Complementary pathways of dissolved organic carbon removal pathways in clear-water Amazonian ecosystems: photochemical degradation and bacterial uptake

Dissolved organic carbon (DOC) photochemical reactions establish important links between DOC and planktonic bacteria. We hypothesize that seasonal changes in DOC quality, related to the flood pulse, drive the effects of light-DOC interactions on uptake by planktonic bacteria uptake in clear-water Amazonian ecosystems. Water samples from two ecosystems (one lake and one stream) were incubated in sunlight during different hydrological periods and were then exposed to bacterial degradation. Photochemical and bacterial degradation were driven by seasonal DOC inputs. Bacterial mineralization was the main degradation pathway of autochthonous DOC in the lake, while allochthonous DOC was more available for photochemical oxidation. We suggest that sunlight enhances the bacterial uptake of refractory DOC but does not alter uptake of labile forms. We also observed a positive relationship between sunlight and bacterial degradation of DOC, instead of competition. We conclude that photochemical reactions and bacteria complementarily degrade the different sources of DOC during the flood pulse in Amazonian clear-water aquatic ecosystems.     

Effect of Campsurus notatus on NH+4, DOC Fluxes,O2 Uptake and Bacterioplankton Production in Experimental Microcosms with Sediment‐Water Interface of an Amazonian Lake Impacted by Bauxite Tailings

The aim of this study was to evaluate the influence of Campusurus notatus Eaton 1868 (Ephemeroptera: Polimitarciydae) and the impact of bauxite tailings on ammonium (NH₄⁺) and dissolved organic carbon (DOC) fluxes, oxygen uptake and bacterioplankton production in the sediment‐water interface of Lake Batata, a shallow Amazonian floodplain lake. Mesocosms were constructed from natural and impacted areas of Lake Batata, to reproduce the sediment‐water interface. The cores were incubated with 0 to 2,388 ind m^–2 of Campsurus notatus nymphs, and the changes in NH₄⁺, DOC, O₂ concentration and bacterioplankton production in the overlying water column were measured. Ammonium efflux (F = 9.8, p < 0.05, multiple regression) and oxygen uptake (F = 11.8, p < 0.05) showed a significant correlation with the density of C. notatus in the cores with natural sediment. No differences on DOC release were observed in cores with natural or impacted sediment. In the cores incubated with natural sediment and nymphs of C. notatus, a significant increase (Two‐way ANOVA, p < 0.05) in bacterial production (0.44 μg C l^–1 h^–1) was observed after 3 hours of incubation. In cores incubated with sediment impacted by bauxite tailings, there was no difference in bacterial production with and without C. notatus. We conclude that C. notatus is an important bioturbator in Lake Batata, increasing the turnover rate of nitrogen (NH₄⁺) at the sediment‐water interface and bacterial production in cores incubated with natural sediment. It is also clear that bauxite tailings reduce the nutrients turnover rates in impacted regions of Lake Batata and influence bacterial production.     

Effects of Inorganic Turbidity on the Phytoplankton of an Amazonian Lake Impacted by Bauxite Tailings

The dumping of bauxite tailings into Batata Lake, an Amazonian clear-water lake, generated high levels of turbidity and caused a serious decrease in phytoplankton densities, which could possibly be the result of a photosynthetic limitation due to light attenuation together with an increase in algal sinking due to the adhesion of clay particles. This study aimed to investigate the sinking process through the addition of different suspended clay concentrations in columns containing Batata lake water. Since no effect of the suspended clays on Batata Lake phytoplankton sinking was observed, it was then evaluated, under laboratory conditions, whether the low conductivity of the Batata Lake water could interfere with the algae-clay aggregation process. Cultures of two algal species known to be capable to aggregate to Batata Lake suspended clays in algal culture medium: Staurodesmus convergens and Phormidium amoenum, were added to both the low conductivity Batata Lake water (14 μS cm^?1) and the high conductivity algal culture media (WC – 300 μS cm^?1 and Z8 – 560 μS cm^?1) together with Batata lake suspended clays. In both algal culture media and Batata lake water the two species had their sinking accelerated due to clay adhesion. It is thus suggested that the decrease in phytoplankton densities recorded in Batata Lake may not be related to an increase in phytoplankton loss rates due to algal-clay aggregation, but rather are a consequence of decreasing growth rates because of light attenuation.     

