Sources of dissolved methane in Lake Biwa

The spatial distribution and seasonal variation in the concentration and carbon isotopic composition of dissolved methane in a river–lake ecosystem were studied in Lake Biwa, Japan, and its tributary rivers. Methane concentrations in all subsystems examined were supersaturated with respect to the atmosphere. The epilimnion showed higher concentrations of dissolved methane than the hypolimnion in the pelagic zone. Peak methane concentrations were observed at the thermocline. The largest amount of methane in the pelagic water column was recorded at the end of a stagnant period, at which the bottom water of the sublittoral zone (30m in depth) exhibited increased methane concentration. Transect observation of dissolved methane revealed three methane peaks at different water depths in the lake, and river water and the sediments in littoral and sublittoral zones were suggested to be the corresponding sources. Water at the river mouth was replete with dissolved oxygen but also contained a high concentration of methane. The present results suggest that river water and littoral sediment are potential sources of dissolved methane in Lake Biwa, and other sources, such as internal waves, are responsible for increased methane in the pelagic zone at the end of stagnant periods. Carbon stable isotope analysis indicated that there were different sources of dissolved methane, although it was difficult to identify the origins due to high variation of the isotopic composition of methane from different sources.     

Distribution of putative denitrifying methane oxidizing bacteria in sediment of a freshwater lake, Lake Biwa

Methane oxidation coupled to denitrification is mediated by 'Candidatus Methylomirabilis oxyfera', which belongs to the candidate phylum NC10. The distribution of putative denitrifying methane-oxidizing bacteria related to "M. oxyfera" was investigated in a freshwater lake, Lake Biwa, Japan. In the surface layer of the sediment from a profundal site, a phylotype closely related to "M. oxyfera" was most frequently detected among NC10 bacteria in PCR analysis of the 16S rRNA gene. In the sediment, sequences related to "M. oxyfera" were also detected in a pmoA gene library. The presence of NC10 bacteria was also confirmed by catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). Denaturing gradient gel electrophoresis (DGGE) and quantitative real-time PCR indicated that the abundance of the "M. oxyfera"-related phylotype was higher in the upper layers of the profundal sediment. The horizontal distribution of the putative methanotrophs in lake sediment was also analyzed by DGGE, which revealed that their occurrence was restricted to deep water areas. These results agreed with those in a previous study of another freshwater lake, and suggested that the upper layer of the profundal sediments is the main habitat for denitrifying methanotrophs.     

Biotic and abiotic methane releases from Lake Biwa sediment slurry

To determine the rate and mechanism of CH₄ production in Lake Biwa sediment, slurry was prepared and incubated. Surface sediment (sed) slurry (1.5–6cm) showed a CH₄ release rate (4.9–9.5nmolg-dry-sed^–1 day^–1) higher than that observed in the 5- to 10-cm sediment slurry (0.2–2nmolg-dry-sed^–1 day^–1). Methane release from the surface (1.5–6cm) sediment slurry was biotic and was inhibited by addition of 2-bromoethanesulfonate (BES, an inhibitor of CH₄ production), whereas that from 5- to 10-cm sediment slurry was abiotic. The addition of BES, HNO₃, and O₂ showed no effect on the CH₄ release rate from the 5- to 10-cm sediment slurry. In addition, tracers (NaH¹³CO₃, ¹³CH₃COONa) were not incorporated into the released CH₄. However, ¹³C of CH₄ released from the 5- to 10-cm sediment slurry (–74.0 ± 0.6) indicated that this CH₄ was produced by bacterial metabolism in the past, stored by adsorption on the surface of clay minerals in the sediment, and then released abiotically by desorption from the sediment slurry as a result of a decrease in hydraulic pressure and CH₄ concentration in the pore water. This CH₄ stored by adsorption could be extracted by autoclaving. In the sediment below 5cm, bacterial activity for CH₄ production ceased, possibly because of the limitated availability of H₂. To clarify the mechanism of CH₄ production in the sediment, biotic CH₄ production and the abiotic CH₄ release found here should be estimated separately.     

Distribution of dissolved organic carbon and dissolved fulvic acid in mesotrophic Lake Biwa, Japan

The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation–emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260–270/430–440 nm; peak B, Ex/Em = 300–310/420–430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4–8.9 min and the apparent molecular weight was estimated at 22–87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4–8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r² = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25–0.43 mg C l⁻¹ and accounted for 15%–41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.     

