Extracellular dissolved organic carbon released from phytoplankton as a source of carbon for heterotrophic bacteria in lakes of different humic content

The quantitative importance of photosynthetically produced dissolved organic carbon (PDOC) released from phytoplankton as a source of carbon for pelagic, heterotrophic bacteria was investigated in four temperate Swedish lakes, of which two had low (≈20 mg Pt 1⁻¹), and two moderately high (60–80 mg Pt 1⁻¹) humic content. The bacterial assimilation of PDOC was estimated with the ¹⁴C method, and the total production of the heterotrophic bacteria was estimated with the [3H]thymidine incorporation method. The release of PDOC from natural communities of phytoplankton was not restricted to periods of photosynthesis, but often continued during periods of darkness. Heterotrophic bacteria often assimilated the labile components of the PDOC at high rates (up to 73% of the released PDOC was assimilated during the incubation in our experiments). The contribution of PDOC to bacterial production exhibited large within-lake seasonal variations, but PDOC was at certain times, both in humic and non-humic lakes, a quantitatively very important carbon source for the heterotrophic bacteria. Under periods of comparatively low primary production, heterotrophic bacteria in humic lakes appear to utilize allochthonous, humic substances as a substrate.     

Effects of different molecular weight fractions of dissolved organic matter on the growth of bacteria,algae and protozoa from a highly humic lake

Effects of different molecular size fractions (< 1000 MW, < 10 000 MW, < 100 000 MW and <0.1 μm) of dissolved organic matter (DOM) on the growth of bacteria, algae and protozoa from a highly humic lake were investigated. DOM from catchment drainage water as well as from the lake consisted mostly (59–63%) of high molecular weight (HMW) compounds (> 10 000 MW). With excess inorganic nutrients, the growth rate and yield of bacteria were almost identical in all size fractions. However, in < 1000 MW fractions and with glucose added, a longer lag phase occurred. Without added nutrients both the growth rates and biomasses of bacteria decreased towards the smaller size fractions and the percentage of dissolved organic carbon (DOC) used during the experiment and the growth efficiency of bacteria were lower than with excess nutrients. The growth efficiency of bacteria was estimated to vary between 3–66% in different MW fractions, largely depending on the nutrient concentrations, but the highest growth efficiencies were observed in HMW fractions and with glucose. The growth of algae was clearly lowest in the < 1000 MW fraction. In dim light no net growth of algae could be found. In contrast, added nutrients substantially enhanced algal growth and in deionized water with glucose, algae achieved almost the same growth rate and biomass as in higher MW fractions of DOM. The results suggested that bacteria and some algae were favoured by DOM, but protozoans seemed to benefit only indirectly, through bacterial grazing. The utilization of DOM by bacteria and algae was strongly affected by the availability of phosphorus and nitrogen.     

Effects of humic stress on the zooplankton from clear and DOC‐rich lakes

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Dissolved organic matter quantity and quality from freshwater and saltwater lakes in east-central Alberta

Concentrations of dissolved organic matter (DOM) in surface waters of sub-humid to semi-arid lakes in east-central Alberta increase with increasing salinity and water residence time from about 20 to 330 mg L^–1 as dissolved organic carbon (DOC). This pattern is opposite to that observed among freshwater lakes spanning a gradient in water residence times, and is probably caused by evaporative concentration of refractory DOM. The proportion of total DOC, operationally defined as humic substances using XAD-8 resin, was high, though similar to surface waters typically referred to as "humic", and independent of salinity. Very long water residence times (hundreds of years) in saline lakes favors evapoconcentration of low-color, low molecular weight DOM, with N-content characteristic of allochthonous DOM.     

Size distribution of dissolved (<200 nm) organic carbon and aluminium in alkaline and humic lakes

Chemical equilibrium calculations treating humic substances as the simple diprotic phtalic acid, predicted no Al-humus at pH>6.5 in humic lake water. However, dissolved (<200 nm) organic carbon (DOC) and dissolved Al appeared to be linearly (r=0.597, P<0.001) correlated in samples from five different humic surface waters in The Netherlands with a DOC range of 10–36 mg.l^–1 and a (mean) pH range of 6.85–8.47. Yet, organic carbon (C_org) and Al did no exhibit similar size distributions between 5 and 200 nm revealed by ultrafiltration. Averaged 25% of the C_org and >50% of the Al occurred in the fraction <5 nm. Only in this fraction the C_org and Al were linearly correlated (r=0.515, P<0.001). This result suggests the presence of organic ligands in the DOC pool of humic waters having smaller molecular sizes and higher Al stability constants than the humic substances used to model aquatic Al speciation.     