Bacterial Growth in Mixed Cultures on Dissolved Organic Carbon from Humic and Clear Waters

Interactions between bacterial assemblages and dissolved organic carbon (DOC) from different sources were investigated. Mixed batch cultures were set up with water from a humic and a clear-water lake by a 1:20 dilution of the bacterial assemblage (1.0 μm of prefiltered lake water) with natural medium (sterile filtered lake water) in all four possible combinations of the two waters and their bacterial assemblages. Bacterial numbers and biomass, DOC, thymidine incorporation, ATP, and uptake of glucose and phenol were followed in these cultures. Growth curves and exponential growth rates were similar in all cultures, regardless of inoculum or medium. However, bacterial biomass produced was double in cultures based on water from the humic lake. The fraction of DOC consumed by heterotrophic bacteria during growth was in the same range, 15 to 22% of the total DOC pool, in all cultures. Bacterial growth efficiency, calculated from bacterial biomass produced and DOC consumed, was in the order of 20%. Glucose uptake reached a peak during exponential growth in all cultures. Phenol uptake was insignificant in the cultures based on the clear-water medium, but occurred in humic medium cultures after exponential growth. The similarity in the carbon budgets of all cultures indicated that the source of the bacterial assemblage did not have a significant effect on the overall carbon flux. However, fluxes of specific organic compounds differed, as reflected by glucose and phenol uptake, depending on the nature of the DOC and the bacterial assemblage.     

Virus-Bacterium Coupling Driven by both Turbidity and Hydrodynamics in an Amazonian Floodplain Lake

