Availability of dissolved organic carbon for planktonic bacteria in oligotrophic lakes of differing humic content

Bacterioplankton from 10 oligotrophic lakes, representing a gradient from clearwater to polyhumic, were grown in dilution cultures of sterile filtered lake water. The bacterial biomass achieved in the stationary phase of the dilution cultures was positively correlated with the amount of both humic matter and dissolved organic carbon (DOC) in the lakes. About the same fraction of the total DOC pool was consumed in the dilution cultures of all lakes (average 9.5%, coefficient of variation (CV) 24%), with approximately the same growth efficiency (average 26%, CV 28%). Thus, humic lakes could support a higher bacterial biomass than clearwater lakes due to their larger DOC pools. The relevance of the results to planktonic food webs of humic and clearwater lakes is discussed.     

Effects of Additions of DOC on Pelagic Biota in a Clearwater System: Results from a Whole Lake Experiment in Northern Sweden

An oligotrophic clearwater lake, initially characterized by a pronounced dominance of autotrophic phytoplankton and mostly by one species, the green alga Botryococcus, was subject to additions of dissolved organic carbon in the form of white sugar (sucrose) during two consecutive years. The hypothesis tested was that it is organic carbon per se, and not other possible effects of humic substances, that determines the differences in structure of the planktonic ecosystem between humic and clearwater lakes. The additions of DOC resulted in a significant increase in bacterial biomass and a decrease in the biomass of autotrophic phytoplankton. The biomass of mixotrophic and heterotrophic flagellates instead increased significantly, whereas no effects were found to propagate to higher trophic levels. As a result of the changes among biota, total planktonic biomass also decreased to a level typical of nearby humic lakes. We suggest that it is the carbon component of humic material and its utilization by bacterioplankton that determines the structure and function of the pelagic food web in humic lakes.     

Accounting for littoral primary production by periphyton shifts a highly humic boreal lake towards net autotrophy

相似文献   

Does ionic strength affect the configuration of aquatic humic substances,as indicated by gel filtration?

SUMMARY. 1. The effect of ionic strength on the configuration of aquatic humic substances was studied by gel filtration and dialysis of water from small. Finnish forest lakes of varying colour.
2. Sephadex gel filtration of water from the most humic lake gave similar elution profiles of UV absorbance and dissolved organic carbon (DOC). Gel filtration of unconcentrated samples from all three lakes under natural conditions of ionic strength (I = 1.7 × 10⁻⁴) and pH (5.5–6.0) gave similar fractionation of humic substances, despite their widely differing colour (30–350 mg Pt l⁻¹) and DOC (5–25mg C l⁻¹).
3. Increasing the ionic strength by two orders of magnitude caused considerable retardation on the Sephadex columns of the humic substances, suggesting a decrease in their molecular size and/or shape.
4. Dialysis experiments strongly indicated that ionic strength-induced changes in the configuration of the aquatic humic substances are indeed real. Hence it is probable that the elution behaviour of aquatic humic substances on Sephadex gels has previously been wrongly attributed to ionic strength-dependent interactions between the gel and the humic substances.     

Dissolved organic carbon in a humic lake: effects on bacterial production and respiration

Allochthonous matter was the main source of carbon for pelagic bacteria in a humic lake, accounting for almost 90% of the carbon required to support observed bacterial growth. The estimated contribution from zooplankton excretion was of the same magnitude as direct phytoplankton release, both accounting for 5–7% of bacterial demands for dissolved carbon. Bacteria were an important source of carbon both for heterotrophic phytoplankton and for filter feeding zooplankton species, further stressing the role of humus DOC in overall lake productivity. The high contribution of allochthonous DOC implies a stoichiometry of dissolved nutrients with a surplus of C relative to P. The high P cell quota of bacteria suggest that under such conditions they are P-limited and act like net consumers of P. Excess C will be disposed of, and bacterial respiration rate will increase following a transition from carbon-limited bacterial growth towards mineral-nutrient-limited growth. Thus the high community respiration and frequent CO₂-supersaturation in humic lakes may be caused not only by the absolute supply of organic C, but also by the stoichiometry of the dissolved nutrient pool.     