Lability of organic carbon in lakes of different trophic status

1. We used first‐order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BOD_ult), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 μg L⁻¹, labile organic carbon (OC_L = 0.3BOD_ult) from 0.75 to 2.95 mg C L⁻¹ and k from 0.044 to 0.14 day⁻¹. 3. Our data showed that there were greater concentrations of OC_L but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OC_L was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27–55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long‐term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications.     

吉安地区典型景观湖泊浮游植物群落特征及其与水环境因子的关系

2012年8月至2013年7月,对吉安地区将军湖、龙湖、庐陵湖和挹翠湖4个景观湖泊水体浮游植物群落及其主要水环境因子进行了调查,并利用生物多样性指数法(Shannon指数H、Margalef指数D和Pielou指数J)和主成分分析(PCA)法分别对湖泊水质和水环境因子进行了评价。结果表明:4个湖泊共鉴定浮游植物7门82属163种,主要优势种为硅藻或绿藻;浮游植物细胞丰度呈季节性变化,秋夏居高,冬春季偏低,平均丰度变化范围为25.45×10~6~54.04×10~6cells·L~(-1);将军湖、龙湖、庐陵湖和挹翠湖的H值分别为1.26~2.08、1.82~2.61、2.27~2.62和1.10~2.32;D值分别为2.03~3.51、2.36~3.71、2.48~3.93和3.12~3.96;J值分别为0.45~0.69、0.59~0.80、0.67~0.77和0.50~0.84。综合评价结果显示,4个湖泊处于富营养化状态、中等污染水平。而对污染较重的庐陵湖水环境因子PCA分析结果表明,水温(WT)、溶解氧(DO)和总氮(TN)是影响小型封闭景观水体浮游植物群落变化的主要因素。建议对小型封闭景观水体进行必要的治理和生态修复。     

Microbial control of dissolved organic carbon in lakes: research for the future

The dissolved organic carbon (DOC) of lakes dominates any budget of organic carbon in these systems. Limnologists are still limited by techniques and particularly by the lack of measurements of rates of microbial transformation and use of this DOC. There are now four different approaches to the study of the microbial control of DOC in lakes. The first is through measurements of the total DOC. Recent advances in measurement with high temperature combustion will likely lead to higher and more consistent measurements in freshwaters than previously. It is possible that a biologically active fraction may be identified. The second approach is through measurements of microbial incorporation and respiration of ¹⁴C-labeled organic matter. The kinetics of this process are well known but advances in measurement of the size of the substrate pool are still being made. The third approach is to use bacterial growth in batch or continuous flow experiments in order to understand how much of the total DOC can be decomposed by microbes. The assay in this approach may be microbial growth (thymidine incorporation, biomass changes) or change in the DOC (total concentrations, specific compounds, or fractions of the DOC by molecular weight). These methods are promising but are not developed enough for routine use. For example, growth measurements in the laboratory are all subject to experimental artifacts caused by changes in the DOC and in the bacterial populations. Finally, the fourth approach is through the use of isotopes of the natural DOC. In the sea this approach has given the age of the bulk DOC (¹⁴C data). In freshwaters it has great potential for differentiating between bacterial use of terrestrial DOC vs. use of algal-derived DOC (13C data). Stable isotopes are also useful for experimentally labeling DOC produced by algae and following the use of this material by bacteria.     

The relation between bacterial carbon and dissolved humic compounds in oligotrophic lakes

Abstract The mean annual biomass of planktonic bacteria showed large variations both within and between lakes. The lowest bacterial biomass was found in acidified lakes (7.8–12.1 μg C · 1⁻¹), and tended to increase with increasing water colour (up to 44.1 μg C · 1⁻¹). The highest recorded bacterial biomass was 138 μg C · 1⁻¹. The mean annual bacterial biomass equalled 23–45% of the algal biomass. Zooplankton biomass was high, compared to algal biomass (40–50%). Multiple regression analysis of 10 variables showed a strong positive correlation between bacterial biomass and humic content ( r = 0.74, P < 0.001), while other parameters, except pH, showed no correlation. The observation thus strongly supports the role of humic compounds in aquatic secondary production.     

