Comparison of solutes, nutrients, and bacteria inputs from two types of groundwater to the Rhône river during an artificial drought

Solute, nutrient and bacterial inputs to the River Rhône from the interstitial habitat of a gravel bar and the floodplain aquifer were investigated during an artificial drought. Eight springs were investigated: four groundwater-fed springs in the floodplain, located at the bottom of the bank; and four interstitial-fed springs located at the downstream end of a gravel bar. During this period, the inflows of groundwater to the river represented an average input of 0.77 mg l^–1 of nitrogen (of which 93.3% were nitrates), 0.0187 mg l^–1 of total phosphorus (of which 42.2% was orthophosphate), 3.56 mg l^–1 of silica, 2.315 ± 0.703 mg l^–1 of dissolved organic carbon (DOC, of which 47% was biodegradable) and 7.3 × 10⁴ ± 3.7 × 10⁴ bacteria per ml (of which 8.8% were active). Silica, DOC, biodegradable DOC, and bacteria concentrations displayed temporal variations during the study, which seem to be linked to the biological activity of the groundwater biofilm. There was a strong heterogeneity between the two types of groundwater that flow to the river: concentrations of calcium and alkalinity were higher in bank springs than in gravel bars springs. In these latters, sulfate, sodium, nitrogen, phosphorus were significantly higher.     

Do parafluvial zones have an impact in regulating river pollution? Spatial and temporal dynamics of nutrients,carbon, and bacteria in a large gravel bar of the Doubs River (France)

The changes in both physical and chemical properties of interstitial water were investigated within a large gravel bar to determine if the parafluvial zone was a hotspot for nutrient transformations in a lowland eutrophic river, the Doubs (Eastern France). Interstitial water was sampled in 30 piezometers along five transects across the gravel bar, surface water was sampled in main and chute channels. Five campaigns of sampling were performed among summer and winter. In both interstitial and surface samples, water chemistry (electrical conductivity, pH, chloride, dissolved oxygen concentrations—DO) and nutrient concentrations (nitrate—NO₃ ⁻, ammonium—NH₄ ⁺, soluble reactive phosphorus—SRP, dissolved organic carbon—DOC) were measured. Moreover, temperature, water level, sediment grain size distribution and total bacterial abundance were assessed along a flowpath through a lateral gravel bar of the Doubs River. Measurements of water table elevation in the bar, main and chute channels indicated that the parafluvial flowpath was perpendicular to the main channel. Very low changes in chloride concentration and electrical conductivity showed minor groundwater input along the flowpath. The parafluvial zone was 0.9 m thick under the mean piezometric level, hydraulic gradients along the flowpath were 0.3%, and the discharge of interstitial water through the bar calculated at low flow was 40.6 m³ day⁻¹. Most changes in interstitial nutrients occurred during the warm season, suggesting that biotic mechanisms occurred. Along the flowpath, DO, DOC, and bacterial abundance declined, while phosphate increased. Temporal trends of nitrate were less clear, exhibiting a slight increase during spring (organic matter biodegradation) and a decrease during summer (denitrification). Using the parafluvial discharge and nutrient concentrations of water infiltrating into and seeping from the gravel bar, the retention capacity of the bar was 171 gC day⁻¹ for DOC and 48.3 gN day⁻¹ for nitrate. Phosphate production ranged from 0.65 to 2.3 gP day⁻¹. These values were low compared to the nutrient fluxes in the river Doubs, suggesting a minimal local impact of the parafluvial zone in regulating of river pollution. Handling editor: J. Padisak     

Nutrient transfer between parafluvial interstitial water and river water: influence of gravel bar heterogeneity

1. Nutrient concentrations in interstitial water springs at the downstream ends of two gravel bars in the River Rhône were measured in order to estimate the importance of nutrient inputs [nitrate, dissolved organic carbon (DOC), biodegradable and refractory fractions of DOC] from the parafluvial zone (saturated sediments adjacent to the wetted channel, i.e. interstitial habitats of gravel bars) to the river as well as seasonal variations in these inputs.
2. Compared with surface water, parafluvial water had lower concentrations of DOC (especially the refractory fraction) and generally higher concentrations of nitrate. These differences were at their lowest during winter.
3. The influence of gravel bar geomorphological heterogeneity (grain size and spatial distribution of fine sediments) on parafluvial inputs was studied in terms of nutrient content and biofilm characteristics along transects across the gravel bars.
4. A gravel bar located in a stable area of the river had low transversal heterogeneity and acted as a sink for DOC and as a source of nitrate. The low fine sediment content and the high oxygenation of interstitial water must have enabled aerobic processes such as mineralization of organic matter and nitrification to occur.
5. In contrast, gravel bars located in a degradation zone of the river had a spatially variable structure, acting as a sink for DOC and nitrate because localized accumulation of fine sediment and ensuing hypoxic conditions induced anaerobic processes such as denitrification.
6. This study highlights the important influence of geomorphological heterogeneity in gravel bars for nutrient transformations as well as nutrient exchanges between parafluvial interstitial systems and the adjacent river over a seasonal cycle.     