The importance of viruses in aquatic ecosystem functioning has been widely described. However, few studies have examined tropical aquatic ecosystems. Here, we evaluated for the first time viruses and their relationship with other planktonic communities in an Amazonian freshwater ecosystem. Coupling between viruses and bacteria was studied, focusing both on hydrologic dynamics and anthropogenic forced turbidity in the system (Lake Batata). Samples were taken during four hydrologic seasons at both natural and impacted sites to count virus-like particles (VLP) and bacteria. In parallel, virus-infected bacteria were identified and quantified by transmission electron microscopy (TEM). Viral abundance ranged from 0.5 × 10⁷ ± 0.2 × 10⁷ VLP ml⁻¹ (high-water season, impacted site) to 1.7 × 10⁷ ± 0.4 × 10⁷ VLP ml⁻¹ (low-water season, natural site). These data were strongly correlated with the bacterial abundance (r² = 0.84; P < 0.05), which ranged from 1.0 × 10⁶ ± 0.5 × 10⁶ cells ml⁻¹ (high water, impacted site) to 3.4 × 10⁶ ± 0.7 × 10⁶ cells ml⁻¹ (low water, natural site). Moreover, the viral abundance was weakly correlated with chlorophyll a, suggesting that most viruses were bacteriophages. TEM quantitative analyses revealed that the frequency of visibly infected cells was 20%, with 10 ± 3 phages per cell section. In general, we found a low virus-bacterium ratio (<7). Both the close coupling between the viral and bacterial abundances and the low virus-bacterium ratio suggest that viral abundance tends to be driven by the reduction of hosts for viral infection. Our results demonstrate that viruses are controlled by biological substrates, whereas in addition to grazing, bacteria are regulated by physical processes caused by turbidity, which affect underwater light distribution and dissolved organic carbon availability.Viruses are the most abundant and dynamic components of the aquatic microbial community (6, 31, 32). Viruses influence many biogeochemical and ecological processes, including nutrient cycling, system respiration, particle-size distribution, bacterial and algal biodiversity, species distribution, algal blooms, and genetic transfer between microorganisms (21, 49). In addition, viruses play a major role in aquatic microbial food webs by releasing carbon trapped in host cells to the dissolved organic carbon (DOC) pool and ultimately back to the bacterial community (11, 21). The action of viruses is an important mechanism of bacterial regulation in aquatic ecosystems, acting directly on bacterial populations and indirectly on bacterial diversity by decreasing the density of dominant bacterial species (31). Studies based on viral decay rates and electron microscopy analyses have shown that viruses can cause up to 40% of bacterial mortality and more than 10% of phytoplankton mortality in aquatic systems (11, 22, 48, 50, 54, 55). It also has been suggested that viral lysis and protistan grazing cause similar bacterial mortality in aquatic ecosystems (22, 40).Several environmental factors, including solar radiation and temperature, can influence viral abundance. Exposure to solar radiation decreases viral abundance in aquatic ecosystems, while low temperatures decrease their virulence (33, 56). However, the majority of the studies on virus ecology have been performed in temperate or polar regions, where seasonal changes in solar radiation and water temperature are more pronounced (26, 30, 32). Viral abundances have been little investigated in tropical aquatic ecosystems (6, 39) and particularly in the Amazonian region, where the abundance and activity of aquatic viruses have not been studied.The greatest watershed in the world is located in the Amazonian region. It is composed of clear-water, black-water, and turbid freshwater ecosystems, which are seasonally influenced by the flood pulse. The hydrologic pulse is characterized by a pronounced change in water level, defining the flood seasons. Nutrient sources and stocks and species dynamics vary according to the water level (25). During the high-water season (flood season), the tight connection between terrestrial and aquatic environments results in an increase in allochthonous DOC input and the dilution of inorganic nutrients and organisms. During the low-water season, there is an increase in nutrient concentrations, organism abundances, and the importance of autochthonous DOC. These seasonal changes differently impact ecosystem functions and aquatic community dynamics (5, 9, 18). For instance, bacterioplankton abundance is less changeable than phytoplankton abundance throughout the hydrological cycle due to the alternative sources of DOC (allochthonous in the high-water period and autochthonous in the low-water period) for bacterial communities (5, 24).Lake Batata is a clear-water Amazonian floodplain lake located in the watershed of the Trombetas River, a tributary of the Amazon River. As a clear-water Amazonian ecosystem, it contains low concentrations of suspended particles and inorganic nutrients (47). Lake Batata is distinct because it was impacted by bauxite tailings for 10 years (1979 to 1989), affecting 30% of the lake''s area. The tailings caused a huge increase in turbidity; large amounts of tailings settled on the sediment surface and often are resuspended by physical mixing or biotic movements (28). The presence of tailings resulted in a clear spatial variation in the lake, forming impacted and natural sites. Furthermore, tailing particles can directly act as a substrate for attaching bacteria and also can adsorb DOC (5).Previous studies on bacterio-, phyto-, and zooplankton communities have shown that flood pulse acts as the primary driver of plankton community structure in Lake Batata (5, 9, 24). Bauxite tailings also affect microbial processes in impacted sites, such as bacterial growth and production (5), photosynthesis rates and primary production (43), or the availability of food for zooplankton (8). However, there still is no evidence indicating a complementary (flood pulse and forced turbidity) effect among these factors in any microbial community. Based on published data, we assume that (i) bacterioplankton abundance is less changeable through the hydrologic cycle than phytoplankton abundance (5, 24), and (ii) tailing particles can act as a substrate for attaching bacteria and also can adsorb organic matter, which is controlled by the flood pulse (5). Therefore, we hypothesized that the relationship between viruses and bacteria in Lake Batata is modulated by a synergistic effect between the hydrological cycle and turbidity.     

Flood pulse influence and anthropic impact on the chemical composition and energy content of Oryza glumaepatula in an Amazonian lake.

The aim of this research was to study the flood pulse influence and the anthropic impact caused by bauxite tailings on the chemical composition of O. glumaepatula in Batata lake (PA, Brazil). Sampling was carried out in stands of O. glumaepatula in the low-water, filling, high-water, and drawdown periods in impacted and natural areas of Batata lake. During the low-water and drawdown periods the stands of O. glumaepatula were exposed, and in the filling and high-water periods the water depth was respectively 1.4 and 3.8 m. The collected material was dried at 70 degrees C, ground, and concentrations of total phosphorus, total nitrogen, organic carbon, and energy content were determined. The results indicate that the biomass increase, caused by the rise in water level, has a dilution effect on nitrogen and phosphorus concentrations in O. glumaepatula. The energy contents did not present significant differences in any of the studied periods. The results suggest that from the low water to filling period, nitrogen becomes more limiting to O. glumaepatula in the impacted area, whereas phosphorus becomes more limiting in the natural area. The population of O. glumaepatula contributes to the recovery of the impacted area of Batata lake as the detritus from this species accumulates over the sediment. This accumulation impedes future re-suspension of the bauxite tailings and increases the organic matter and nutrient concentrations in the impacted sediment.     