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.     

Demonstration of extraction and PCR amplification of DNA from phytoplankton of lakes with high humic acid content

Lakes with high levels of dissolved organic matter are common in northern temperate regions. These habitats are sensitive to environmental and climate change, but the molecular ecology and eco-physiology of their phytoplankton have been under studied, in part because the DOM interferes with molecular and biochemical analyses of these populations. We developed a procedure which uses polyvinylpyrrolidone to adsorb interfering material during the extraction of nucleic acids from these populations. This procedure does not depend on enzymatic lysis and yields DNA of sufficient quality to serve as a template for PCR amplification of specific genetic sequences. In particular, we detected fragments of the conserved rbcL gene encoding the large subunit of the Rubisco enzyme from the phytoplankton of lakes with high DOM. Our extraction procedure may be useful to other workers studying similar populations.     

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso‐ to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed. 2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes. 3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non‐algal and detrital C to the POC pool was high. 4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition.     

海水中藻菌共培养体系对碳氮磷的吸收转化

海洋环境中,细菌和微藻之间的物质交换是生源要素在自然界中迁移转化的重要方式。为进一步了解生源要素的生物地球化学循环,在实验室模拟条件下,研究了共培养体系中营养盐和有机物在细菌和微藻之间的转换。通过纯培养中肋骨条藻(Skeletonema costatum)、东海原甲藻(Prorocentrum donghaiense)、天然海水中的细菌以及藻菌混合培养,分析了营养盐和有机物随藻菌生物量的变化情况,并计算了溶解有机碳(DOC)和溶解有机氮(DON)的浓度比值[(DOC/DON)a]。结果发现,在共培养体系中,细菌对中肋骨条藻的生长有抑制作用,对东海原甲藻影响不明显;中肋骨条藻有利于细菌生长,东海原甲藻抑制细菌生长,这种不同可能与微藻的粒径有关。海洋细菌在2种藻的指数生长均期均会促进微藻吸收氨氮(NH_4-N),但在生长末期NH_4-N以释放为主。硝氮(NO_3-N)的浓度与藻的生长呈负相关,但在衰亡期NO_3-N略有增加,表明NO_3-N再生所需时间较长。细菌对硝氮的吸收量较少,但对其再生有贡献。细菌和中肋骨条藻对磷酸盐(PO_4-P)的吸收存在竞争,但与东海原甲藻的竞争关系不明显。不同培养体系中DOC浓度变化不同,在藻菌共培养体系中增加较快,纯藻培养体系中增加缓慢,在纯菌培养体系中缓慢减少。通过对DOC与DON浓度比值的分析,发现用判断颗粒有机碳(POC)来源的方法可以分析DOC的来源。     

Will an increased greenhouse impact in Fennoscandia give rise to more humic and coloured lakes?

A large-scale increase in the discharge of humic material in rivers, as well as a darkening of Swedish lakes during the last 15 years, are illustrated and discussed. Increased precipitation was responsible for the greatest changes, but a marked part (10–40%) of the increase in river transport of coloured material was independent of discharge. Possible explanations for this increase may be: increased ground water level leading to increased runoff in superficial soil levels, a flow in direct contact with the organic soils; humus fractions in these layers may also be rich in iron resulting in more coloured water; increased primary production giving rise to increased amounts of litter and humic material; reduced capacity of soils to absorb organic matter because of acidification; and effects of modern forest management. Models of global climate predict a doubling of atmospheric-CO₂, an increase in mean annual temperature of up to 1.5 to 4.5 °C, and in Fennoscandia possibly an increase in precipitation. The factors responsible for the observed large-scale change in humus flux may be strengthened by the increasing greenhouse impacts, and may result in more humic and coloured lakes in Fennoscandia during the next century.     

