Microbial activity facilitates phosphorus adsorption to shallow lake sediment

Purpose  

The phosphorus (P) level in sediments is one of the major indicators of the eutrophication of lakes. Little information is available on the effect of microbial activity on P adsorption in shallow lake sediments. In this study, we evaluated the effect of microbial activity on the adsorption of P to sediments in a shallow lake.     

Vertical distributions of PAHs in the sediments of four lakes in Japan

Purpose

The purpose of this study was to elucidate historical trends, spatial variations, and the sources of polycyclic aromatic hydrocarbons (PAHs) pollution in several Japanese lakes.

Materials and methods

The vertical distributions of PAHs in the core samples of sediments taken at several points in lakes Kasumigaura, Suwa, Kizaki, and Shinji were determined using a gas chromatograph equipped with a mass selective detector and combined with chronological information and the physical/elemental properties of the sediment.

Results and discussion

Seventeen related compounds (congeners) typically had concentration peaks at sediment depths corresponding to the 1960s to 1970s. In Lake Shinji and one bay of Lake Kasumigaura, there was a tendency for PAH concentrations to increase downstream; in contrast, another bay of Lake Kasumigaura showed the reverse trend. During big flood events, the fluxes of PAHs increased due to large inputs of particulate matter, although PAH concentrations were reduced. For the four study lakes and other similar lakes, PAH concentrations of surface sediments were approximately proportional to population densities in the respective watersheds, while the total input of PAHs to the lakes were correlated with their population and watershed area. The source apportionment analysis using isomer ratios for the congener profiles indicated that the principal sources of the PAHs in the lake sediments were gasoline and/or diesel engine exhausts and biomass burning.

Conclusions

The observed concentration peaks showed a deterioration of the chemical quality of atmospheric conditions around 1960?C1970 and a recent tendency for their amelioration. Between-lake differences suggest that the influence of human activity in the watersheds influences sediment PAH concentrations. The PAH sources were identified to be of pyrogenic origin.     

Silicon budget of eutrophic Lake Kasumigaura, Japan

Purpose

Determining the dynamics of silicon in lakes, one of the essential nutrients for diatoms, is valuable for understanding aquatic environmental problems. The dissolved silicon (DSi) and biogenic silicon (BSi) budgets in Lake Kasumigaura, a shallow eutrophic lake in Japan, during the last three decades were assessed based on the analysis of dated sediment cores and a water quality database.

Materials and methods

Sediment cores (100?cm long) were taken at the center of Lake Kasumigaura in 2005, 2007, and 2009 and at two other sites in 2007. BSi contents of the dated sediments were determined by wet alkaline digestion. The net sedimentation rates of BSi were defined as the difference between the DSi load from inflowing rivers and the DSi and BSi loads from the outflow of the lake, calculated using DSi concentrations and diatom abundances in the lake from 1980 to 2007 and DSi concentrations of the inflowing rivers during 1994, 2007, and 2009. The gross sedimentation rates of BSi were estimated by multiplying BSi concentrations in lake water by the diatom sinking rate reported by previous studies.

Results and discussion

Budgetary calculations based on the database showed that 60?C70?% of DSi inputs from the inflowing rivers during the 27?years could ultimately be accumulated as diatom frustules in bottom sediments in the lake. The sediment analysis revealed that the amount of BSi accumulated in the lake from 1980 to 2007 was 2.0?C2.6?×?10¹¹?g, similar to the amount based on the database of 1.3?C2.4?×?10¹¹?g. Although the gross sedimentation rates of BSi likely increased, the net sedimentation rates of BSi decreased significantly from 6?C10?×?10⁹?g?year^?1 in the 1980s to 2?C6?×?10⁹?g?year^?1 in the 2000s, suggesting a fast recycling of BSi in recent years caused by an increase in sediment resuspension and regeneration.

Conclusions

The sediment core information and the water quality database can be used for calculating the long-term silicon budgets in Lake Kasumigaura. An increase in the DSi release rates was identified, which is consistent with recent sediment resuspension. Comparing the sediment core information with the database suggests the long-term dissolution of sediment BSi; however, analysis of the BSi content in sediment cores representing a much longer time period is needed to confirm this.     