Humic substance mineralization in a tropical oxbow lake (São Paulo,Brazil)

We evaluated the mineralization rates of humic substances in Infernão oxbow lake (State of São Paulo, Brazil). Experiments were conducted under aerobic and anaerobic conditions using fulvic acid and humic acid from four sources: Scirpus cubensis and Cabomba piauhyensis leachate submitted to a 120-day degradation process, sediment, and dissolved organic matter from the lake water. A fixed amount of substrate was added to 450 ml of water from Infernão lake, filtered over glass wool. After adding substrate, the flasks were incubated at 21.0°C under aerobic and anaerobic conditions. The dissolved organic carbon was monitored during 95 days. The results were fitted to first-order kinetics model, which pointed to one labile and one refractory fraction. The refractory fractions predominated, ranging from 71.4 to 84.3% for fulvic acid and from 73.4 to 85.0% for humic acid. Mineralization rates of the labile fractions of dissolved organic carbon were higher under aerobic than anaerobic conditions, while the converse was true for the refractory fractions.     

Sources,concentrations and characteristics of organic matter in softwater lakes and streams of the Swedish forest region

18 Swedish forest lakes covering a wide range of dystrophy were studied in order to quantify and characterize the organic matter in the water with respect to origin (allochthonous or autochthonous), physical state (particulate or dissolved) and phosphorus content. Samples were collected repeatedly during a two-year period with unusually variable hydrological conditions. Water from three different depths and from tributaries was analysed with standard monitoring methods, including water colour, Secchi disk transparency, total organic carbon (TOC), COD_Cr, COD_Mn, total phosphorus and molybdate reactive phosphorus. Interrelationships were used to compare different methods and to assess the concentration and composition of organic matter. It is estimated that in remote softwater lakes of the Swedish forest region, autochthonous carbon is typically < 5 g m⁻³. Most lakes in this region receive significant amounts of humic matter originating from coniferous forest soils or peatland in the catchment area. In most humic lakes with a water colour of ≥ 50 g Pt m⁻³, more than half of the organic carbon in the surface water is of allochthonous origin, and in polyhumic lakes (> 200 g Pt m⁻³) the proportion can exceed 90%. Secchi depth readings were related similarly to organic matter from both sources and provided good estimates of TOC with a single optical measurement. Water colour was used to distinguish allochthonous and autochthonous matter. High concentrations of phosphorus were found in humic waters, most of it being molybdate reactive, and probably associated with humic matter rather than as dissolved free inorganic forms. COD_Mn yielded only 25–60% of TOC and appears to include mainly truly dissolved substances of low molecular weight.     

Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolved organic carbon (DOC) between 15 and 20 mg L^–1 were fertilized with inorganic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake was unfertilized and served as a control. In addition to this lake fertilization experiment, data from different regional surveys were used to assess the role of different limiting factors.
2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inorganic nutrient limitation of bacterioplankton was a function of DOC concentration in water of the investigated region and nutrient-limited bacteria were found only in lakes with DOC concentrations less than around 15 mg L^–1
3. The fertilization experiments demonstrated that the DOC-rich experimental lakes contained a bioavailable pool of P that was not utilized to its full potential under natural conditions. The overall mobilization of energy (bacterioplankton plus phytoplankton) in the experimental lakes was restricted by lack of inorganic N.     

Browning affects pelagic productivity in northern lakes by surface water warming and carbon fertilization

Global change impacts important environmental drivers for pelagic gross primary production (GPP) in northern lakes, such as temperature, light, nutrient, and inorganic carbon availability. Separate and/or synergistic impacts of these environmental drivers on pelagic GPP remain largely unresolved. Here, we assess key drivers of pelagic GPP by combining detailed depth profiles of summer pelagic GPP with environmental and climatic data across 45 small and shallow lakes across northern Sweden (20 boreal, 6 subarctic, and 19 arctic lakes). We found that across lakes summer pelagic GPP was strongest associated with lake water temperatures, lake carbon dioxide (CO₂) concentrations impacted by lake water pH, and further moderated by dissolved organic carbon (DOC) concentrations influencing light and nutrient conditions. We further used this dataset to assess the extent of additional DOC-induced warming of epilimnia (here named internal warming), which was especially pronounced in shallow lakes (decreasing 0.96°C for every decreasing m in average lake depth) and increased with higher concentrations of DOC. Additionally, the total pools and relative proportion of dissolved inorganic carbon and DOC, further influenced pelagic GPP with drivers differing slightly among the boreal, subarctic and Arctic biomes. Our study provides novel insights in that global change affects pelagic GPP in northern lakes not only by modifying the organic carbon cycle and light and nutrient conditions, but also through modifications of inorganic carbon supply and temperature. Considering the large-scale impacts and similarities of global warming, browning and recovery from acidification of lakes at higher latitudes throughout the northern hemisphere, these changes are likely to operate on a global scale.     

