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Vertical transport caused by mixing is essential for understanding physical processes in lakes. However, mixing processes in shallow lakes are not well understood because of the lack of turbulence measurements. This study presents observations of vertical mixing and oxygen flux in a shallow lake, Lake Kitaura of the Lake Kasumigaura continuous lake system, which is located along the central eastern coast of the Japanese mainland. Mooring and microstructure surveys were conducted in August 2020. The vertical eddy diffusivity was estimated from the Ellison scale using high-frequency sampled temperature data from the mooring location, and the estimations were consistent with those observed from the microstructure profiler. The estimated eddy diffusivity revealed a mixing structure and oxygen flux in the lake during the study period. The daily cycle of stratification and mixing was caused by daily heating due to solar radiation and by winds from daily sea breezes, respectively. The daily stratification maximum occurred around noon, which suppressed vertical mixing. Vertical mixing was intensified due to sea breezes in the afternoon, which led to movement of oxygen from the surface layer to the bottom layer. The maximum vertical mixing was observed at 18:00. The oxygen concentration did not increase during nighttime when the negative surface heat flux was observed; thus, nighttime cooling may not primarily contribute to the vertical oxygen supply.

During the last four decades, substantial changes have been observed not only in tripton concentrations but also in other limnological parameters in Lake Kasumigaura, a shallow eutrophic lake. A previous study reported that tripton concentration showed a positive correlation with sediment water content (WC) and a negative one with sediment ignition loss. To explain the changes in sediment WC, we conducted an analysis of the changes in mineral concentrations and sediment WC during 1979–2016 to determine their relationship. High mineral concentrations in water throughout the lake were observed in the latter half of the 1990s and higher WC in sediments followed with a time lag of a few years, resulting in significant correlations. The relationship between them was confirmed in laboratory experiments that indicated swelling of sediments (slowdown of sediment compaction) by the addition of NaCl into the water-sediment system. In contrast, significantly lower SS concentrations in overlying waters observed in higher mineral conditions were similar to previous studies. The significantly low annual precipitation during 1994–1997 probably caused high Na+ and Cl- concentrations in the lake waters in the latter half of the 1990s. The combined influence of sea-spray aerosol and evaporation was suspected to be the cause of high salt concentrations. The intrusion of salty water through the downstream gate and/or surrounding construction structures was another possibility.

In shallow polymictic lakes where the resuspension of sediments frequently occurs, tripton (non-living particulate materials) can affect physical, chemical and biological processes. Over the last four decades, substantial changes in tripton concentrations and therefore limnological parameters have been observed in Lake Kasumigaura, a shallow eutrophic lake. To build a conceptual model of the various relationships between these, we analyzed data on monthly water and yearly sediment quality at several stations together with meteorological information. Large short-term and decadal variations were characteristics for tripton. Annual averages in tripton concentration changed substantially (e.g., from 3 to 25 mg l−1 at the lake center), peaking in the early 2000s, when phosphorus and silicon in water were high, and we observed a large proportion of diatoms in phytoplankton and low light penetration. Tripton concentration showed a positive correlation with sediment water content (WC) and a negative one with sediment ignition loss (IL) with a few years’ delay, confirming the results of previous hydraulic experiments. Using these parameters, multiple regression models for estimating tripton concentrations were constructed and showed good performances (e.g., adjusted r2 = 0.61 at the center). Furthermore, changes in inorganic ions in water, probably resulting from meteorological variations, were inferred to bring about changes in sediment WC. On the other hand, the amount of Microcystis sp. in water was suspected to govern the sediment IL. We reveal many processes (physical, chemical, biological and meteorological phenomena) related to tripton, and thus, their relationships could be considered a conceptual model for a shallow lake.

To investigate differences in culturable freshwater bacterioplankton between summer and winter, we examined specific bacterioplankton in the low temperature surface water of two shallow, hypereutrophic, temperate lakes in Japan over two winters, and compared our results with the previously published data. We used the size-exclusion assay method (SEAM), a cultivation-based approach that simply and effectively isolates typical freshwater bacterioplankton. The specific clusters detected in the winter samples were the Rhodoferax sp. BAL47 cluster (LimA and LimC), a predominant and ubiquitous freshwater lineage, and the LiUU-5-340 cluster. To confirm tolerance to low water temperature of winter-specific groups, we also compared growth rates at 5°C among several pure strains of typical freshwater bacterioplankton clusters belonging to the Rhodoferax sp. BAL47, Polynucleobacter necessarius, GKS98, LiUU-5-340, and IRD18C08 of the class Betaproteobacteria, and the Luna-1 and Luna-2 of the class Actinobacteria. Specific detectability of freshwater bacterioplankton clusters/subclusters by SEAM in the winter sample substantially correlated with the low temperature-specific growth characteristics of each isolate. Response to water temperature is a key control factor in freshwater bacterioplankton assemblage composition. These results provide important insights into the specific response to water temperature of several ubiquitous culturable freshwater bacterioplankton clusters inhabiting a temperate climate zone.