Shifts in the structure and function of the microbial community in response to metal pollution of fresh water sediments in Finland

Purpose

Mining is a common source of metals in aquatic ecosystems. Metal loading in the environment is thought to be a selective pressure that induces compositional and functional changes within the affected microbial community in the sediment. This study aims to explore shifts in the diversity, structure, and functional gene abundance of microbial communities in the sediment of the copper mining-induced contaminated lakes in Finland.

Materials and methods

The sediment microbial community structures and abundance of the functional groups involved in carbon/nitrogen/sulfur cycling in four lakes located downstream from metal mines (Kirkkoselkä (KS), Junttiselkä (JS), Laakajärvi (LJ), and Sysmäjärvi (SJ)) and one reference lake (Parkkimanjärvi (PJ)) in Finland were compared using high throughput sequencing and quantitative PCR.

Results and discussion

Compared to the PJ reference lake sediment, the relative abundances were higher for Bacteroidetes, Gemmatimonadetes, Acidobacteria, and Nitrospirae but lower for Firmicutes and Alphaproteobacteria in the mine-contaminated sediment samples. The number of copies of copper-resistant genes (copA) in the two copper-contaminated sediments (5.34 × 10⁶ and 4.95 × 10⁶ copies ng^?1 DNA for KS and JS, respectively) was significantly higher than that in the PJ sediment (1.33 × 10⁶ copies ng^?1 DNA). Methanogens (mcrA gene) accounted for 5.09–11.5% of the total archaea (16S rRNA) in these lake sediments. In addition, ammonia-oxidizing archaea (amoA gene) in the LJ sediment accounted for 36.0% of the total archaea but only 0.83–1.63% in the sediment of other lakes. The abundance of eight investigated functional groups accounted for 28.8% of the total bacteria in the PJ sediment but less than 1.3% in the metal-contaminated sediments. The canonical correspondence analysis showed that the microbial community structure of Lake LJ was scattered far from the other lakes and was significantly correlated with nitrate; the community structural change in the JS and KS sediments was positively correlated with copper or negatively correlated with nitrate concentration.

Conclusions

These results indicate that the sedimentary indigenous microbial community may shift its composition and structure as well as its function to increase its adaptability and/or resistance to metal-contaminated freshwater sediments.

     

Relationship between mercury and organic carbon in sediment cores from Lakes Qinghai and Chenghai, China

Purpose

Factors such as organic matter can significantly influence the distribution of mercury (Hg) in aquatic environments. Recent studies in Arctic and sub-Arctic lakes in Canada have investigated whether scavenging of Hg by phytoplankton significantly affects distributions of Hg in sediments. This study examined the relationships between Hg and organic components in two contrasting lakes (Lakes Qinghai and Chenghai) in low and middle latitudes of China.

Materials and methods

Sediment cores from the less-polluted, oligotrophic Lake Qinghai (QH) and from the polluted, eutrophic Lake Chenghai (CH) were collected by a gravity corer. The cores were sectioned and transported on ice to the laboratory where they were stored at ?20 °C. Subsamples were dried in a vacuum freeze dryer and grounded with a mortar and pestle prior to analyses. Total concentrations of Hg were quantified using cold vapor atomic absorption spectrometry. Total organic carbon (TOC) was quantified using an elemental analyzer after removal of carbonate. The Rock-Eval 6 pyrolysis technique (Vinci Technologies, Rueil-Malmaison, France) was used to deconvolute TOC in sediments into S1, S2, and RC components; S2 was further separated into S2a and S2b.

Results and discussion

Different relationships between Hg and TOC were found in the two lakes, which suggest that different types of organic compounds might play completely different roles in the distribution of Hg in lakes. S1 (the soluble organic matter (SOM)) was found to significantly control distributions of Hg in sediments of both lakes, while S2 and S2a were not. Combining the synchronous fluctuations of Hg and the oxygen index in the QH sediment core and in recent sections of CH suggested that allochthonous SOM derived from the terrestrial environment had an important influence on the distribution of Hg in both lakes and a large portion of Hg that originated from the lake catchment.