Carbon steady-state model of the planktonic food web of Lake Biwa, Japan

1. A steady‐state model of carbon flows was developed to describe the summer planktonic food web in the surface mixed‐layer of the North Basin in Lake Biwa, Japan. This model synthesised results from numerous studies on the plankton of Lake Biwa. 2. An inverse analysis procedure was used to estimate missing flow values in a manner consistent with known information. Network analysis was applied to characterise emergent properties of the resulting food web. 3. The system strongly relied on flows related to detrital particles. Whereas primary production was mainly by phytoplankton >20 μm, microzooplankton were active and mainly ingested detritus and bacteria. 4. The main emergent property of the system was strong recycling, through either direct ingestion of non‐living material by zooplankton, or ingestion of bacteria after degradation of detritus to release dissolved organic carbon.     

Population dynamics and production of Jesogammarus annandalei, an endemic amphipod, in Lake Biwa, Japan

SUMMARY 1. Population dynamics and production of Jesogammarus annandalei , an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning in early autumn. They preferred low temperature (<12 °C) and their spatial distribution varied seasonally and accordingly.
2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m⁻² and decreased towards summer and the biomass (0.01∼3.6 g C m⁻²) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago.
3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m⁻² year⁻¹) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo- and mesotrophic lakes.
4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.     

The response of the sedimentological regime in Lake Kinneret to lower lake levels

The surface level of Lake Kinneret is regulated to remain between –209 m and –213 m msl. During the stratified period, soluble reactive phosphorus (SRP), ammonium (NH _inf4 ^sup+ ) and dissolved sulphide (H₂S) accumulate in the hypolimnion. The concentration of these solutes, which are direct and indirect products of the decomposition of organic matter, increase considerably in summers with lower lake levels. A numerical model describing depth-averaged hypolimnion and epilimnion current velocities for high and low lake levels was adapted for Lake Kinneret. Simulated hypolimnetic currents were shown to be stronger for low lake levels as a result of the fact that low lake levels are characterized by a thinner hypolimnion while the thickness of the epilimnion remains unchanged. We suggest that the stronger hypolimnetic currents have the following consequences: 1. turbulence is induced, 2. the enhanced turbulence results in higher resuspension, 3. because SO⁴= is available to bacteria on resuspended particles, mineralization rates are enhanced, and 4. focusing of fine sediments and associated organic matter to the pelagic zone is enhanced.     

The water bloom of Cyanobacterial picoplankton in Lake Biwa,Japan

Unicellular autofluorescent picoplankton ranging from 0.4 to 1.5 µm in diameter were found to be a significant component of phytoplankton in the North Basin of Lake Biwa during early summer in 1989 and 1990. The abundance of these picoplankton varied seasonally by about three orders of magnitude with one maximum of up to 10⁶ cells ml^–1. Bloom-forming picoplankton were isolated by dilution and further cultivated in liquid medium. Three clones were found to be representative species of the bloom. Using epifluorescence and electron microscopy as well as absorption and fluorescence emission spectroscopy, we examined these clones according to shape and pigment composition. They have ringlike thylakoids, are photosynthetically active and have no nuclear envelope. The cyanobacterial clones isolated represent three types containing phycobilisomes with either phycocyanin or phycoerthrin as the dominant accessory pigment. They are described here as three new species, two phycoerythrin-rich types and one phycocyanin-rich type, all of them belonging to the Synechococcus group. The differences found by fluorescence emission of isolated clones are discussed with respect to in situ strain identification.     

Seasonal Distribution of Cyprinid Herpesvirus 3 in Lake Biwa,Japan

The seasonal distribution of the cyprinid herpesvirus 3 (CyHV-3) in Lake Biwa, Japan, was investigated. CyHV-3 was distributed all over the lake 5 years after the first outbreak. The mean concentration of CyHV-3 in water showed annual oscillation, with a peak in the summer and a trough in winter. Our results suggested that CyHV-3 is present at high density in reductive environments, such as reed zones and turbid or eutrophic water.A novel fatal disease of fish caused by cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus or carp interstitial nephritis and gill necrosis virus, which infects the common carp (Cyprinus carpio carpio) and ornamental koi (Cyprinus carpio koi) was reported at the end of the 1990s, and it has spread rapidly worldwide (13). In 2003, the first mass mortality in Japan was observed in Lake Kasumigaura, Ibaraki Prefecture (17), and the disease immediately spread all over the country.Lake Biwa is the largest and oldest freshwater lake in Japan. It occupies 670 km², and the total length of the coastline is 241 km. The first and relatively small-scale mortality caused by CyHV-3 was observed in the autumn of 2003 in Lake Biwa, and mass mortality occurred in the following spring, in which more than 100,000 carp died (10). Thus, the disease is a great threat not only to the cultivation industry and koi collectors but also to the natural carp population. Carp is regarded as an ecological engineer that has an impact on freshwater ecosystems, and therefore, mass mortality will affect the entire lake ecosystem (10).The pathogenesis and diagnosis of the disease have been studied intensively, but the dynamics of CyHV-3 has not been clarified. Recently, the major portal of CyHV-3 entry was reported to be fish skin (2), which means that infection via water is possible. Thus, to determine the method by which the virus spreads and to evaluate the infectious risk in the environment, one must clarify the CyHV-3 dynamics in the natural environment. In the present study, we surveyed the seasonal distribution pattern of CyHV-3 in Lake Biwa, Japan, using a quantitative method.     