Summary The free-living, cosmopolitan, freshwater betaproteobacterial bacterioplankton genus Polynucleobacter was detected in different years in 11 lakes of varying types and a river using the size-exclusion assay method (SEAM). Of the 350 strains isolated, 228 (65.1%) were affiliated with the Polynucleobacter subclusters PnecC (30.0%) and PnecD (35.1%). Significant positive correlations between fluorescence in situ hybridization and SEAM data were observed in the relative abundance of PnecC and PnecD bacteria to Polynucleobacter communities (PnecC + PnecD). Isolates were mainly PnecC bacteria in the samples with a high specific UV absorbance at 254 nm (SUVA254), and a low total hydrolysable neutral carbohydrate and amino acid (THneutralCH + THAA) content of the dissolved organic matter (DOM) fraction, which is known to be correlated with a high humic content. In contrast, the PnecD bacteria were abundant in samples with high chlorophyll a and/or THneutralCH + THAA concentrations, indicative of primary productivity. With few exceptions, differences in the relative abundance of PnecC and PnecD in each sample, determined using a high-sensitivity cultivation-based approach, were due to DOM quality. These results suggest that the major DOM component in the field, which is allochthonously or autochthonously derived, is a key factor for ecological niche separation between PnecC and PnecD subclusters.

The relationship between the formation of river water quality and the land use of river watershed at Lake Kasumigaura (Ls. Nishiura and Kitaura) was investigated in this study using measured in situ data and data calculated using a geographic information system (GIS). NO3-N concentrations in river water were high, particularly in L. Kitaura watershed. The NO3-N concentrations increased at a low-water-temperature period and decreased with dilution after rainfall. The relationship between river water quality and land use characteristics in river watershed was examined by principal component analysis. The nitrogen concentrations in river water were correlated with the number of livestock, upland field and forest area in their watershed at ordinary flow. A high correlation was confirmed particularly between the number of pigs and NO3-N concentration. Paddy field and urban area were correlated with phosphorus, organic matter and particulate matter concentrations. During flood due to rainfall, the nitrogen concentrations in small-scale rivers showed a high correlation with only the number of pigs.

Binuclear ruthenocenes bridged by ethenes and thiophene derivatives, Rc–CH CH–Z–Rc and Rc ∗ –CH CH–Z–CH CH–Rc ∗ (Z = thiophene, thieno[3,2- b ]thiophene, and 2,2′-bithiophene; Rc = ruthenocenyl, R ∗ = 1′,2′,3′,4′,5′-pentamethylruthenocenyl) were prepared. These complexes showed a one-step two-electron redox wave in the cyclic voltammograms, in contrast to the benzenoid-bridged dinuclear ruthenocenes. The chemical oxidation of the Rc–CH CH–Z–Rc complexes gave no stable oxidized species. The two-electron oxidized species of the Rc ∗ –CH CH–Z–CH CH–Rc ∗ complexes were comparably stable and contained a fulvene-complex type structure.

生体試料中のジフェニルアルシン酸（DPAA）定量法として，試料のアルカリ分解液中のDPAAを溶媒抽出及び固相抽出法を利用して選択的かつ効率よく抽出し，黒鉛炉原子吸光法に供する方法を開発した．試料のアルカリ分解液中のDPAAは，抽出補助剤として，ヨウ化カリウム及びシステインを添加することにより効率よく抽出でき，固相抽出法により共存する他の無機及び有機ヒ素化合物等の夾雑物からDPAAを単離できた．本法で作成した検量線は，0～50 ng/mlの範囲で直線性を示し（r ＝0.999，n ＝3），10 ng/mlの標準液を5回繰り返し抽出した際の測定値の相対標準偏差は約3％ であった．また，マウス脳及びDPAA投与マウス脳，肝，腎を用いての回収率は87～98％ で，測定値の相対標準偏差は15％ 以下であり，臓器試料での検出下限値は50 ng/gである．