Phosphorus transformations in the epilimnion of humic lakes: biological uptake of phosphate

SUMMARY. 1. The hypothesis that dissolved humic material (DHM) stimulates bacterial involvement in phosphorus transformations and may thus lead to decreased accessibility of phosphorus to algae was investigated by studying three small forest lakes in southern Finland representing a wide range of concentrations of DHM. 2. Other chemical differences between the three lakes were slight, although the most humic lake exhibited higher concentrations of total phosphorus and of molybdate-reactive phosphorus. Bacterial biomass did not differ significantly between the lakes, but algal biomass was significantly lower at higher DHM concentrations. Consequently the ratio of algal biomass to bacterial biomass was significantly lower in the most humic lake. 3. Uptake of phosphorus from added ³³PO₄ was partitioned between algal and bacterial size fractions by differential filtration. No significant variation between lakes was found in the proportion of particulate ³³P recovered from the algal fraction. 4. Turnover times for phosphate were significantly longer in the most humic lake and also showed lower variability. In general turnover times were long in comparison with values reported from many other lakes. Only briefly in mid summmer did turnover times in two of the lakes shorten to values which would indicate that demand for phosphate was outstripping supply. 5. Short-term storage of samples from the most humic lake stimulated biological incorporation of ³³P, but additions of nitrogen and iron had little effect on phosphate uptake. 6. In these small forest lakes it is probable that no single nutrient consistently limits plankton development. Since no evidence was found that DHM shifts the balance of plankton phosphate uptake away from algae towards bacteria, the influence of DHM on phosphorus transformations may rather be through chemical regulation of free phosphate availability.     

Late Holocene changes in the humic state of a boreal lake and their associations with organic matter transport and climate dynamics

An important factor in the ontogeny of boreal lakes is the development of their humic state through terrigenous input of organic matter (carbon) that affects strongly the functioning and structure of these ecosystems. The long-term dynamics and role of humic substances for these systems in relation to climate are not clear. In this study, a boreal lake from southern Finland was investigated using paleolimnological methods, including diatom, chironomid and geochemical analyses, for Late Holocene changes in the humic state. The aim was to examine the relationship between sediment biogeochemistry and climate variation. Consistent trends were found in diatom-inferred total lake-water organic carbon (TOC) and in the ratio of humic/oligohumic chironomids. Sediment geochemistry provided further evidence for the limnological development of the lake and related long-term climate trends in the region. The results indicated three distinct phases with differing humic state; the beginning of the record at ca. 4,500 cal year BP was characterized by extremely humic conditions coinciding with warm and dry climatic conditions, a meso-oligohumic period between ca. 3,000–500 cal year BP with increasing allochthonous organic matter transport and cooler and wetter climate, and recent period with polyhumic (TOC >10 mg L^?1) lake status and warming climate. This study shows that instead of straightforward linear development, boreal lakes evolve through dynamic humic stages related to climate and lake-catchment coupling processes. As the changes in the humic state are ultimately climate-driven, the ongoing climate change probably has a major influence on boreal lakes through direct and indirect effects on organic carbon transport, utilization and accumulation.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

On the importance of dissolved organic matter in the nutrition of zooplankton in some lake waters

Summary A steady state, radiotracer technique was used to study the original source of the carbon in zooplankton. The experiments were started in filtered lake water with added inorganic radiocarbon. At the beginning of the experiments, a proportionally insignificant volume of unfiltered water was introduced into the culture, together with some ovigerous zooplankton individuals. Since the radioactivity: carbon ratio in the dissolved inorganic carbon was kept constant, a similar ratio would be expected to develop in the autotrophic phytoplankton. The same ratio would then be expected to develop in the zooplankton, if its sole carbon source was autotrophic phytoplankton.According to the results of this approach dissolved organic matter seems to be an important food resource for zooplankton, particularly in highly humic lakes. This conclusion was confirmed by the finding that zooplankton from these lakes was able to grow and reproduce in experiments started with filtered lake water and conducted in complete darkness.The development of algae was followed over the course of one experiment in highly humic water. The same micro-flagellates reproduced equally well in both light and darkness, which indicates the importance of heterotrophic metabolism in their nutrition. Although there are no direct observations about the food of zooplankton in our experiments, it appears likely that heterotrophic flagellates play an important role as a food of zooplankton in humic waters.The importance of dissolved organic matter in the nutrition of aquatic organisms would seem to be much greater than has generally been recognized. Consequently the prevailing concepts of the structure and functioning of planktonic ecosystem should be thoroughly re-evaluated.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