Influence of the Hydrological Cycle on the Bacterioplankton of an Impacted Clear Water Amazonian Lake

Abstract Free-living and attached bacterial population sizes were determined fortnightly from December 1991 to December 1992 in natural and disturbed areas of an Amazonian clear water lake (Batata Lake, Pará, Brazil) impacted by bauxite tailings. The bacterioplankton showed distinct patterns during different phases of the hydrological cycle. Total bacterial population size and rates of thymidine incorporation (measured during high and low water phases) were high during low water, with values ranging from 3.3 × 10⁵ to 1.1 × 10⁶ cells ml⁻¹, and from 0.28 to 4.01 μg C l⁻¹ h⁻¹, respectively. The population size of free-living bacteria was larger at the natural station, while no differences were observed between attached bacterial populations at both stations. However, production and turnover rate of attached bacteria were high at the disturbed area. During low water, bacterial growth appeared to be driven mainly by the input of dissolved organic carbon (DOC) from phytoplankton origin. During high water, bacterial abundance was reduced, probably as the result of dilution and the input of less labile DOC from floodplains. The presence of bauxite tailings seems to influence bacterial dynamics in an indirect way, probably due to shading of phytoplankton cells and, hence, reducing the DOC supply for bacterial growth. This study, the first on the microbial ecology of an Amazonian clear water lake, demonstrated that water level variations exert a strong influence on the bacterioplankton dynamics. Received: 9 January 1996; Accepted 6 November 1996     

Phytoplankton–bacterioplankton interactions in a neotropical floodplain lake (Laguna Bufeos,Bolivia)

Laguna Bufeos is a floodplain lake of the river Ichilo, a tributary of the Amazon basin situated in Bolivia. Nutrient addition assays involving whole water (<200 μm) as well as fractionated water (<0.8 μm) treatments were carried out in incubation tubes to test whether bacterial growth is limited by the availability of inorganic nutrients and to test whether bacteria are able to utilize inorganic nutrients directly or are stimulated by inorganic nutrients through increased production of phytoplankton. The assays were carried out during two extreme hydrological conditions, the high-water and the low-water period. During the high-water period experiment, neither N or P limited bacterial growth rates. During the low-water period, bacterial growth was P limited. Bacterial growth was stimulated in the fractionated as well as in the whole water treatments, indicating that bacterial growth was directly stimulated by P. Bacterial growth corrected for grazing losses (determined by means of dilution experiments) was significantly higher in the fractionated water containing only bacteria when compared to the whole water containing also grazers and phytoplankton. This suggests that bacterial growth was suppressed by competition with phytoplankton rather than stimulated through the production of dissolved organic matter by phytoplankton.     

Effect of exposure to sunlight and phosphorus-limitation on bacterial degradation of coloured dissolved organic matter (CDOM) in freshwater

This study reports on the interacting effect of photochemical conditioning of dissolved organic matter and inorganic phosphorus on the metabolic activity of bacteria in freshwater. Batch cultures with lake-water bacteria and dissolved organic carbon (DOC) extracted from a humic boreal river were arranged in an experimental matrix of three levels of exposure to simulated sunlight and three levels of phosphorus concentration. We measured an increase in bacterial biomass, a decrease in DOC and bacterial respiration as CO(2) production and O(2) consumption over 450 h. These measurements were used to calculate bacterial growth efficiency (BGE). Bacterial degradation of DOC increased with increasing exposure to simulated sunlight and availability of phosphorus and no detectable growth occurred on DOC that was not pre-exposed to simulated sunlight. The outcome of photochemical degradation of DOC changed with increasing availability of phosphorus, resulting in an increase in BGE from about 5% to 30%. Thus, the availability of phosphorus has major implications for the quantitative transfer of carbon in microbial food webs.     

Potential denitrification in submerged natural and impacted sediments of Lake Batata,an Amazonian lake

Lake Batata is one of many clear water lakes located on the floodplain of the Trombetas River in the northern Brazilian Amazon. Lake Batata is distinctly different from other lakes of this region because, for a period of 10 years, its waters received tremendous amounts of aluminum ore tailings from a bauxite mining operation. Approximately 30% of the sediments of the upper basin of the 2100 hectare lake were covered by tailings before dumping was curtailed. The goal of this research was to identify factors controlling denitrification in the natural and impacted sediments of Lake Batata. Rates of denitrification in sediments were estimated in the laboratory by the acetylene blockage method. Denitrification was measured under four conditions: without amendment; amended with glucose; amended with nitrate; and amended with glucose and nitrate. Denitrification was observed only in assays amended with nitrate suggesting that availability of nitrate is a principle factor for controlling denitrification in the sediments of Lake Batata. Effects of nitrate amendments are most pronounced when the water level is low, i.e. during the hydroperiods of draw-down and low-water.     