Influence of Hydrological Pulse on Bacterial Growth and DOC Uptake in a Clear-Water Amazonian Lake

This study was conducted to evaluate: (1) the bacterial growth and the dissolved organic carbon (DOC) uptake in an Amazonian lake (Lake Batata) at high-water and low-water periods of the flood pulse; (2) the influence of nitrogen and phosphorus (NP) additions on bacterial growth and DOC uptake in Lake Batata at two flood pulse periods; and (3) the bioavailability of the main DOC sources in Lake Batata. Lake Batata is a typical clear-water Amazonian lake, located in the watershed of Trombetas River, Central Amazon, Brazil. Bacterial batch cultures were set up with 90% 0.2-μm filtered water and 10% inoculum from Lake Batata. N-NH₄NO₃ and P-KH₂PO₄, with final concentrations of 50 and 5 μM, respectively, were added to the cultures, except for controls. Extra sources of DOC (e.g., algal lysate, plant leachates) were added to constitute six distinct treatments. Bacterial response was measured by maximum bacterial abundance and rates of bacterial production, respiration, DOC uptake, and bacterial growth efficiency (BGE). Bacterial growth and DOC uptake were higher in NP treatments than in controls, indicating a consistent nutrient limitation in Lake Batata. The composition of DOC also seems to be an important regulating factor of bacterial growth in Lake Batata. Seasonally, bacterial growth and DOC bioavailability were higher at low-water period, when the phytoplankton is a significant extra source of DOC, than at high-water period, when the forest is the main source of DOC. DOC bioavailability was better estimated based on the diversity and the diagenetic stage of carbon compounds than on single classes of labile compounds. Changes in BGE were better related to CNP stoichiometry in the water, and the “excess” of organic substrates was oxidized in catabolism, despite the quality of these compounds for bacterial growth. Finally, we conclude that bacterial growth and DOC uptake vary throughout the flood pulse in clear-water Amazonian ecosystems as a result of changes in nutrient concentration and in DOC composition.     

Dissolved organic carbon and its utilization in a riverine wetland ecosystem

Variations in dissolved organic carbon (DOC) concentrations of surface waters and subsurface interstitial groundwater of riparian and wetland soils to 1.2 m depth were evaluated in a riverine wetland ecosystem over one year. DOC was monitored at seven sites within the wetland pond, two sites on the inflow stream, and one site on the outflow stream. Surface concentrations in the inflow stream ranged from 0.74 to 11.6 mg C L^–1 and those of the outflow from 2.1 to 8.0 mg C L^–1 Average DOC from stream floodplain hydrosoils (3.1 to 32.1 mg C L^–1 was greater than DOC from the sediments below the stream channel (1.6 to 6.8 mg C L^–1 Surface DOC within the wetland varied seasonally, with greatest fluctuations in concentrations through the summer and autumn (range 4.8 to 32.6 mg C L^–1 ) during intensive macrophyte growth and bacterial production. DOC was less variable during the winter months (1.7 to 3.3 mg C L^–1 Within the wetland pond, average DOC concentrations (7.1 to 48.2 mg C L^–1) in the subsurface waters were significantly greater (p < 0.05) than average surface concentrations. The microbial availability of surface and subsurface DOC to bacteria was evaluated from losses of DOC by wetland bacteria grown on the DOC. Bacterial growth efficiencies ranged from 5 to 20% and were negatively correlated to the percentage of DOC removed by bacteria (r²=0.93). Throughout the ecosystem, DOC concentrations were greatest in the subsurface waters, but at most depths this DOC was a less suitable substrate than surface DOC for utilization by bacteria.     