How light and nutrients affect the relationship between autotrophic and heterotrophic biomass in a tropical black water periphyton community

This study examines how nutrients and light affect the relationship between autotrophic biomass and non-autotrophic periphyton organic matter in a tropical black water lake biofilm community. We hypothesized that there is no positive correlation between autotrophic and non-autotrophic organic matter in the periphytic community of a black water humic lake, where non-algal components of periphyton can rely on carbon sources external to the periphyton matrix and where nutrient availability is low. Second, we sought to test our hypothesis that non-autotrophic periphyton organic matter will benefit from nutrient enhancement in a lake where the availability of DOC is high. We performed a field experiment using in situ lake mesocosms to manipulate nutrient concentrations and light availability in a 2 × 2 factorial design. Control treatments (no nutrient added) and nutrient treatments (N + P) were compared in different light conditions: high light (near surface water) and low light (near bottom). No positive correlation was found between autotrophic biomass and non-autotrophic periphyton organic matter, but a negative correlation was observed in high nutrient and light conditions. The low C:P and N:P ratios revealed that the non-autotrophic organic matter mostly comprised a heterotrophic microbial biofilm. High levels of light and nutrients together caused significant changes in periphyton community properties. The non-autotrophic periphyton organic matter was negatively affected by nutrient addition, whereas autotrophic biomass was positively affected, especially in high light conditions. Our results strongly suggest that non-autotrophic periphyton organic matter in a humic lake is primarily comprised of a bacterial biofilm that directly competes for nutrients with autotrophs in the periphytic community. We also observed no effect of nutrient addition on periphyton growing in light-limited conditions. These results suggest that heterotrophic periphytic organisms might experience carbon limitation despite the high availability, but usually low quality, of dissolved carbon in the water column of humic lakes.     

Scaling of Pelagic Metabolism to Size,Trophy and Forest Cover in Small Danish Lakes

ABSTRACT We tested whether pelagic light and nutrient availability, metabolism, organic pools and CO₂-supersaturation were related to lake size and surrounding forest cover in late summer–autumn measurements among 64 small (0.02–20 ha), shallow seepage lakes located in nutrient-rich, calcareous moraine soils in North Zealand, Denmark. We found a strong implicit scaling to lake size as light availability increased significantly with lake size while nutrient availability, phytoplankton biomass and dissolved organic matter declined. Forest lakes had significantly stronger net heterotrophic traits than open lakes as higher values were observed for light attenuation above and in the water, dissolved organic matter, pelagic community respiration (R) relative to maximum gross primary production (R/GPP) and CO₂-supersaturation. Total-phosphorus was the main predictor of phytoplankton biomass (Chl) despite a much weaker relationship than observed in previous studies of larger lakes. Maximum gross primary production increased with algal biomass and decreased with dissolved organic matter, whereas community respiration increased with dissolved organic matter and particularly with gross primary production. These results suggest that exogenous organic matter supplements primary production as an energy source to heterotrophs in these small lakes, and particularly so in forest lakes experiencing substantial shading from the forest and dissolved humic material. This suggestion is supported by 20–30-fold CO₂ supersaturation in the surface water of the smallest forest lakes and more than sixfold supersaturation in 75% of all measurements making these lakes among the most supersaturated temperate lakes examined so far.     

Within- and between-Lake Variability in the Composition of Bacterioplankton Communities: Investigations Using Multiple Spatial Scales

This study examined the similarity of epilimnetic bacterial community composition (BCC) across several within- and among-lake spatial scales, and the environmental factors giving rise to similar bacterial communities in different lakes were also explored. Samples were collected from 13 northern and southern Wisconsin lakes representing gradients in lake size, productivity, dissolved organic carbon and humic acid contents, and pH. Hypotheses regarding patchy distribution of bacterial communities in lakes were tested by comparing samples collected from nearby (tens of meters) and distant (hundreds of meters) sampling sites in the same lake. BCC was characterized by using a molecular fingerprinting technique, automated ribosomal intergenic spacer analysis (ARISA). Overall, samples collected at the 10-m, 100-m, and between-lake scales differed by 13, 17, and 75%, respectively. Variation at these last two scales was significant. The development of within-lake variation in BCC appeared to depend on the isolation of water by lake shoreline features such as bays or narrow constrictions. ARISA profiles from northern lakes had fewer peaks and were less similar to each other than were those of the southern lakes, suggesting that regional features do not necessarily lead to the development of similar bacterial communities. Lakes at similar positions on productivity and dissolved organic carbon concentration gradients had similar bacterial communities, and bacterial diversity was positively correlated with lake productivity and water temperature. Factorial studies taking into account these gradients, as well as regional spatial scales, should provide much insight into the nature of aquatic bacterial biogeography.     