Conclusions

This study provides further evidence that organic matter is one of the most important factors that influences distributions of Hg in lake sediments and that SOM was the primary form of carbon associated with sedimentation of Hg. The results also suggest that Hg in lake sediments might not accurately represent its pollution history as it could also be influenced by land use, such as agriculture or other human activities in the catchment.     

Characterization of sediment contamination patterns by hydrophobic pesticides to preserve ecosystem functions of drainage lakes

Purpose

Many drainage basins are terminal recipients of hydrophobic contaminants such as pesticides. To minimize adverse effects of the contaminated sediments on wildlife, it is important to understand sediment contamination patterns. This study was conducted at the Salton Sea, which is a heavily polluted large lake in southern California, USA, with the purpose to identify areas with minimal contamination so as to support species conservation.

Materials and methods

We investigated the horizontal and vertical distribution of 14 organochlorine pesticides (OCPs) and 12 currently used pesticides (CUPs) in playas at locations near the drainage outfalls. The data were subjected to spatial analysis using Kriging interpolation and converted to contour maps. Statistical comparisons were made among different areas, between different sediment depths, and between air-exposed and submerged sediments.

Results and discussion

Various OCPs were found near two drainage inlets, with mean concentrations of 6?C30???g?kg^?1 in air-exposed sediments and 3?C18???g?kg^?1 in submerged sediments. Chlordane (detected frequency, DF?=?77?%) and DDT derivatives (DF?=?100?%) were among the most frequently detected OCP. Significantly higher concentrations were found in air-exposed sediments than in submerged sediments, and in subsurface sediments than in surface sediments (P?<?0.01), suggesting historical deposition and burial. Sediments at many locations exceeded the threshold levels for DDE. A total of seven CUPs were detected with the maximum ??CUPs concentration of up to 27???g?kg^?1. Bifenthrin was the dominant CUP contaminant, representing more than 60?% of ??CUPs for most samples with the highest concentration of 26???g?kg^?1.

Conclusions

Findings from this study provide a snapshot of the spatial distribution in both horizontal and vertical directions of hydrophobic pesticides in a drainage-dominated lake, and such information and the method of investigation may be used for identifying areas of minimal contamination as alternative habitats for this and other impacted lakes.     

Effects of lake liming on phytoplankton production in acidic Adirondack lakes

Phytoplankton community characteristics were monitored prior to and following CaCO₃ addition to two small, highly acidic lakes (Cranberry Pond and Woods Lake) and one larger, less acidic lake (L. Simon Pond). Data were also collected from a control site (Dart's Lake) exhibiting chemical characteristics similar to the pretreatment conditions observed at the experimental sites. In the two small, most acidic lakes, base addition was associated with higher chlorophyll levels during the first summer following treatment. Woods Lake was maintained at a circumneutral pH for 3 yr and exhibited increased phytoplankton abundance throughout the posttreatment period. In contrast, Cranberry Pond reacidified within 1 yr following based addition. Reacidification was accomplished by a decrease in lake chlorophyll levels to pre-treatment levels. At the larger, less acidic lake (L. Simon Pond), liming was associated with lower chlorophyll levels during the first summer after treatment. Reductions in chlorophyll levels at L. Simon Pond reflect the absence of the spring phytoplankton peak and a decrease in phytoplankton growth below the depth of the thermocline. At Cranberry Pond, annual differences in phytoplankton production did not correspond to changes in lake acidity and phytoplankton abundance. Productivity in Woods Lake exhibited an increasing trend during the 3 yr following treatment. Interpretation of treatment effects on productivity was confounded by high between-year variability at the control site.     