Flow variability and longitudinal patterns in parafluvial water chemistry,aquatic invertebrates and microbial activity

相似文献   

Effects of the emergent macrophyte Juncus effusus L. on the chemical composition of interstitial water and bacterial productivity

Release of oxygen from the roots ofaquatic macrophytes into anaerobic sediments canaffect the quantity of interstitial dissolved organicmatter and nutrients that are available to bacteria. Nutrient and dissolved organic carbon (DOC)concentrations were compared between subsurface(interstitial) waters of unvegetated sediments andsediments among stands of the emergent herbaceousmacrophyte Juncus effusus L. in a lotic wetlandecosystem. Concentrations of inorganic nitrogen(NH₄ ⁺, NO₃ ^-, and NO₂ ^-)were greater from sediments of the unvegetatedcompared to the vegetated zone. DOC concentrations ofinterstitial waters were greater in sediments of theunvegetated zone both in the winter and springcompared to those from the vegetated zone. AlthoughDOC concentrations in hydrosoils collected from bothzones increased from winter to spring, bacterialproductivity per mg DOC in spring decreased comparedto winter. Greater initial bacterial productivityoccurred on DOM collected from the vegetated comparedto the unvegetated zone in winter samples (days 1 and4), with increased bacterial productivity on samplescollected from the unvegetated zone at the end of thestudy (day 20). Bacterial productivity wassignificantly greater on all sampling days on DOM fromvegetated samples compared to unvegetated samples. In nutrient enrichment experiments, bacterialproductivity was significantly increased (p < 0.05)with phosphorus but not nitrogen only amendments.     

Labile dissolved organic carbon and water temperature as regulators of heterotrophic bacterial activity and production in the lakes of Sub-Antarctic Marion Island

Summary The objectives of the 3 year study were to determine the relationship between bacterial numbers and phytoplankton standing crops (chlorophyll a) in sub-antarctic Marion Island lakes (33) and to determine the relative importance of labile dissolved organic carbon and water temperature as regulators of heterotrophic bacterial activity and production. Bacterial activity (the incorporation and respiration rates of ¹⁴C-labelled substrates) and production (the rate of [methyl-³H]thymidine incorporation into DNA) were measured in oligotrophic Lava Lake and Gentoo Lake, an elephant seal wallow. Samples were incubated under ambient conditions as well as at increased temperature and with additions of labile dissolved organic carbon (DOC). Bacterial numbers ranged from 2.13 × 10⁵ cell ml^–1 to 15.17 × 10⁶ cells ml^–1 in the lake survey. The chlorophyll range was 0.18 to >75 g 1^–1. Bacterial numbers were not correlated to chlorophyll concentration in waters where the chlorophyll content was 5 g 1^–1 but were correlated in waters with larger algal contents. Heterotrophic bacterial activity and production, which were similar to rates recorded for equivalent lower latitude systems, were higher in Gentoo Lake than in Lava Lake. As a result of qualitative and quantitative differences in the DOC pools, DOC was the stronger regulator of bacterial activity and production in Lava Lake, while temperature was the stronger factor in Gentoo Lake.     

Bacterial Contribution to Dissolved Organic Matter in Eutrophic Lake Kasumigaura,Japan

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter⁻¹ day⁻¹), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using d-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed.     

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among thehighest rates of atmospheric nitrogen deposition in the United States. Somestreams draining Catskill catchments have shown dramatic increases in nitrateconcentrations while others have maintained low nitrate concentrations. Streamsin which exchange occurs between surface and subsurface (i.e. hyporheic) watersare thought to be conducive to nitrate removal via microbial assimilationand/ordenitrification. Hyporheic exchange was documented in the Neversink River inthesouthern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate(NO₃ ^–) losses along hyporheic flowpaths werenegligible. In this study, Neversink River water was amended with natural,bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series ofexperimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamicswere examined before and throughout a three week BDOC amendment. In addition,bacterial production, dissolved oxygen demand, denitrification, and sixextracellular enzyme activities were measured to arrive at a mechanisticunderstanding of potential DOC and NO₃ ^– removalalong hyporheic flowpaths. There were marked declines in DOC and completeremoval of nitrate in the BDOC amended mesocosms. Independent approaches wereused to partition NO₃ ^– loss into two fractions:denitrification and assimilation. Microbial assimilation appears to be thepredominant process explaining N loss. These results suggest that variabilityinBDOC may contribute to temporal differences in NO₃ ^–export from streams in the Catskill Mountains.     