Within- and between-lake variability in the composition of bacterioplankton communities: investigations using multiple spatial scales

This study examined the similarity of epilimnetic bacterial community composition (BCC) across several within- and among-lake spatial scales, and the environmental factors giving rise to similar bacterial communities in different lakes were also explored. Samples were collected from 13 northern and southern Wisconsin lakes representing gradients in lake size, productivity, dissolved organic carbon and humic acid contents, and pH. Hypotheses regarding patchy distribution of bacterial communities in lakes were tested by comparing samples collected from nearby (tens of meters) and distant (hundreds of meters) sampling sites in the same lake. BCC was characterized by using a molecular fingerprinting technique, automated ribosomal intergenic spacer analysis (ARISA). Overall, samples collected at the 10-m, 100-m, and between-lake scales differed by 13, 17, and 75%, respectively. Variation at these last two scales was significant. The development of within-lake variation in BCC appeared to depend on the isolation of water by lake shoreline features such as bays or narrow constrictions. ARISA profiles from northern lakes had fewer peaks and were less similar to each other than were those of the southern lakes, suggesting that regional features do not necessarily lead to the development of similar bacterial communities. Lakes at similar positions on productivity and dissolved organic carbon concentration gradients had similar bacterial communities, and bacterial diversity was positively correlated with lake productivity and water temperature. Factorial studies taking into account these gradients, as well as regional spatial scales, should provide much insight into the nature of aquatic bacterial biogeography.     

Small lakes dominate a random sample of regional lake characteristics

1. Lakes are a prominent feature of the Northern Highland Lake District (NHLD) of Wisconsin, covering 13% of the landscape. Summarising the physical, chemical, or biological nature of NHLD lakes at a regional scale requires a representative sample of the full size distributions of lakes. In this study, we selected at random 168 lakes from the full size distribution of lakes in the NHLD and sampled each lake for a broad suite of limnological variables.
2. Most lakes were small. The median lake area was 1.1 ha, however, half of the surface area of water was in a relatively small number of lakes larger than 162 ha. Smaller lakes tended to be low in dissolved inorganic carbon (DIC) and high in dissolved organic carbon (DOC). Inclusion of small lakes (<4 ha) in the survey resulted in an acid neutralising capacity (ANC) median (76.5  μ Eq L⁻¹) much lower than previous estimates, and a DOC median (10.1 mg L⁻¹) about 50% higher than it would have been without the smaller lakes. Unlike DOC, total P tended to be evenly distributed across lake sizes.
3. The implications of these findings are that regional summaries of lake characteristics for the NHLD are influenced by the inclusion of small lakes in the sample, even though most of the water surface area is in lakes larger than 162 ha. Excluding small lakes introduces bias in the estimates of organic carbon and inorganic carbon values, for example. Similar biases may be introduced for lake characteristics at the global scale if small lakes are not sampled, because the size distribution of lakes globally is dominated in number by small lakes.     

Long-Term CO2 Variability in Two Shallow Tropical Lakes Experiencing Episodic Eutrophication and Acidification Events

Here we report the long-term (13-year) dynamics of surface pCO₂ and its response to episodic eutrophication and acidification events in two contrasting tropical coastal lakes, one clear-water and the other humic. A short-term nutrient addition experiment was also conducted in mesocosms in the humic lake where in situ eutrophication was moderate. Our objective was to elucidate the response of pCO₂ to interannual changes in key limnological conditions, such as nutrient concentrations and pH. The humic waters showed a median pCO₂ almost ninefold higher across the 13-year study than the clear waters, supporting pCO₂ values about tenfold above atmospheric equilibrium. Eutrophication of the clear-water lake resulted in a decrease in pCO₂ to median values below atmospheric equilibrium, producing a strong sink for atmospheric CO₂. In contrast, pCO₂ increased by over tenfold in both lakes during the acidification phase, resulting in very large CO₂ emissions to the atmosphere. Experimental nutrient additions in the humic lake showed a strong persistence of high pCO₂. The extreme variability in pCO₂ observed here might be a characteristic of tropical lakes and may have important consequences for regional carbon budgets.