Paddy herbicide inputs in the entire river inflow reaching Lake Biwa, Japan

This study estimated the inputs of four paddy herbicides in the entire river inflow reaching Lake Biwa, the largest lake in Japan, which serves as a water resource for 14 million people. The Uso River and the Hino River, the main contaminated rivers among the inflow rivers, were selected as daily and hourly monitoring sites to provide data on the seasonal trends in the concentration and load of herbicides and to determine the effect of rainfall events on load. The monitoring was also performed four times in 15 inflow rivers. The total input to the lake was calculated from the loads during fine weather conditions and additional loads during rainfall events. The former based on the lumped load from the two rivers and by prorating for the 15 rivers, and the latter was estimated from the relation between precipitation and increased load rate. The annual losses of herbicide from the basin to Lake Biwa were estimated to be 14.5% for bromobutide, 3.0% for pretilachlor, 5.2% for molinate, and 8.8% for simetryn. The loads caused by rainfall events accounted for 9%–18% of the total annual loads.     

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     

Determining suitable submerged macrophyte biomass in terms of dissolved oxygen concentration and biodiversity in the South Basin of Lake Biwa,Japan

Limnology - After a regime shift in eutrophic lakes from a turbid water state with phytoplankton blooms to a clear water state, troublesome issues caused by excessive growths of aquatic plants...     

Dynamics of methane in mesotrophic Lake Biwa,Japan

As a part of a core project of IGBP (International Geosphere-Biosphere Programme), distribution, production, oxidation and transport processes of methane in bottom sediments and lake water in a mesotrophic lake (Lake Biwa) have been studied with special reference to the spatial heterogeneity of each process. In this study, we attempted to synthesize previously reported results with newly obtained ones to depict the methane dynamics in the entire lake. The pelagic water column exhibited subsurface maxima of dissolved methane during a stratified period. Transect observation at the littoral zone suggested that horizontal transportation may be a reason for the high methane concentration in epilimnion and thermocline at the offshore area. Tributary rivers and littoral sediments were suggested to be the source. Observations also showed that the internal wave caused resuspension of the bottom sediment and release of methane from the sediment into the lake water. The impact of the internal waves was pronounced in the late stage of a stratified period. The littoral sediment showed much higher methanogenic activity than the profundal sediments, and the bottom water of the littoral sediments had little methanotrophic activity. In the profundal sediment, most of the methane that diffused up from the deeper part was oxidized when it passed through the oxic layer. Active methane oxidation was also observed in the hypolimnetic water, while the lake water in the epilimnion and thermocline showed very low methane oxidation, probably due to the inhibitory effect of light. These results mean a longer residence time for methane in the epilimnion than in the hypolimnion. Horizontal inflow of dissolved methane from the river and/or littoral sediment, together with the longer residence time in the surface water, may cause the subsurface maxima, which have also been observed in other lakes and in the ocean.     

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.     

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.     

Seasonal and site-specific variability in terrigenous particulate organic carbon concentration in near-shore waters of Lake Biwa, Japan

Identifying sources of particulate organic matter (POM) is important for clarifying fundamental mechanisms by which lake food webs are sustained. We determined carbon and nitrogen stable isotope ratios of POM in near-shore waters of Lake Biwa, a large, meso-eutrophic lake in Japan, to estimate relative contributions of terrigenous particulate organic carbon (T-POC), plankton-derived POC (P-POC) and epilithon-derived POC (E-POC) to POC in near-shore waters. Samples were collected during different months (November, February, May and July) at 29 sites located near the mouth of tributary rivers with different discharge and catchment land use. The data revealed that POC mainly consisted of P-POC and T-POC, with relative contributions varying widely over season and among locations. E-POC generally contributed little to the near-shore POC. Path analyses revealed that the concentration of riverine POC whose isotopic signatures were similar to those of rice straws increased with a larger %paddy field area in the catchment of tributary rivers, which subsequently enhanced T-POC inputs to near-shore waters through riverine transportation. Furthermore, our results suggested that T-POC contribution was influenced, with a time lag, by wave-driven turbulence and shore topography, which appear to affect sedimentation and resuspension of T-POC.     

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.     