The budget of dissolved trace metals in Lake Biwa,Japan

A comprehensive study on the dynamics of dissolved elements (Mg, Al, Si, P, Ca, V, Cr, Mn, Fe, Ni, Zn, As, Sr, Y, W, and U) in Lake Biwa was carried out using a clean technique. Lake water samples (n = 523) were collected from six stations in the North Basin and three stations in the South Basin. River water samples (n = 178) were collected from 14 major rivers flowing into the North Basin. Rainwater samples (n = 89) were collected at Otsu. The river water was enriched with Mn, Al, Fe, P, and Zn and the rainwater was enriched with Zn, Al, Fe, and Mn compared to North Basin water during winter mixing. The residence times of dissolved species were estimated on the basis of input through the rivers and rain. The residence times for Ca, Mg, and Sr were about 8 years, the same as that for water. Mn, Al, Fe, and Zn showed the shortest residence times (0.05–0.19 year). A budget calculation suggested that more than 60% of the input of dissolved Si, P, V, Cr, Mn, Fe, Ni, and Zn was scavenged and retained in the lake sediments and/or discharged as suspended particles.     

The production-to-respiration ratio and its implication in Lake Biwa,Japan

Production-to-respiration (P:R) ratio was estimated at an offshore site of Lake Biwa in order to examine whether the plankton and benthic community is subsidized with allochthonous organic carbon, and to clarify the role of this lake as potential source or sink of carbon dioxide. The respiration rate of protozoan and metazoan plankton was calculated from their biomass and empirical equations of oxygen consumption rates, and that of bacterioplankton was derived from their production rate and growth efficiency. In addition, the carbon mineralization rate in the lake sediments was estimated from the accumulation rate of organic carbon, which was determined using a ²¹⁰Pb dating technique. On an annual basis, the sum of respiration rates of heterotrophic plankton was comparable to net primary production rate measured by the ¹³C method. However, when the mineralization rate in the lake sediments was included, the areal P:R ratio was 0.89, suggesting that Lake Biwa is net heterotrophic at the offshore site with the community being subsidized with allochthonous organic carbon. Such a view was supported by the surface water pCO₂ that was on average higher than that of the atmosphere. However, the estimate of net CO₂ release rate was close to that of carbon burial rate in the sediments. The result suggests that the role of Lake Biwa in relation to atmospheric carbon is almost null at the offshore site, although the community is supported partially by organic carbon released from the surrounding areas.     

Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa(Kütz.) Kütz. and M. wesenbergii(Komárek) Komárek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystisfluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystispopulation in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystisblooms in Lake Biwa. Microcystiscolonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystiscolonies in the deep North Basin of Lake Biwa (70 – 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystiscells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystiscolonies may survive for a long time under stable conditions of low temperature (ca. 8 °C) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.     

Seasonal variation of dissolved aluminum concentration in harmonic-type Lake Biwa, Japan

Monthly observations performed on a typical harmonic-type lake, Lake Biwa (latitude 35°15′ N, longitude 136°05′ E, Japan), showed that the particulate aluminum concentration varied around a mean value of 0.8 μM, with occasional extraordinarily high values, whereas the dissolved aluminum concentration varied, in the surface layer of the whole lake, between a minimum of 0.01 μM and a maximum of 0.30 μM, depending on the season. Although the variation in dissolved aluminum paralleled the variation in the pH of the lake water, the variation in dissolved aluminum lagged behind the variation in pH by approximately 1 month. A series of laboratory incubation experiments suggested that the supply of dissolved aluminum from, and its removal by, the suspended particulate matter involved a slow, pH-dependent reaction. The stoichiometry and the apparent equilibrium constant of this reaction were evaluated by adopting a zeolite-like structure for the surfaces of the suspended particulates. Received: December 28, 2000 / Accepted: August 22, 2001     