Base neutralizing capacity of sediments from an acidic lake

The base neutralizing capacity (BNC), or alkalinity consumption, of acidic lake sediments may influence the amount of neutralizing agent required to neutralize a lake if the sediment BNC is large relative to the BNC of overlying waters. The extent ofin situ sediment BNC in acidic Bowland Lake (pH 5.0) was inferred by (1) measuring the loss of Ca-45 to acidic sediments from labeled lake water neutralized with CaCO₃, and (2) measuring exchangeable Ca in sediments collected prior to and following neutralization of Bowland Lake with calcite (CaCO₃). The sediment BNC derived from the Ca-45 radiolabeling experiment was 0.01 mg CaCO₃ g^?1 w wt. The mean losses of Ca-45 from the aqueous phase of neutralized and untreated sediment/water mixtures were not significantly different. The mean pH of both neutralized and untreated mixtures decreased to 4.0 during the incubation, possibly because of oxidation of reduced sediments. Sediment BNC estimates derived from literature data for several lakes may be overestimated because of the inclusion of anoxic sediments containing significant amounts of reduced Fe. There was no significant difference in exchangeable Ca between sediments from untreated Bowland Lake and sediments collected 10 m after whole-lake neutralization indicating that little of the supplied alkalinity had been lost to the sediments. Hence,in situ sediment BNC was probably small in Bowland Lake.     

Effects of acidity and doc on phytoplankton community structure and production in three acid lakes (Nova Scotia)

Phytoplankton community structure varied between the three lakes and between years within lakes. The Beaverskin Lake phytoplankton community was dominated by cyanophytes and chlorophytes in the summer and chrysophytes in the winter. Kejimkujik Lake was dominated by bacillariophytes in the summer of 1979 but no single group dominated in 1980 or 1981. Pebbleloggitch Lake phytoplankton consisted mainly of chlorophytes in 1979 but low biomass and no dominant groups characterized this lake during the growing season of 1980. Daily integral planktonic primary production measured simultaneously in the three lakes showed that in both years annual planktonic primary production was highest in the clear water lake, Beaverskin Lake, which also had lower dissolved organic carbon (DOC) concentration compared to the two dystrophic lakes. In the clear water lake annual production was similar between years but in the two colored lakes annual production was 40% higher in the second year. The observed increases in annual production between years in the colored lakes were largely due to changes in euphotic depth resulting from variations in hydrology and DOC export from the lake catchments. Lower discharges in the colored lakes in 1980 were accompanied by significantly lower in lake DOC concentrations, water color, light extinction coefficients and increased euphotic depth. Similar changes in discharge accompanied by lower DOC concentration in the clear water lake did not produce significant changes in water color, light extinction coefficient nor annual production between years. Rates of primary production at light optimum (P-max) were consistently higher in the most colored, acidic lake indicating that relatively high rates of autotrophic production will occur under acidic conditions if nutrient supply is maintained.     

Sudbury Sediments Revisited: Evaluating Limnological Recovery in a Multiple-Stressor Environment

Paleolimnological techniques were utilized to determine whether diatom and scaled chrysophyte assemblages in Daisy, Swan, and Tilton lakes (Sudbury, Ontario) have recovered toward their preimpact conditions as a result of reduced inputs of anthropogenic pollutants (SO ₄ ^2? and metals) or whether other environmental stressors have affected recovery trajectories. In addition, geochemical analysis was used to track trends in sedimentary nickel and copper concentrations through time. Preindustrial algal assemblages were primarily dominated by circumneutral to alkaline and pH-indifferent taxa. However, with the onset of open-pit roasting and smelting operations, there was a stratigraphic shift toward acid-tolerant species. With wide-scale smelter emission reductions commencing in the 1970s, scaled chrysophyte assemblages in Swan and Daisy lakes have started to show signs of biological recovery in ～1984 and ～1991, respectively. Although the scaled chrysophyte assemblage in Tilton Lake has not recovered toward the predisturbance assemblage, the decline in acidophilic taxa and increase in circumneutral taxa in recently deposited lake sediments indicate that the community is responding to increased lake water pH. Conversely, diatom assemblages within each of the study lakes have not begun to recover, despite well-documented chemical recovery. It is suspected that biological recovery in Sudbury area lakes may be impeded by other environmental stressors such as climate warming. Copper and nickel concentrations in lake sediments increased with the onset of mining activities and subsequently declined with emission controls. However, metal concentrations in lake sediments remain elevated compared to preindustrial concentrations. Together, biological and geochemical evidence demonstrates the clear environmental benefits associated with smelter emission controls.     