A method for the determination of diphenylarsinic acid (DPAA) in plants and biological samples by high-performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS) using a stable isotope labeled compound as an internal standard is described. After the sample was decomposed in 2 M sodium hydroxide by heating, an internal standard (DPAA-13C12) was added to the solution. The decomposed solution was shaked with a mixture of chloroform and n -hexane (1 : 4). After centrifugal separation, the organic layer was discarded. The aqueous layer was then adjusted to ca. 0.5 M hydrochloric acid solution, and cysteine and potassium iodide were added in order to extract DPAA efficiently in the chloroform. Extraction was carried out twice, and the chloroform layers were combined and evaporated. The extracts were dissolved in methanol or water in case of plants. When a colored solution was obtained, it was purified by an Oasis HLB solid-phase cartridge. In biological materials, the extracts were dissolved in a small volume of methanol, and then 0.5% nitric acid solution was added. When the mixture got cloudy, it was passed through a 0.20 μm of membranfilter. DPAA in the final solution was analyzed by HPLC/MS/MS. HPLC separation was performed with an ODS column using a linear gradient elution of an acetonitrile-water system containing 0.1% trifluoroacetic acid as the mobile phase. MS/MS was used in multiple reaction monitoring (MRM), employing electrospray ionization. The proposed method could determine DPAA in plants and biological samples with an average recovery of 99.4%. The coefficient of variation was as low as 6%. The calibration curve was linear between 0 and 30 ng/ml.

固相抽出法により簡便にジフェニルアルシン酸（DPAA）を単離・濃縮し，はん用性の高い黒鉛炉原子吸光法で測定する検討を行った．DPAA溶液20 μlと0.025％ マグネシウム液 20 μlを黒鉛炉原子吸光光度計に導入することで高感度に測定することが可能になった．固相にエタノール5 mlを負荷後，精製水 20 mlを負荷し，固相を活性化させた．次に，試料液を負荷後，精製水15 mlを負荷し固相を洗浄した後，エタノール6 mlを負荷しDPAAを溶出した．夾雑物として各種ヒ素化合物，陽イオン，陰イオン，フミン酸を添加しDPAAの測定に対する影響を検討した結果，フェニルアルソン酸は1 μgまでが許容範囲でそれ以上では，固相に吸着した．更に，鉄イオンがDPAAの回収率を低下させたが，EDTAを試料液に添加することで解決した．井戸水に対する添加回収率は97％ で変動係数は2.3％，海水では102.7％ で変動係数は3.4％ と良好であった．また，検出限界は井戸水で0.028 ppb（ヒ素換算値0.008 ppb），海水で0.175 ppb（ヒ素換算値0.05 ppb）であった．

The inhabitants living in a specific region of Kizaki area in Kamisu-town, Ibaraki Prefecture exhibited uncommon clinical central nervous system symptoms. A graphite furnace atomic absorption spectrophotometer detected markedly elevated concentration of arsenic (4.5 ppm) in their drinking well water. Further investigation using HPLC, GC/MS and HPLC/ICP/MS demonstrated that the structures of the arsenic were bis(diphenylarsine)oxide (BDPAO), diphenylarsinic acid (DPAA) and phenylarsonic acid (PAA), compounds that can be derived from the chemical warfare agents, diphenylchloroarsine (DA) and diphenylcyanoarsine (DC). The predominant form of the arsenic compound in the well water was DPAA (maximum 15 ppm), so that it was calculated that the inhabitants ingested 11‐30 mg of DPAA daily. This is the first report of inhabitants that were injured by drinking well water contaminated with organic arsenic compounds that were likely derived from chemical weapons.

The free-living, cosmopolitan, freshwater betaproteobacterial bacterioplankton genus Polynucleobacter was detected in different years in 11 lakes of varying types and a river using the size-exclusion assay method (SEAM). Of the 350 strains isolated, 228 (65.1%) were affiliated with the Polynucleobacter subclusters PnecC (30.0%) and PnecD (35.1%). Significant positive correlations between fluorescence in situ hybridization and SEAM data were observed in the relative abundance of PnecC and PnecD bacteria to Polynucleobacter communities (PnecC + PnecD). Isolates were mainly PnecC bacteria in the samples with a high specific UV absorbance at 254 nm (SUVA(254) ), and a low total hydrolysable neutral carbohydrate and amino acid (THneutralCH + THAA) content of the dissolved organic matter (DOM) fraction, which is known to be correlated with a high humic content. In contrast, the PnecD bacteria were abundant in samples with high chlorophyll a and/or THneutralCH + THAA concentrations, indicative of primary productivity. With few exceptions, differences in the relative abundance of PnecC and PnecD in each sample, determined using a high-sensitivity cultivation-based approach, were due to DOM quality. These results suggest that the major DOM component in the field, which is allochthonously or autochthonously derived, is a key factor for ecological niche separation between PnecC and PnecD subclusters.