Bacterioplankton Growth and Nutrient Use Efficiencies Under Variable Organic Carbon and Inorganic Phosphorus Ratios

We carried out enclosure experiments in an unproductive lake in northern Sweden and studied the effects of enrichment with different dissolved organic carbon (glucose)/inorganic phosphorous (DOC/Pi) ratios on bacterioplankton production (BP), growth efficiency (BGE), nutrient use efficiency (BNUE), growth rate, and specific respiration. We found considerable variation in BP, BGE, and BNUE along the tested DOC/Pi gradient. BGE varied between 0.87 and 0.24, with the highest values at low DOC/Pi ratios. BNUE varied between 40 and 9 g C g P⁻¹, with high values at high DOC/Pi ratios. More DOC was thus allocated to growth when bacteria tended to be C-limited, and to respiration when bacteria were P-limited. Specific respiration was positively correlated with bacterial growth rate throughout the gradient. It is therefore possible that respiration was used to support growth in P-limited bacteria. The results indicated that BP can be limited by Pi when BNUE is at its maximum, by organic C when BGE is at its maximum, and by dual organic C and Pi limitation when BNUE and BGE have suboptimal values.     

Responses of the fish community to the flood pulse and siltation in a floodplain lake of the Trombetas River,Brazil

The distributions of the fish species were examined in relation to environmental variables, to evaluate the effects of environmental degradation on the fish community of Batata Lake, a typical Amazonian clearwater lake. From 1979 to 1989, tailings composed of water and clay, extracted from bauxite by water jets, were discharged into Batata Lake. The tailings spread into about 30% of the lake’s area, where the level of the lake bottom rose and turbidity increased. In the present study, multivariate analyses were performed on data for environmental parameters and fish density and biomass. Fish were collected with gillnets during the annual hydrological cycle (filling, flood, drawdown and dry periods), in the silted area, the partly silted area (intermediate) and the natural area. Values of the Shannon index, density and biomass were compared among areas and periods to evaluate the effects of the tailings on community structure. Sediment resuspension, which reduces transparency, is accentuated in shallow water, and was the main factor regulating differences in the community structure between the natural and silted areas. The decrease in transparency occurs mainly during the filling period in the silted area and during the low-water period in the silted and intermediate areas, when sediment resuspension increases concentrations of nutrients and chlorophyll-a. The strong influence of migratory and piscivore species in low-transparency waters is likely associated with the greater bacterioplankton productivity and turnover rate observed by other authors in the silted area, increasing the importance of the heterotrophic food chain in Batata Lake. Reduction of transparency in the silted area was a selective factor for fish species. The death of part of the flooded forest vegetation was decisive in lowering densities of the igapó-associated species in the silted area. The unconsolidated substrate, the death of part of the igapó forest and the negative effects of low transparency – inhibiting resident visually oriented species in the affected areas – are the main factors causing the low diversity in the silted area as a whole. The correlations between CPUEs, conductivity and nutrients and chlorophyll-a concentrations do not appear to reflect cause–effect relationships, indicating that these environmental parameters are poor predictors of fish density in Batata Lake.     

Low bacterial growth efficiency in the oligotrophic eastern Mediterranean Sea: a modelling analysis

Bacterial growth efficiency (BGE) is generally related to thetrophic status of marine systems, with values as low as 0.15or less associated with the most nutrient-depleted areas. Asimple steady-state model is used to examine whether the observedratio of bacterial to primary production (BP:PP) of 0.22 inthe eastern Mediterranean Sea, an ultra-oligotrophic system,can be sustained given a BGE of 0.15. There is no a priori reasonto believe that BGE should be higher in this area relative toother open ocean environments, although accurate measurementsare required to investigate this possibility. The model includesprimary production [both particulate and exudation of dissolvedmaterial expressed as a percentage of extracellular release(PER)] and terriginous dissolved organic carbon (DOC) as sources,cycling via zooplankton, viruses and bacteria, and bacterialand zooplanktonrespiration as sinks for carbon. Results indicatethat a BP:PP of 0.22 cannot be maintained if bacterial carbondemand is met solely with DOC of autochthonous origin, givena BGE of 0.15. Sufficient carbon is, however, supplied to maintainthis ratio if a high phytoplankton exudation of DOC (PER = 40%)is permitted, in conjunction with a BGE of 0.16. A BGE of 0.28is required if PER takes on a more typical value of 10%. Thepossibility of BP being significantly enhanced by DOC of terrestrialorigin is discounted, there being no major riverine sourcesnear the Cretan Sea. Our analysis emphasizes the need for accurateestimates of BGE, and an improved understanding of sources andsinks of organic carbon, in marine systems.     