Phytoplankton community of an acidified,heavy metal—Contaminated lake near Sudbury,Ontario: 1973–1977

Phytoplankton data for 1973 to 1977 from Clearwater Lake, an acid- and heavy metal—contaminated lake near Sudbury, Ontario are presented.Peridinium inconspicuum (Lemmermann) comprised between 30 and 55% of the average, ice-free period biomass of 0.33 to 0.73 mg l^?1 and is considered indicative of acidic lakes if it forms a substantial portion of the total biomass. The data were compared with those from three other contaminated and ten uncontaminated lakes in Ontario. The phytoplankton communities of all contaminated lakes were dominated byP. inconspicuum while chrysophytes dominated the uncontaminated lakes. Community biomass was better correlated with phosphorus concentration than with hydrogen ion concentration. There was some evidence of reductions of biomass in lakes with the highest heavy metal concentrations. Limitations of phytoplankton data collected in synoptic surveys are discussed.     

Chrysophyte scales in lake sediments provide evidence of recent acidification in two Quebec (Canada) lakes

Several lake surveys have indicated that many lakes in Quebec are sensitive to acidification, but no historical data are available to provide conclusive evidence of lake acidification. Paleolimnological studies can provide such evidence. Chrysophycean algal microfossils were analyzed from the sediments of Lake Bonneville and Lake Truite Rouge in Quebec. The chrysophyte flora in the study lakes was stable until 40 yr ago, but since then the taxa characteristic of acidic or slightly acidic waters increased in abundance. The study provides evidence of recent lake acidification in Quebec.     

Impact of Environmental Conditions on the Phytoplankton Structure in Mediterranean Sea Lagoon, Lake Burullus, Egypt

Lake Burullus is one of the northern Delta lakes, connected with the Mediterranean Sea through El-Boughaz opening. The main objective of the present work is to study the physical and chemical properties of the lake and its relation with the phytoplankton communities, chlorophyll-a as well as diversity and Eutrophication index. The studies revealed that, the pH values lie in alkaline side, with wide range of fluctuation (7.15–8.5), the total suspended matter was relatively high (18.2–149 mg l^?1), high values of dissolved oxygen (5.17–11.29 ml l^?1) and high concentration of nutrient salts, (nitrate ranged between (3.4–44.7 μmole l^?1), nitrite (0.42–15.95 μmole l^?1), ammonia (1.46–50.60 μmole l^?1), phosphate (0.63–14.83 μmole l^?1), and silicate (6.54 to 119.3 μmole l^?1). Chlorophyll-a concentration showed a wide range of fluctuation (13.8–127.4 μg/l) and the highest value were recorded at the western region of the lake. Phytoplankton counts showed a pronounced increase (1.9 × 10⁶ units l^?1) as compared with previous studies, a total of 170 species were identified represented mainly by Bacillariophyceae (44.8% to the total community) comprising 68 species, Chlorophyceae (39.99%) 54 species, followed by Cyanobacteria (9.52%) 26 species, Euglenophyceae (5.63%) 15 species. Phytoplankton standing crop attained the highest counts at the western sector particularly at station X during spring and summer (6.7×106 and 4.1×10⁶ units l^?1 respectively) due to the dominance of Bacillariohycean species Stephanodiscus phytoplankton diversity was high and showed widely range (1.47–3.66 nats). A series of stepwise regression equations describing the dependence of phytoplankton standing crop, its main groups and diversity index on the changes of the most biotic prevailing conditions were given and discussed. In general, the results showed that the Lake is considered as a eutrophic area. This phenomenon observed in the body of the lake water that receives large influxes of nutrients, which reflects high phytoplankton counts, diversity index and Chlorophyll-a     

Exploring the relationship between polycyclic aromatic hydrocarbons and sedimentary organic carbon in three Chinese lakes

Purpose  

Previous studies have shown a positive correlation between concentrations of polycyclic aromatic hydrocarbons (PAHs) and total organic carbon (TOC) in lake sediments. However, with respect to the complex organic matter in recent sediments, it is still unclear which part of TOC plays a key role in controlling PAHs distributions in natural sediments. The aim of this study was to examine the relationships between PAHs and TOC components of different origins in lake sediments.     

