to examine the effects of inorganic and organic nutrients on viral and prokaryotic standing stocks and activities, and on prokaryotic community composition as assessed by fluorescent in situ hybridization (FISH) method. Experiments were performed during periods of severe phosphate (P)-limiting conditions in the oligotrophic Sep Reservoir (Massif Central, France). In the absence of nutrient addition, the presence of grazers in microcosms stimulated prokaryotic growth and viral proliferation, likely through nutrient and substrate enrichment. Addition of nutrients had a stronger effect on viral infection of prokaryotes than grazing. Addition of P led to the most pronounced increase in prokaryotic abundance, production and growth efficiency, thus providing direct evidence of P limitation of prokaryotes. Enhanced prokaryotic activity in P treatments also stimulated viral abundance and viral-induced lyses of prokaryotes. Changes in prokaryotic community composition due to nutrient additions were evident in the grazer-free samples. Prokaryotic populations hybridizing for the probes bacteria, beta-Proteobacteria and alpha-Proteobacteria responded to nutrient enrichment with significant increases in their relative abundances, whereas cells hybridizing for Archaea and Cytophaga-Flavobacterium (now known as Bacteroidetes) probes failed to show any functional response. Cells hybridizing for the latter cluster increased towards the end of incubation period in the control samples (that is, without nutrient additions) with grazers present, suggesting the development of grazing resistant forms. From our nutrient enrichment microcosm experiments, we conclude that the presence of grazers is a stimulating factor for prokaryotic growth and viral proliferation in the plankton, probably through nutrient regeneration process.

Bacterioplankton heterotrophic activity in relation to the phytoplankton compartment in a recently formed reservoir

Seasonal and spatial variations in bacterial abundance, biomass and potential heterotrophic activity in a recently flooded reservoir were measured for two consecutive years, in conjunction with phytoplankton biomass (chlorophyll-a concentration) and activity (primary production). The mean value of primary production remained constant between the two study years, while those of the chlorophyll-a concentrations, bacterial abundance, bacterial biomass and bacterial heterotrophic activity decreased. The observed trends in the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter, in addition to selective grazing activity on bacterioplankton. Multivariate regression analyses identified bacterial abundance (29%) and temperature (17%) as dominant correlates of the bacterial potential heterotrophic activity. We concluded that, in a new reservoir, organic matter, other than that from phytoplankton, might be of great importance for bacterioplankton metabolism. Furthermore, grazing activity on bacteria by metazoa in a new reservoir represents, on occasion, an important trophic link between the top consumer and otherwise unavailable dissolved organic carbon sources. Finally, even if little energy is transferred to larger consumers, the microbial route is still important in re-mineralizing organic matter in Sep Reservoir.

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.

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-Dome, 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.

Succession of bacterial community composition over two consecutive years in two aquatic systems: a natural lake and a lake-reservoir

The succession in bacterial community composition was studied over two years in the epilimnion and hypolimnion of two freshwater systems: a natural lake (Pavin Lake) and a lake-reservoir (Sep Reservoir). The bacterial community composition was determined by cloning-sequencing of 16S rRNA and by terminal restriction fragment length polymorphism. Despite large hydrogeological differences, in the Sep Reservoir and Pavin Lake the dominant bacteria were from the same taxonomic divisions, particularly Actinobacteria and Betaproteobacteria. In both ecosystems, these major bacterial divisions showed temporal fluctuations that were much less marked than those occurring at a finer phylogenetic scale. Nutrient availability and mortality factors, the nature of which differed from one lake to another, covaried with the temporal variations in the bacterial community composition at all sampling depths, whereas factors related to seasonal forces (temperature and outflow for Sep Reservoir) seemed to account only for the variation of the hypolimnion bacterial community composition. No seasonal reproducibility in temporal evolution of bacterial community from one year to the next was observed.

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 x 10(10) 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 (R(2) = 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 (R(2) = 0.20) and bacterial production (R(2) = 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.

Relative importance of nutrients and mortality factors on prokaryotic community composition in two lakes of different trophic status: Microcosm experiments

The effect of nutrient resources (N and P enrichment) and of different grazing communities on the prokaryotic community composition (PCC) was investigated in two freshwater ecosystems: Sep reservoir (oligomesotrophic) and lake Aydat (eutrophic). An experimental approach using microcosms was chosen, that allowed control of both predation levels, by size fractionation of predators, and resources, by nutrient amendments. Changes in PCC were monitored by fluorescent in situ hybridization (FISH) and terminal-restriction fragment length polymorphism (T-RFLP). The main mortality agents were (i) heterotrophic nanoflagellates and virus-like particles in Aydat and (ii) cladocerans in Sep. All the nutritional elements assayed (N-NO 3, P-PO 4 and N-NH 4) together with prokaryotic production (PP) always accounted for a significant part of the variations in PCC. Overall, prokaryotic diversity was mainly explained by resources in Sep, by a comparable contribution of resources and mortality factors in lake Aydat and, to a lesser extent, by the combined action of both.