Short-term effects of phosphorus addition and pH rise on bacterial utilization and biodegradation of dissolved organic carbon (DOC) from boreal mires

Natural mires and forested peatlands are known to be very significant sources of dissolved organic carbon (DOC) to aquatic ecosystems. Peatland management operations (e.g., forestry operations, restoration of drained mires and peat mining) and extreme hydrological events may increase the DOC runoff. We hypothesized that an increase in phosphorus (P) leaching, together with near-neutral conditions in recipient lakes will accelerate decomposition of DOC that originates from acidic, nutrient-poor mire waters. The efficiency of DOC utilization was evaluated by measuring microbial respiration and bacterial production (BP) in short-term laboratory experiments with runoff waters from six boreal mire sites. Mere inorganic phosphorus (PO₄-P) addition did not affect the rate of respiration or the proportion of decayed DOC. However, in the nutrient-poor bog waters, P addition slightly promoted BP and bacterial growth efficiency (BGE). In contrast, the elevation of pH alone, and the elevation of pH and PO₄-P level together, caused a significant increase in respiration and in the proportion of decayed DOC, but did not affect net BP. Elevated pH alone, however, depressed BGE when compared to that under the combined elevation of pH and PO₄-P. These results suggest that the increased P availability, e.g., after mire restoration, would slightly benefit bacterial net growth in P-limited waters. However, in near-neutral recipient lakes, the increased microbial decomposition of mire-originated DOC contributes more to carbon dioxide (CO₂) supersaturation than potentially supporting detritus-based food chains.     

Phosphorus and DOC availability influence the partitioning between bacterioplankton production and respiration in tidal marsh ecosystems

The organic carbon consumed by aquatic bacteria (BCC) is partitioned between bacterial production (BP) and respiration (BR), but the factors that determine BCC and its partition into BP and BR are not well understood. We explored the coupling between BR, BR and BCC, and their links to dissolved organic carbon (DOC) and nutrient availability in natural and restored tidal marshes and in the adjoining waters of Delaware Bay estuary. Labile DOC (LDOC) ranged from 3% to 22% of the DOC pool, and explained more of the variance in both BR and BCC than did bulk DOC. Bacterial growth efficiency (BGE) was highly variable (0.09-0.58), and natural Spartina alterniflora marshes had consistently higher BGE than both restoration marshes and tidal floodwaters. BGE was negatively related to the ratio of LDOC to total dissolved phosphorous, which was highest in natural marshes. The enhancement of BP observed in the marshes relative to the estuarine floodwaters had different origins: In natural marshes it was mostly due to increases in BGE, whereas in restored marshes it followed increased BCC. These results highlight the importance of P in regulating microbial metabolism in coastal areas, and the need to understand the pathways that lead to BP in these systems.     

Nutrient limitation of bacterial production in clear water Amazonian ecosystems

The aim of this research was to determine the main limiting nutrient (carbon, nitrogen or phosphorus) to bacterial production in different clear water Amazonian ecosystems during the high water period, when there is influence of the flooded land, mainly as sources of organic matter. Five stations were sampled in three clear water ecosystems: Trombetas River, Lake Batata and Caranã Stream. We estimated in each station the nutrient concentration, bacterial production and bacterial abundance. The experiment was set up with GF/F filtered water from all stations together with additions of glucose (400 M C), KNO₃ (15 M N) and KH₂PO₄ (5 M P) in accordance with each treatment (C, N, P ,CN, CP, NP, CNP and no amends). Bacterial production was estimated after 24 h of incubation. We observed that the values of bacterial production after additions of phosphate alone (P treatment) were 2- to 6-fold greater than the values measured in control flasks. Additions of nitrate (N treatment) and glucose alone (C treatment) had no effect on the bacterial production in four out of five ecosystems studied. However, additions of glucose with phosphate (CP treatment) strongly stimulated bacterial production in all ecosystems studied, including treatments with phosphate addition only. We conclude that phosphorus is the main limiting nutrient to bacterioplankton production in these clear water Amazonian ecosystems during the high water period. In addition, we conclude that, together with phosphorus, additions of glucose stimulated the bacterial production mainly due to the low quality of the carbon pool present in these ecosystems.     