Fluorescence spectroscopy of sedimentary pore-water humic substances: a simple tool for retrospective analysis of lake ecosystems

Purpose

The feasibility of applying the method of single-scan fluorescence emission spectra of humic substances (HSs) without chemical pretreatments of sediment pore-water samples was tested to ascertain the past productivity and sources of organic matter of lakes.

Materials and methods

Sediment samples were collected from ten Estonian lakes (located between 57°36′ and 59°25′N and 22°12′ and 26°59′E) covering all levels of the trophic scale. The height (fluorescence intensity), location (fluorescence maximum) and shape (fluorescence index, the ratio of intensities at the emission wavelengths 450 and 500 nm) of the fluorescence emission spectrum at an excitation of 340 nm were under consideration.

Results and discussion

Pore-water humic substances (pwHSs) from sediments of eutrophic lakes had generally a high fluorescence intensity and fluorescence index and their fluorescence maximum was located at shorter wavelengths. Characteristic features of pwHSs from oligotrophic lakes were low fluorescence intensity, emission of maximum fluorescence at shorter wavelengths and high fluorescence index values. Pore-water humic substances from sediments of dystrophic lakes were characterized by a low fluorescence intensity and fluorescence index and their spectral peak was shifted to longer wavelengths. The study also demonstrated that a shift in the peak location of pwHSs fluorescence was accompanied with a change in the C/N ratios of sedimentary organic matter, and the alteration in the fluorescence index of pwHSs was synchronous with the changes in their molecular weight.

Conclusions

The obtained results suggest that fluorescence spectroscopy of pwHSs without using chemical pretreatments has a great potential in the reconstruction of past lake conditions.     

Physical and chemical characteristics of three acidic,oligotrophic lakes and their watersheds in Kejimkujik National Park,Nova Scotia

Southwestern Nova Scotia receives acidic precipitation (average pH 4.5 to 4.6), and there are many waterbodies that are susceptible to acidification. This study characterizes the physical and chemical features of three remote, oligotrophic lakes and their watersheds in this region, in order to provide baseline information against which assessments can be made of changes caused by atmospheric depositions. Two of the lakes are small (<0.5 km²) and on headwater watersheds: Beaverskin Lake has an almost completely forested watershed and is moderately acidic and clear (pH 5.3, 5 Hazen units), while the watershed of Pebbleloggitch Lake is about 2/3 forested and 1/3 covered by a Sphagnum-heath bog, and its water is very acidic and highly colored (pH 4.3, 87 Hazen units). Kejimkujik Lake is much larger, its watershed is mostly forested but also contains some boggy terrain, its water is intermediate in acidity and color (pH 4.9, 65 Hazen units), and because it drains a much larger area of watershed it has relatively large concentrations of Ca, Mg, Na, Cl, and SO₄.     

Natural acidification of lakes and rivers in Japan: The ecosystem of Lake Usoriko (ph 3.4–3.8)

Most of the acid lakes and rivers in Japan have been influenced by acid waters of volcanic origin. Acidophilic and acidotolerant organisms are observed in these natural acid lakes and rivers, each of which has its own ecosystem and history. Lake Usoriko (pH 3.4–3.8) is a lake which.has been acidified by strongly acid water containing sulfuric acid and hydrochloric acid. The ecosystem of Lake Usoriko consists of aquatic vascular plants, aquatic bryophytes (Drepanocladus fluitans, etc.), algae, fish (Tribolodon hakonensis), insects, Zooplankton, fungi and bacteria. The paleosediment distributed along Lake Usoriko's northern shore is partly silicified and it contains fossils of a moss (Drepanocladus fluitans) and diatoms such as Pinnularia braunii var. amphicephala which are also distributed in the present lake. ¹⁴C dating of the specimens in the paleosediment, shoots of D. fluitans and the wood and cones of Picea glehnii gave values in the range 11500–34000y B.P. This record shows how long Lake Usoriko has been an acid lake.     