Relations between production and biomass of phytoplankton in four Swedish lakes of different trophic status and humic content

Production and biomass values from phytoplankton populations in four different Swedish lakes were analysed. The production in all lakes was directly proportional to biomass during homothermal periods. When the lakes were stratified there was a strong negative relation between specific growth rate and biomass. The data fitted to a logistic density dependent growth equation of the form: dB/ dt = µ_mB(1-B · K^–1) where B is the biomass, µ_m the maximum specific growth rate and K the carrying capacity. The equation was used to derive the parameters µ · µ_m ^–1 and carrying capacity (the maximum possible biomass). These parameters were then discussed in relation to light climate, phosphorus concentration and humic content.     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta

SUMMARY 1. The seasonal dynamics of light attenuation, and the relative roles of total suspended solids (TSS), dissolved organic carbon (DOC) and chlorophyll as light attenuators among two sets of lakes in the Mackenzie Delta, were assessed during the open‐water periods of 1998 and 1999. 2. The first set consisted of 40 spatially discrete lakes where the frequency of flooding with river water was controlled by sill height (‘sill‐set lakes’). The second set consisted of a chain of six lakes connected to a main river channel (frequently flooded, all with same frequency), but where riverine influence was controlled by the distance from the channel connection point (‘chain‐set lakes’). 3. As the flooding frequency of lakes decreased (sill‐set), and as the distance from the channel connection point increased (chain‐set), lake water became increasingly transparent and the stability (decreasing temporal variability) of underwater light increased. 4. The effect of flooding on transparency was greater in years with a high minimum summer water level. However, the effect of river flooding on lake water transparency was damped more by an increase in the frequency and duration of flooding than by an increase in distance from the channel connection point. 5. The index of scattering was linearly related to TSS over the common range of concentrations in both sets of lakes. The specific attenuation coefficient for TSS (and scattering) increased substantially from the most turbid to the most transparent waters. 6. During the summer, DOC provided an approximate index of water colour in the sill‐set lakes but not in the chain‐set lakes, where the gradient of DOC ran counter to the gradient of water colour. The specific attenuation coefficient for water colour was roughly constant among both sets of lakes. 7. Calculations of partial attenuation show that, during the spring flood peak, TSS is the dominant attenuator among most lakes, other than those with high sills or positioned far from channel connection points. During the lengthy summer period of open water, however, water colour appeared to be the most important light attenuator among almost all of the lakes in the central delta, with chlorophyll a of only minor importance. 8. Lakes of the Mackenzie Delta may be quite sensitive to changes in climate and ultraviolet‐b (UV‐b) radiation in the circumpolar arctic because of the role of DOC as an attenuator of photosynthetically active radiation and UV‐b irradiance and as an energy source for microbial foodwebs in this system.     

Oscillations of algal biomass,nutrients and dissolved oxygen as a measure of ecosystem stability

Graphic presentation of weekly rates of change of algal biomass (expressed as chlorophyll a) and nutrient and dissolved oxygen concentrations can be regarded as harmonic oscillation motion. Maximum amplitudes of these oscillations provide a useful tool to assess the degree of stability of aquatic ecosystems in relation to their trophic state. Data sets from seven different lakes ranging from hypereutrophic to oligo-mesotrophic were processed using a computerized method. The high values of oscillation amplitudes of approximately 150 g l^–1 wk^–1 chlorophyll a, 500 g l^–1 wk^–1 ammonia nitrogen, 50 g l^–1 wk^–1 soluble reactive phosphorus and 10 mg l^–1 wk^–1 dissolved oxygen, indicated strong ecosystem instability, while low values of less than 10 g l^–1 wk^–1 of chlorophyll a, 20 g l^–1 wk^–1 ammonia nitrogen, 2 g l^–1 wk^–1 soluble reactive phosphorus, and 3 mg l^–1 wk^–1 dissolved oxygen represented a stable system. Oscillation amplitudes of the chlorophyll a values were found to be the most representative indicator of ecosystem stability.     

Determination of the trophic state by qualitative and quantitative phytoplankton analysis in two gravel pit lakes

The phytoplankton of two gravel pits with comparable nutrient concentrations but different chlorophyll-a concentrations and phytoplankton biovolume was investigated qualitatively and quantitatively over a period of two years. This study deals with the phytoplankton-rich ‘Rotter See’ and the phytoplankton-poor ‘Paulsmaar’, both South of Cologne. The trophic state of both lakes was determined comparatively according to Ryding &; Rast (1989) and the categories used by Brettum (1989). The results were compared with each other. The trophic lake index according to Hörnström (1981) was modified slightly and subsequently determined for both lakes. A combination of qualitative and quantitative phytoplankton analysis made a proposal of a rehabilitation for the Rotter See possible.