Grazer effects on prokaryotes and viruses in a freshwater microcosm experiment

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 41:115-124 (2005) - doi:10.3354/ame041115 Grazer effects on prokaryotes and viruses in a freshwater microcosm experiment T. Sime-Ngando1,*, A. S. Pradeep Ram1,2 1Laboratoire de Biologie des Protistes, Université Blaise Pascal (Clermont-Ferrand II), UMR CNRS 6023, 63177 Aubière Cedex, France2Present address: Centre for Ecological Research, Kyoto University, 509-3 Otsuka, Kamitanakami-hirano, Otsu 520-2113 Shiga, Japan *Email: telesphore.sime-ngando@univ-bpclermont.fr ABSTRACT: We conducted a short-term course experiment using a size-fractionation approach to manipulate grazers and test for their effects on viral and prokaryotic standing stocks and activities and on prokaryotic community composition, as assessed by the FISH (fluorescence in situ hybridisation) method. Experimental samples were collected in the Sep Reservoir during severe P-limiting conditions. The presence of grazers in microcosms appeared to be a stimulating factor for prokaryotic growth and viral proliferation, likely through the related nutrient and substrate enrichments. About 60% of the total prokaryote abundance was detected by FISH, with a dominance of Eubacteria and beta-Proteobacteria throughout the experiment, i.e. in the presence and in the absence of grazers. The relative abundances of the minor phylotypes remained unchanged in the absence of grazers, but significantly increased with time when grazers were present, indicating a grazer-mediated reduction in resource competition in prokaryotic assemblages. However, grazer effects resulted in an apparently greater alteration of the bacterial size structure (i.e. the occurrence of large, grazing-resistant cells) than of the relative abundances of the phylotypes analysed. The main findings suggest that, at least on a short-term scale (i.e. ≤1 prokaryotic net generation time): (1) There is a synergy between grazer, prokaryotic and viral activities in oligotrophic conditions, through a cascading effect from grazer-mediated resource enrichment, with (2) a higher impact on the prokaryotic size structure than on the relative abundances of the major phylotypes, and (3) a significant acquisition of competitive advantages by the less adundant phylotypes. These findings are discussed in light of the recent ‘phage kills winner’ theory. KEY WORDS: Plankton · Lakes · Microcosms · Viruses · Prokaryotes · Community composition · Grazer effects · Microbial ecology Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 41, No. 2. Online publication date: November 25, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Bottom-up and top-down control of bacterial community composition in the euphotic zone of a reservoir

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 35:259-273 (2004) - doi:10.3354/ame035259 Bottom-up and top-down control of bacterial community composition in the euphotic zone of a reservoir Ludwig Jardillier1, Maryline Basset1, Isabelle Domaizon1,2, André Belan1, Christian Amblard1, Mathilde Richardot1, Didier Debroas1,* 1Université Blaise Pascal, Laboratoire de Biologie des Protistes UMR CNRS 6023. 63177 Aubière, France 2Present address: Université de Savoie, CARRTEL, 73376 Le Bourget du Lac, France *Corresponding author. Email: didier.debroas@free.fr ABSTRACT: Temporal changes in the bacterial community composition (BCC) and the impact of resources and predation on this community composition have been studied in the euphotic zone of the Sep reservoir (France), using terminal-restriction fragment length polymorphism (T-RFLP) and fluorescent in situ hybridization (FISH) for Eubacteria (EUB338) and eubacterial subgroups. Net growth and grazing rates of the various subgroups were computed from experiments conducted in dialysis bags in the presence or absence of predators. There was a significant difference between the grazing rates of different bacterial groups and subclasses, the alpha-proteobacteria (ALF1b) were subjected to the highest grazing rate (max. 1.76 d-1). In contrast, the beta-proteobacteria (BET42a) seemed to be little consumed by bacterivorous organisms, and predation led to large variations in the grazing rates of Cytophaga-Flavobacterium (CF319a). The different composition of bacterial consumers (Dinobryon sp. and Cladocera) during the study could explain the observed differences in grazing impact. The mean net growth rates of total bacteria, EUB338, BET42a ALF1b, and CF319a in dialysis bags without predators were 0.21, 0.69, 0.45, 0.50 and 0.84 d-1, respectively. Various statistical analyses indicated that the BCC could depend on the organic matter excreted by the phytoplankton. There were also positive correlations not only between net growth and/or net production of ALF1b and CF319a with the primary production and biomass of the main phytoplankton groups, but also between phytoplankton biomass, primary production (PP) and some operational taxonomic units (OTUs). Among the physical variables, input and output of water in the reservoir seem to play a role in determining the BCC in this ecosystem. The results suggest that the BCC depends on the combined impact of dominant substrate sources and selective predation by bacterial consumers. KEY WORDS: Bacterial community composition · Top-down · Bottom-up · Reservoir · Terminal-restriction fragment length polymorphism · Fluorescent in situ hybridization Full article in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 35, No. 3. Online publication date: May 19, 2004 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2004 Inter-Research.