太湖、巢湖、龙感湖沉积物中磷的地球化学形态

A sequential extraction method for the fractionation of phosphorus （P） in lake sediments was used to analyze phos- phorus fractions of sediments taken from three large, shallow. eutrophic freshwater lakes of China-Talhu Lake. Chaohu Lake, and Long.an Lake. All three lakes are located in the lower reaches of the Changjiang River （Yangtze River）. In Taihu Lake and Chaohu Lake, algae blooms occurred every year, while Longgan Lake was a macrophyte-dominated lake. Results showed that exchangeable phosphorus fractions were much higher in the eutrophic lake sediments than in the macrophyte-flourishing lake sediment. Also, the ratio of Fe：P in the sediments of the algae-predomlnant lakes was generally much lower than that in the macrophyte-predominant lakes. Thus, the geochemical fractions of phosphorus in sediments had a closer relationship with the type of aquatic vegetation.     

Detection of methane biogenesis in a shallow urban lake in summer

Purpose

Sediments are significant methane (CH₄) sources of the atmosphere. However, the mechanisms of CH₄ generation remain unclear in sediments of shallow urban lakes. The aims of this investigation were to study the characterization of environmental parameters, CH₄ generation, and methanogen populations in Wulongtan Lake, China, which is affected solely by nonpoint pollution.

Materials and methods

The concentrations of CH₄ in the atmosphere and of vertical sediment profiles and the methane flux at the air–water interface were monitored in the summer of 2012. Environmental parameters in the water column and in the vertical sediment profiles were assayed. The activities of cellulose, saccharase, polyphenol oxidase, and urease enzyme and 16S rRNA gene copy numbers of Archaea (ARC), Methanobacteriales (MBT), Methanococcales (MCC), Methanomicrobiales (MMB), Methanosarcinales (MSL), Methanosarcinaceae (MSC), and Methanosaetaceae (MST) were determined in the vertical sediment profiles. The abundance of methyl–coenzyme A reductase (ME) gene was also determined to evaluate the total activities of methanogens.

Results and discussion

High CH₄ concentrations were detected in the atmosphere above the lake, and the mean CH₄ flux at the air–water interface was 6.21 mM m^?2 h^?1. Dissolved oxygen decreased with an increase of water depth. Eh values and CH₄ contents increased, but total nitrogen, water content, and total organic carbon (TOC) decreased with an increase of sediment depth. Cellulose, saccharase, polyphenol oxidase, and urease activities were detected in all sedimentary layers. The copy number of 16S rRNA gene (wet weight) for Archaea reached the highest value in the surface sediment. Copy numbers of ME were higher at 12–33 cm than at 0–6 cm. In general, abundances of MMB, MBT, and MSL were higher than that of MCC in the same sedimentary layer. 16S rRNA gene copy numbers of MST decreased with increasing depth, while MSC was higher at 18–27 cm than that at other sections. These indicate that hydrogenotrophic, aceticlastic, and methylotrophic pathways coexisted in these sediments. Principal component analysis revealed that in the sediments, the level of CH₄ was closely related with several parameters including saccharase, urease, ME, and MBT, while TOC content was related to CEL, MST, ARC, water content, and Eh.

Conclusions

High CH₄ release potential was detected in this shallow urban lake and can be ascribed to the anaerobic aquatic environment, bacterial enzyme activities, and methanogens. The orders MMB, MBT, and MSL were dominant in sediments for CH₄ production. The presence of orders or families of methanogens might be determined by the types of available substrates in lake sediments.     

Specific rates of net methylmercury production in lake sediments

Specific rates of Hg (²⁰³HgCl₂) methylation and McHg (¹⁴CH₃HgI) demethylation in aerobic and anaerobic conditions were determined in samples of surface sediments (0 to 2 cm) taken from five small headwater lakes in Southern Finland. The highest rates of methylation were measured in anaerobic conditions. However, the importance of aerobic methylation increased with increasing Fe and Mn content in sediment. There was little difference between aerobic and anaerobic demethylation. The results demonstrate that the net McHg production in lake sediments depends on the individual characteristics of the lake, particularly pH and and sediment properties. These characteristics seem to affect demethylation in anaerobic conditions and methylation in aerobic conditions.