Microbiology and chemistry of acid lakes in Florida: I. Effects of drought and post-drought conditions

Short term changes in acid loading and dissolved organic carbon (DOC) content were studied in relation to water column bacteria of ten acid lakes on the Katharine Ordway Preserve, Florida. Five clear oligotrophic lakes and five dark dystrophic lakes were sampled during and after a drought period in July and September, 1985. Water column bacterial densities, light extinction, chlorophyll a, DOC, pH, dissolved oxygen, nutrients, and other chemical variables were measured. Significant positive correlations existed among DOC, chlorophyll a, pH, and water column bacterial densities during the drought period.There were no significant changes in water column bacterial densities or pH of clear lakes in the post-drought period, despite a 4.6 fold increase in acid loading from rainfall. A 3 fold increase of DOC, a decline in pH, and decreased bacterial densities in dark lakes suggested inhibition of bacteria by DOC and pH. A decrease in the relationship of DOC to bacterial numbers in all lakes was also noted. The correlations among DOC, chlorophyll a, and pH were no longer significant.Using data from both time periods significant polynomial regressions were observed between DOC and bacterial density and DOC and chlorophyll a. Maximum bacterial numbers occurred at 20 mg C 1^–1 of DOC. Above this bacterial numbers decreased also suggesting an inhibitory effect of DOC. Because pH was lower after DOC had increased in the dark lakes, the increase in acid conditions may have enhanced this inhibitory effect. The short term effects of DOC on the dark-lake bacteria greatly exceeded the influence of acid loading on clear-lake bacteria.     

Patch distribution of interstitial communities: prevailing factors

1. Community structure of interstitial invertebrates was studied in the sediments of two gravel bars in a bypass section of the Rhone River (France), in relation to hydrological patchiness and to major geomorphological characteristics of the sampling areas. Hydrological patchiness was characterized by upweiling or downwelling zones while geomorphological characteristics were concerned mainly with position towards the head or tail of each bar, which presented different environmental conditions. 2. Longitudinal profiles of the two bars, at 0.5 and 1.0m below the substrate surface, were compared. At each station 10-l samples were taken using mobile standpipes and a Bou—Rouch pump. During low-water conditions, replicate samples were taken in both the head-bar and tail-bar regions to confirm the results. Physicochemical parameters were used to differentiate surface water from ground water (conductivity, alkalinity, silica, nitrates, sulphates and temperature) and to detect hydrological patterns. 3. Two different situations were observed within the gravel bars. In the first one, the upweiling zone was situated in the tail bar and downwelling zone in the head bar. The reverse was observed in gravel bar 2 where the head-bar region formed the upweiling zone and the tail-bar region formed the downwelling zone. 4. In the first situation, the distribution of interstitial assemblages corresponded with the observed hydrological patterns: epigean fauna characterized the downwelling zone and stygofauna characterized the upweiling zone. The interstitial fauna seemed to be strictly linked to the mode of water movement within the sediments. Hydrological patchiness therefore appeared to be an important factor for the structure of interstitial assemblages. 5. In the second case there was no relationship between the hydrological and the biological pattern. Epigean fauna and hypogean fauna were scattered all along the gravel bar. These results suggest that geomorphology is a second important factor governing the composition and structure of interstitial assemblages. A precise structure was obtained when hydrological and geomorphological characteristics were superimposed (case 1). When they were opposed (case 2) all the species responded according to their individual ecological requirements.     

Longitudinal patterns of dissolved organic carbon concentration and suspended bacterial density along a blackwater river

Dissolved organic carbon (DOC) is the dominant form of carbon in transport in blackwater rivers, and bacteria are the major biological agents of its utilization. This study describes longitudinal patterns in DOC concentration and relates them to suspended bacterial populations in the channel. Concentrations of total DOC, three molecular weight fractions, and bacterial numbers were determined at 12 sites along the Ogeechee River in 1985–1986 and 1989 during periods of low and high discharge. Suspended bacterial populations were compared with DOC concentrations to determine if differences in bacterial abundance were related to longitudinal patterns of DOC concentration. Three distinct longitudinal patterns were observed: (1) The longitudinal pattern followed by both total and intermediate molecular weight DOC concentrations was a linear function of the geographic distance along the river. (2) During low flow conditions, there was a high degree of correspondence between patterns of bacterial numbers and low MW DOC (< 1000 apparent MW). (3) During periods of high discharge, the proportion of high (> 10,000) and intermediate (1000–10,000) MW fractions increased, and there was no longer a clear relationship between bacterial cells and low MW DOC.     

Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Alternation of Factors Affecting Bacterioplankton Abundance in the Danube River Floodplain (Kopački Rit, Croatia)

The purpose of this investigation was to determine the influence of different phases of the hydrological cycle on the abundance of colony-forming units (CFU) of bacteria in the Kopački Rit floodplain and to assess temporal and spatial variations in the proportion of eutrophic and oligotrophic bacteria (r- and K-strategists) as a biological indicator of dissolved organic carbon (DOC) availability. Determination of bacterial abundance was performed with abiotic variables (water temperature, depth, Secchi disc transparency, dissolved oxygen, pH, electrical conductivity, ammonia, nitrates+nitrites, total nitrogen and total phosphorus) and one biotic (chl-a) variable, and dimensionality reduced with principal component analysis (PCA). The component scores were used as independent variables in a multiple regression and the relationship between scores (indirect variables) and bacterial abundance was examined. An elevated bacterial abundance was observed during the limnophase (floodplain isolated from the river) and potamophase (flood). During the limnophase also the highest chl-a concentrations were found and in Sakadaš Lake during September 2003 significantly higher numbers of eutrophic than oligotrophic bacteria emerged (p = 0.026). During potamophase the opposite state was established. Elevated bacterial abundance occurred in parallel with minimal chl-a concentrations and during July 2004 in the Čonakut Channel a significantly higher number of oligotrophic than eutrophic bacteria emerged (p < 0.001). Differentiation in the number of eutrophic and oligotrophic bacteria suggests the existence of a higher concentration of labile DOC during September 2003 than during July 2004. Multiple regression analysis explains 13.6% of the variation in abundance of eutrophic bacteria. The regression model for oligotrophic bacteria is not significant. The results suggest two different mechanisms control bacterioplankton numbers to some extent in the Kopački Rit floodplain. In addition, two sources of DOC differentiating the quality of organic matter predominate under totally different hydrological regimes. Under these conditions, eutrophic bacteria may be partially bottom-up controlled, whereas it is not clear how the abundance of oligotrophs is controlled.     

Protist abundance and carbon concentration during a Phaeocystis-dominated bloom at an Antarctic coastal site

Summary Changes in the concentrations of bacteria, phytoplankton, protozoa, dissolved organic carbon (DOC), particulate organic carbon (POC), particulate carbohydrate (PCHO) and particulate organic nitrogen (PON) were followed throughout the summer at an Antarctic coastal site. The colonial prymnesiophyte Phaeocystis pouchetii was the first major phytoplankton species to bloom, reaching concentrations of 6 × 10⁷ cells · 1^–1 and remained numerically dominant for most of the summer. During the P. pouchetii bloom the concentration of most other autotrophs did not increase. Microheterotroph abundance peaked during or immediately after the Phaeocystis bloom. Their peak coincided with very high concentrations of organic carbon, particularly DOC which exceeded 100 mg · 1^–1, and low bacterial abundance. Maximum bacterial abundance was reached after the decline in microheterotroph numbers. Bacterial utilization of carbon substrates and microheterotroph grazing of bacteria and uptake of DOC may form an important link to higher trophic levels during Antarctic Phaeocystis blooms.     

Microbial heterotrophic utilization of dissolved organic matter in a piedmont stream

SUMMARY.

• 1 The microbial heterotrophic utilization of dissolved organic matter (DOM) was determined experimentally in microcosms using stream water and stream-bed sediments from a third order reach of White Clay Creek (Pennsylvania, U.S.A.).
• 2 Sources of DOM for the experiments included White Clay Creek water at baseflow and stormflow and cold water extracts of jewel weed (Impatiens capensis L.) and spicebush (Linderu henzoin (L.) Blume).
• 3 The heterotrophic activity of the sediments was measured as uptake of the following: dissolved organic carbon (DOC), molecular weight fractions within the DOC pool, carbohydrates, amino acids and peptides, phenolics, and dissolved oxygen (DO), all in the overlying water.
• 4 Concentrations of adenosine triphosphate (ATP), and direct microscopic counts of bacteria were used to estimate bacterial biomass in the surface sediments.
• 5 The microcosm experiments showed that specific DOC molecular size classes and DOM functional groups were selectively removed from solution, exposure to one DOM source affected responses to a different DOM source and certain DOM sources were more readily utilized than others.
• 6 Continued exposure to a DOM source increased microbial heterotrophic activity (a condition which persisted even after removal of the DOM source for several days).
• 7 Rates of biotic DOC uptake ranged from 3.6 to 242.8 mg Cm^-2h^-1.
• 8 Indirect estimates of biosynthesis calculated from DOC and DO data ranged from 1.6 at baseflow and 2.6–61.2 at stormflow to as high as 192.6 mg C m^-2 h^-1 when the community was repeatedly exposed to enriched DOM sources.
• 9 The mean generation times of bacteria in sediments, determined from direct microscopy data, ranged from 12.5 to 46.2 h at 15°C.