Size‐Fractionated Phytoplankton and Relationships with Metazooplankton in a Newly Flooded Reservoir

AbstractIn order to yield some insights into the planktonic food web structure of new reservoirs, size‐fractionated biomass and productivity of phytoplankton were examined from 1996 to 1997 (following the 1995 flooding of the Sep Reservoir, Puy‐de‐Dôme, France), in relation to nutrients (P, N) and metazooplankton (Rotifers, Cladocera, Copepods). Autotrophic nanoplankton (ANP, size class 3–45 μm) dominated the phytoplankton biomass (as Chlorophyll a) and production, while autotrophic picoplankton (APP, 0.7–3 μm) exhibited the lowest and relatively constant biomass and production. Cells of the autotrophic microplankton (AMP, >45 μm) were considered inedible for planktonic herbivores. The production‐biomass diagram for the different size classes and the positive correlation between APP production and ANP + AMP production suggested that grazing was potentially more important than nutrients in shaping the phytoplankton size structure. Metazooplankton biomass was low compared to other newly flooded reservoirs or to natural lakes with phytoplankton biomass similar to that of the Sep Reservoir. This resulted in low ratios (metazooplankton to edible phytoplankton) both in terms of production (average 0.43% in 1996 and 0.76% in 1997) and biomass, suggesting that only a small fraction of phytoplankton was directly consumed by metazooplankton. We suggest that the observed low ratios in the Sep Reservoir, reflect possible low metazooplankton inputs in the main influents, changes in hydrologic conditions and a high potential role of microheterotrophs. The latter role was supported by (i) the positive inter‐annual correlation between ciliates and phytoplankton, (ii) the significant and negative correlations between ciliates and metazooplankton, and (iii) the significant and negative correlations between total metazooplankton biomass and total phosphorus (TP), whereas neither TP nor total metazooplankton biomass was correlated with phytoplankton variables.

Food web structure in the recently flooded Sep Reservoir as inferred from phytoplankton population dynamics and living microbial biomass.

Phytoplankton dynamics, bacterial standing stocks and living microbial biomass (derived from ATP measurements, 0.7-200 mm size class) were examined in 1996 in the newly flooded (1995) Sep Reservoir ('Massif Central,' France), for evidence of the importance of the microbial food web relative to the traditional food chain. Phosphate concentrations were low, N:P ratios were high, and phosphate losses converted into carbon accounted for <50% of phytoplankton biomass and production, indicating that P was limiting phytoplankton development during the study. The observed low availability of P contrasts with the high release of "directly" assimilable P often reported in newly flooded reservoirs, suggesting that factors determining nutrient dynamics in such ecosystems are complex. The phosphate availability, but also the water column stability, seemed to be among the major factors determining phytoplankton dynamics, as (i) large-size phytoplankton species were prominent during the period of increasing water column stability, whereas small-size species dominated phytoplankton assemblages during the period of decreasing stability, and (ii) a Dinobryon divergens bloom occurred during a period when inorganic P was undetectable, coinciding with the lowest values of bacterial standing stocks. Indication of grazing limitation of bacterial populations by the mixotrophic chrysophyte D. divergens (in late spring) and by other potential grazers (mainly rotifers in summer) seemed to be confirmed by the Model II or functional slopes of the bacterial vs phytoplankton regressions, which were always <0.63. Phytoplankton biomass was not correlated with phosphorus sources and its contribution was remarkably low relative to the living microbial biomass which, in contrast, was positively correlated with total phosphorus in summer. We conclude that planktonic microheterotrophs are strongly implicated in the phosphorus dynamics in the Sep Reservoir, and thus support the idea that an important amount of matter and energy flows through the "microbial loop" and food web, shortly after the flooding of a reservoir.

The Microbial Food Web in the Recently Flooded Sep Reservoir: Diel Fluctuations in Bacterial Biomass and Metabolic Activity in Relation to Phytoplankton and Flagellate Grazers.