Spatial distribution and seasonal changes of pesticides in Lake Biwa,Japan

The spatial distribution and seasonal variation in the concentrations in Lake Biwa of pesticides used in paddy fields were studied. Lake Biwa is the largest lake in Japan and is a recognized water resource for 14 million people in the Kinki district. Samples were collected nine times from April to December 2001 at ten sites within the lake and at the mouths of six influent rivers. Weekly sampling was also carried out at a single site on an effluent river. Among the 20 pesticides analyzed, the detection frequencies in surface water were almost 100% for simetryn, bromobutide, and isoprothiolane; around 75% for molinate and pyroquilon; around 30% for three herbicides and one fungicide; and almost zero for the remaining substances. The maximum concentrations of pesticides detected frequently in the lake were in the range 0.1–0.4µgl^–1. The occurrence of a few pesticides below the thermocline may be explained by thermal stratification and vertical circulation. Although the thermocline suppressed vertical diffusion in spring and summer during pesticide application periods, a few pesticides remaining at the surface of the lake in winter were transported to the hypolimnion by vertical circulation and remained there even after the reestablishment of the thermocline. The half-lives of pesticides in the lake were estimated to be more than a year for simetryn, half a year for bromobutide, 1.5 months for molinate, and 1 month for dimepiperate. The main cause of elimination for molinate and dimepiperate was estimated to be degradation, that for simetryn was outflow, and for bromobutide both degradation and outflow were significant.     

Horizontal variability and regulation of bacterial production in Lake Biwa,Japan

Limnology - To clarify horizontal variability and regulation of bacterial production (BP), we investigated BP and environmental variables along three east–west transects (lines 12, 15, and...     

Long-term light environment variability in Lake Biwa and Lake Kasumigaura, Japan: modeling approach

Light environment variability was investigated in the two Japanese lakes Biwa and Kasumigaura, which offer a broad range of optical conditions in the water bodies due to their diverse morphometries and limnological characteristics. To elucidate their light environments, Secchi depths (SDs) were related to long-term monitored datasets of concentrations of optically active substances (OASs) using two approaches based on statistical and mechanistic models. A good estimate for the nonphytoplanktonic suspended solids (NPSS) concentration gained using a monthly factor ?? (which represents the phytoplanktonic portion in total suspended solids) from a long-term analysis was utilized to develop robust models. Using the mechanistic model, the contribution of each OAS to the SD can be understood and investigated in more detail than possible with a statistical approach, but the statistical model yields better results in terms of SD prediction. Based on the results of an analysis of the contribution of each OAS to the SD, it was clear that NPSS was the component that exerted the most influence on the light environments in the two lakes; in this respect, this study agrees with other studies that show the importance of suspended inorganic particles as the main contributor to the SD in inland waters. Using ANOVA, we analyzed how specific inherent optical properties may have changed spatially and temporally, and the results indicated that the temporal (monthly) effect was primarily responsible for the loss of accuracy in the models. In addition, the ANOVA analysis suggested that grouping the data improved the predictive performances of the statistical models. Finally, we concluded that combining the two models yields the most reliable results in terms of SD prediction and determining the contribution of each OAS to the SD at present.     

Dissolved oxygen concentration in river sediment of the Lake Biwa tributaries, Japan

The dissolved oxygen concentration in the sediment pore water downstream of rivers in the Lake Biwa basin was measured, and the factors affecting the dissolved oxygen concentration were analyzed. In August 2003, nine rivers (Sakai, Nakanoi, Hebisuna, Anziki, Yasu, Echi, Ane, Oh, and Ohura) were surveyed. The dissolved oxygen was depleted in the sediment pore water of the rivers with a high proportion of particles less than 250 μm in size. For these rivers, the difference between the dissolved oxygen concentrations of the river surface water and the pore water was large, ranging from −9.54 to −5.26 mg L⁻¹. It was found that the proportion of land turned to paddy fields has an effect on the percentage of the particles below 250 μm (standard partial regression coefficient = 0.807, p = 0.023). These results suggest that, in the Lake Biwa basin, the sedimentation of the fine particles released from paddy fields results in poor dissolved oxygen in the river sediment downstream. In addition, the water flow conditions in small- and medium-scale rivers without headwaters also affect the sedimentation of suspended particles.     

Effects of summer drawdown on cyprinid fish larvae in Lake Biwa, Japan

The effects of artificial water-level manipulations in Lake Biwa, initiated in 1992 to prevent flooding, on cyprinid fish larvae were assessed by measuring and estimating the topographic change in the reed zones. The shallow, litter-accumulated water in those zones was inhabited by larvae of two species, Carassius spp. and Cyprinus carpio. More than 70% (by volume) of these shallow areas in Lake Biwa was estimated to be lost when the water level was artificially lowered by 30cm from June to October 1997. During the low water level period, significantly fewer eggs of these cyprinid fishes were collected in a reed zone than in April and May of the same year, a comparison with a previous study suggesting that spawning of the fishes was inhibited during the period of low water level. The artificial reductions in water level probably resulted in a significant decrease in the volume of shallow water in the lake and may be linked to the drastic decline in these cyprinid fishes.