     

Seasonal variation in dissolved carbon concentrations and fluxes in the upper Purus River, southwestern Amazon

One of the less studied components of carbon cycling that could improve our understanding of how and how strongly Amazonian ecosystems act as sinks or sources of carbon is the amount that is carried downstream by rivers. In this paper, we show that a headwater river can carry from 25 to 130 % of the reported sink for Amazonian forests, therefore not being negligible for ecosystem-level carbon budgets. Based on monthly measurements from May 2004 to April 2005 of the upper Purus River, southwestern Amazonia, we found that: water pH, dissolved oxygen, specific electrical conductivity, and dissolved inorganic carbon (DIC) were inversely related to water discharge and precipitation; pCO₂ was directly and strongly related to discharge and precipitation, and to a lesser extent to pH and dissolved oxygen; and dissolved organic carbon (DOC) was not related to any measured variable. Annual flux of dissolved carbon (DIC + DOC) at the sampling site was estimated as 604 ± 55 Gg C a^?1. More than 75 % was in the form of bicarbonate, with the remainder as CO₂ and DOC. This amount is equivalent to 0.15 ± 0.01 Mg C ha^?1 a^?1 in the upstream drainage basin, which is on the same order of magnitude as terrestrial carbon fixation.     

Stimulation of aquatic bacterial activity by cyanobacteria

The time-course response of natural bacterial populations and isolates from lake water to various densities of the filamentous cyanobacteriaAphanizomenon flos-aquae andLyngbya birgei collected from the same lake is reported. The cyanobacteria were separated from the bacteria by dialysis membranes that allowed only dissolved cyanobacterial products to pass. Bacterial³H-thymidine incorporation and cell number were significantly (p<0.05) correlated with cyanobacterial density for both species. Estimated dissolved organic carbon (DOC) utilization, based on bacterial biomass changes over time, were usually significantly (p<0.01) correlated with cyanobacterial density and the decrease in bulk pool DOC for both species. Bacterial volume per cell increased significantly (p<0.05) in response to cyanobacterial density on day 5 of the experiments; cell volume remained unchanged on day 1. Bacterial cell numbers on outer surfaces of the tubular membrane containing the cyanobacteria (on the side exposed to the test bacteria) were significantly (p<0.01) correlated with cyanobacterial density. Statistical analysis inferred that bacteria closely associated with cyanobacteria (i.e. attached) responded more strongly to cyanobacterial products than free-living bacteria. Overall, our results indicate that cyanobacterial products have a potentially important role in regulating bacterioplankton productivity in aquatic systems.     

Bacterial Community of the Largest Oligosaline Lake,Namco on the Tibetan Plateau

Bacterial abundances and diversity in the surface water of Lake Namco, the largest oligosaline lake on the Tibetan Plateau, were examined using flow cytometry approach and constructing 16S rRNA gene clone libraries. Bacterial abundances were from 0.08 × 10⁶ to 1.6 × 10⁶ cells mL^?1, and were in the reported range of other lakes of the Tibetan Plateau and high mountain regions. Bacterial abundances were significantly correlated with the concentrations of chlorophyll a (chl a), but showed no significant relationship with the dissolved organic carbon (DOC), which suggested that the amount of DOC released by algae was the key factor determining the bacterial abundance rather than the total DOC. The total trace elements concentrations also obviously connected with bacterial abundances, and 9 of 20 elements showed significant relationship. Bacterial 16S rRNA gene clone sequences were affiliated to the α-, β-, γ-, δ-, and ?-Proteobacteria, Actinobacteria, Bacteroidetes, Cyanobacteria, Firmicutes, Acidobacteria, Planctomycetes, Verrucomicrobia, Candidate division OD1, or unclassified, and among these the β-Proteobacteria dominated. Bacteria in Lake Namco were most closely related to those retrieved from freshwater habitats. Relatively few sequences were closely related to those recovered from saline habitats. Eleven of 34 typical freshwater bacterial clusters were detected in the oligosaline Lake Namco. Bacterial diversity within the lake varied and was connected with the concentrations of DOC and chl a.