The spatial distribution of the bacterial biomass and production and of potential heterotrophic activity (PHA) were measured every 4 h between 23 July (10:00 h) and 25 July (10:00 h) 1997 in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-de-Dôme, France), in relation to temperature, the phytoplankton biomass and production, and the abundance of heterotrophic flagellates. The temperature varied slightly with time during the study, but the well-established thermal stratification agreed well with vertical distribution of the biological variables that were measured. Only the bacterial production and the PHA showed significant diel changes (t-test, p <0.05), with maxima at 18:00 h and minima at 02:00 h. A significant positive relation was found between bacterial abundance and that of heterotrophic flagellates, which, rather than being an association related to the thermal stratification of the water column, was considered to reflect a trophic relation between these two communities. A carbon balance analysis suggested that at least 30% of the C from primary production measured during the sampling period was used by bacteria, and that 42% of this secondary production, or 6% of the primary production, would be used for the development of the heterotrophic flagellates present. We conclude that the bacterioplankton forms, at least occasionally, an important source of carbon for higher trophic levels, and reject the hypothesis that bacterial production in the Sep Reservoir depends exclusively on organic matter of allochthonous origin.

Primary productivity in the recently flooded Sep Reservoir'(Puy-de-Dome, France)

Seasonal variations in phytoplankton biomass and production were studied in 1996 and 1997 in the newly flooded (1995) Sep Reservoir (France) in relation to various physico-chemical environ- mental factors. No trophic upsurge occurred in the reservoir as has been found in most of the newly flooded reservoirs. Phytoplankton composition changed drastically from 1996 to 1997, the latter year being characterized by a rarity of diatoms. Areal euphotic chlorophyll a concentration and phyto- plankton carbon biomass were typical of mesotrophic lakes in 1996 and of somewhat oligotrophic environments in 1997. In contrast, the primary production (PP) rates were similar in both years, indi- cating a change in the factors controlling phytoplankton populations from 1996 to 1997. The low PP (compared with the algal biomass present) in 1996 seemed to be related to the instability of the water column and the Zmix:Zeu ratio, whose negative effects on the phytoplankton community were appar- ently more pronounced in 1996 than in 1997. Orthophosphate concentrations, and their contribution to total phosphorus, remained low in 1996 but increased with time in 1997. The temporal losses of orthophosphates from June to September 1996 accounted for only 42% of the measured PP, indicat- ing that PP in 1996 was also strongly affected by P availability and probably took place due to other P sources such as regenerated nutrients.

Rates of growth and microbial grazing mortality of phytoplankton in a recent artificial lake

Dilution experiments were conducted from May to September 1998 in the epilimnion of a recently flooded reservoir (Sep Reservoir, Puy-de-Dome, France: 46° 2' N, 3° 1' E), to estimate growth and microzooplankton (20 to 200 µm) grazing mortality of 2 size classes of the phytoplankton community. This community was dominated by 25 to 200 µm size fraction. Micrograzers were largely dominated by ciliated protozoa (86 to 96% of total abundance), and also comprised rotifers and copepod nauplii. Experiments with and without added nutrients (N and P) indicated, together with changes in particulate protein, RNA and DNA, that phosphorus is a limiting element in the Sep Reservoir. Grazing activity of microzooplankton was significant on nanoalgae, averaging 0.38 ± 0.19 d -1 (range 0.16 to 0.66 d -1 ). The grazing activity bal- anced the daily production of the target algal community by 71 ± 11% (range 54 to 88%). Microzoo- plankton herbivory and the growth rates of nanoalgae were strongly correlated, suggesting the exis- tence of an operating homeostatic interaction between the prey and the predators. The high mortality:growth ratio indicated that a substantial fraction of phytoplankton carbon is recycled in surface waters through microbial grazing. We conclude that microzooplankton herbivory provides an effective and substantial link to higher trophic levels in the Sep Reservoir, and might contribute to fueling planktonic communities with the limiting nutrient through regeneration.

Bacterial population dynamics, production, and heterotrophic activity in a recently formed reservoir

Seasonal and spatial fluctuations in abundance, biomass production, and potential heterotrophic activity (i.e., 14C-glucose uptake) of bacterioplankton assemblages in a 1-year-old reservoir (the Sep Reservoir, Puy-de-Dôme, France) were examined concurrently with water temperature, phytoplankton chlorophyll a concentration, and primary production (PP). Based on the values observed for these biological variables, the Sep Reservoir was considered to have evolved to an oligo-mesotrophic state. Spatiotemporal variations of bacterial variables were a consequence of the seasonal evolution of the reservoir coupled with the resource availability. Multivariate regression analyses suggest that about 14 and 26% of the variance in bacterial standing stock and activity may be explained by the physical environment (i.e., temperature) and a resource availability index (chlorophyll a concentration or primary production), respectively. A carbon budget indicated that 4-126% (mean = 20%) of the ambient PP may be channeled through the microbial loop via bacterial biomass production. Heterotrophic bacterial production in the Sep Reservoir may therefore, on occasion, represent a significant source of carbon for higher order consumers.Key words: reservoirs, plankton, bacteria, heterotrophic uptake, primary and bacterial production.