Dissolved oxygen concentration and emergence of sea trout fry from natural redds in tributaries of the River Rhine

A total of 4289 sea trout Salmo trutta alevins swam up from six of eleven covered redds. They emerged only from those redds where the mean dissolved oxygen level was >6·9 mg l⁻¹ and the Sorting coe.cient, an indicator of intragravel void space, was <5. These results are discussed in relation to field studies using artificially buried salmonid eggs but also in relation to the ongoing programme for the reintroduction of Atlantic salmon in tributaries of the River Rhine.     

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.     

Characteristic seasonal variation in dissolved uranium concentration induced by the change of lake water pH in Lake Biwa,Japan

Epilimnion-dominated profiles of dissolved uranium (U) have been observed during summer in an oxygenated Japanese lake, Lake Biwa, contrary to the commonly accepted view that U shows conservative behavior in oxygenated seas and lakes. Monthly observations were conducted to reveal the mechanism for such characteristic distribution and geochemical behavior of dissolved U in the lake. In the surface water, dissolved U concentration started to increase in spring, peaked in summer, and decreased from autumn to winter. In contrast, the concentration remained almost constant in the middle layer (40 m depth) and decreased slightly in the bottom layer (70 m depth) throughout the stagnation period. Mass balance calculations of U suggest that the major mechanism for seasonal variations in the surface layer is the supply of U, not via water inflow from the watershed, but by internal chemical reactions within the lake. A laboratory experiment using the lake water and sediment demonstrated that U was desorbed from and adsorbed onto sediment in response to variations in lake water pH. From these results, it is inferred that the seasonal variation in the concentration of dissolved U in the epilimnion results mainly from the desorptive/adsorptive processes of U between sediment/water interface in response to variation in water pH, which is affected by biological activity in the lake.     

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.     

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...     

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.     

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.     

Occurrence of viable photoautotrophic picoplankton in the aphotic zone of Lake Biwa, Japan

The distribution and abundance of photoautotrophic picoplankton(PPP. Synechococcus group) in the aphotic bottom sediments ofLake Biwa were investigated by direct counting and viable counting(most probable number, MPN) methods. In the surface layer ofbottom sediments (0–1 cm). where large PPP blooms occurredin the past 5 years, >10⁵ cells cm^–3 of PPP were foundto be viable throughout the year. Furthermore, the density ofPPP deposited on the sediment surface (0–0.1 cm) was oneorder of magnitude higher (MPN = 1.3 x 10⁶ cells cm^–3.direct count = 9.9 x 10⁶ cells cm^–3) than that of bulkedsurface sediments (0–1 cm). Even in the deeper layer (13–14cm) of bottom mud, viable PPP were still found (10¹ cells cm^–1.In winter, viable PPP in the aphotic bottom sediments were 10⁴–10⁵times greater per Unit volume than those in the euphotic lakewater. Since the aphotic bottom sediments have high levels ofPPP, as well as high growth potential (high ratio of viablecount/total direct count), they are likely to seed PPP bloomsin the North Basin of Lake Biwa.     

Production Rate of Planktonic Bacteria in the North Basin of Lake Biwa, Japan

Vertical and seasonal variations in the cell number and production rate of planktonic bacteria were investigated at a pelagic site (water depth, ca. 72 m) of the north basin of Lake Biwa during April to October 1986. The [methyl-³H]thymidine uptake rate into a cold trichloroacetic acid-insoluble fraction and the frequency of dividing cells (FDCs) were measured for each sample as indices of the bacterial production rate. The seasonal data of bacterial number, thymidine uptake rate, and bacterial growth rate based on the FDCs were correlated with one another (rank correlation analysis, P < 0.05). These bacterial variables were not correlated positively with the chlorophyll a concentration. Vertically, the maxima of both bacterial number and the thymidine uptake rate were found in the euphotic zone. The direct counting of bacteria and the measurements of thymidine uptake rate combined with the size-fractionation method revealed that more than 90% of the bacterial biomass and production rate were attributed to unattached bacteria throughout the investigation period. The carbon flux estimates of bacterial production were less certain due to the variability of the conversion factor for the thymidine uptake method and that of the calibration for the FDC method, but even when the conservative range of bacterial net production rate was used (5 to 60 μg of carbon per liter per day), it can be suggested that bacterial net production in the investigated area was a significant fraction (ca. 30%) of the level of the primary production rate in the same water basin.     

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.