长江口北支水域浮游动物的研究

依据2003年7月(丰水期)和2004年1月(枯水期)对长江口北支水域环境调查资料,研究了长江口北支水域浮游动物分布变化成国.结果表明,丰水期浮游动物生物量较高,平均为234.38 mg·m^-3,涨、落潮时分别为141.3和327.40mg·m^-3;枯水期为188.81 mg·m^-3,涨、落潮时变化不大,分别为184.69和192.93 mg·m^-3;涨潮时长江口北支西侧水域生物量高于东侧,落潮时则相反;潮汐对种类数的影响不明显;多样性指数(H')值涨潮大于落潮.丰水期涨潮时北岸生物量高于南岸,落潮时相反,而枯水期两岸差别不如丰水期明显.长江口北支浮游动物分布的变化与长江口门外浮游动物数量季节变动和潮流携带有关,与长江径流关系不明显.柯氏力通过潮夕作用,是长江口北支南北两岸生物量差异形成的重要原因.     

Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m² d⁻¹) and accounted for 20%–53% of CFTB and 11%–33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5×) and dissolved P (10×) relative to lake water, peaked in the winter–spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%–20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Received 11 June 2001; Accepted 14 December 2001.     

Pelagic carbon metabolism in a eutrophic lake during a clear-water phase

Dissolved and paniculate organic carbon (DOC and POC, respectively),primary production, bacterial production, bacterial carbon demandand community grazing were measured for 9 weeks in eutrophicFrederiksborg Slotssø. The period covered the declineof the spring bloom, a clear-water phase and a summer phasewith increasing phytoplankton biomass. The process rates andchanges in pools of organic carbon were combined in a carbonbudget for the epilimnion. The POC budget showed a close balancefor both the post-spring bloom and the clear-water phase, whilea surplus was found in the summer phase. Production of POC wasdominated by phytoplankton (2/3) compared to bacteria (1/3)during all phases, and there was a significant correlation betweenphytoplankton and bacterial production rates (r² = 0.48, P <0.039). Bacterial demand for DOC was balanced by productionand changes in the pool of DOC during the decline of the springbloom, but the calculated demand exceeded the supply by 81 and167%, respectively, during the other two periods. The discrepancywas most probably due to an underestimation of bacterial growthefficiency and an overestimation of in situ bacterial productionin carbon units. Production of bacterial substrate by zooplanktonactivity was estimated to be higher than the direct excretionof organic carbon from phytoplankton. The biological successionwas regulated by the balance between area primary productionand community grazing. The clear-water phase was initiated bya combination of low primary production due to low surface irradianceand high community grazing (100 mmol C m^–2 day^–1),which caused a decrease in phytoplankton biomass. However, dueto the high initial phytoplankton biomass, community grazingwas not high enough to cause a significant decrease in areaprimary production. The summer phase was initiated by a decreasein community grazing followed by an increase in phytoplanktonbiomass. Based on these observations and calculations of areaprimary production as a function of chlorophyll concentrations,we suggest that the possibility for zooplankton to regulatephytoplankton biomass in temperate lakes decreases with increasingnutrient level.     

Structure and dynamics of stream fish communities in the flood zone of the lower Purus River,Amazonas State,Brazil

The structure and dynamics of fish communities were characterized by richness, abundance, diversity and stability, during high-water and low-water periods. These analyses were performed on data from the flood zone of four streams in the lower Purus River, in the Brazilian Amazon. A total of 188 species of fish were collected, distributed among 29 families and eight orders. The statistical test showed a difference in community diversity between periods. The high-water period showed higher evenness in comparison to the low-water season. The low-water period was marked by high species abundance. A great variation in community composition between the flood and low-water periods was encountered. The adjustments for species abundance models suggested that stochastic events structure the communities. Most of the species showed a temporal variation of abundance indicating low community stability. Changes in the physico-chemical conditions of the water caused by the seasonal hydrological regime may be influencing the structuring of the fish communities.