Lake Nakaumi Research Articles

中海浚渫窪地の硫化水素発生抑制における石炭灰造粒物の適正施工量

中海浚渫窪地の硫化水素発生抑制における石炭灰造粒物の適正施工量

Vertical profiles of legacy organochlorine pesticides in sediment cores from lake Nakaumi, Japan

Two sediment core samples from the brackish Lake Nakaumi in Japan were analyzed to determine the historical profiles of organochlorine pesticides (OCPs). It was observed that from the 1940s to 2005, the vertical distribution of OCPs in sediment cores reflected the temporal trend of pesticide usage in Japan. Dichlorodiphenyltrichloroethane (DDT) and its metabolites were predominant, with concentrations of 0.008–8.27 ng g−1 dry weight, and their contribution to ΣOCPs was over 58%. The results also confirmed that the DDTs in the sediment cores originated from past input. Further, even though hexachlorocyclohexanes were the most used OCP in Japan, their residual concentrations were lower than those of DDTs and chlordane related compounds (CHLs). The concentrations of CHLs were 0.163–1.539 ng g−1 dry weight, whereas hexachlorobenzene (HCB), drins, heptachlor, and mirex showed very low concentrations. Interestingly, although HCB was never registered as a pesticide in Japan, it was detected in both core samples. This HCB contamination might be attributed to pentachlorophenol. Additionally, the hierarchical cluster analysis results corresponding to both sediment cores could be classified under four groups based on a similarity of over 50%. The results also showed that the OCP burden in Lake Nakaumi for the past 60 years was 130 kg and 1153 kg at Honjo and at the center of Lake Nakaumi, respectively. Overall, the results of this study indicate that the distribution of OCPs in Lake Nakaumi reflects the trend of pesticide usage in Japan.

Determination of Flow Characteristics of Ohashi River through 3-D Hydrodynamic Model under Simplified and Detailed Bathymetric Conditions

The Ohashi River is a narrow water stream that connects two brackish lakes in Japan. Intermittent saline water intrusion often occurs in Lake Shinji from Lake Nakaumi through Ohashi River. In this study, two approaches were discussed to reproduce the hydrodynamic conditions of a morphologically complex river. In the first approach, the river sinuosity was straightened. The straightening of the river resulted in a higher flow velocity and water flux coefficient due to the reduction in the flow path and the resistance, and this approach was found to be appropriate for the reproduction of the flow velocity. However, the river shape was visually quite different from the actual river morphology. In the second approach, the prime focus was given to the shape and bathymetry to quantitively reproduce the flowrate of the saline water intrusion. This approach resulted in an underestimation of the flow velocity, which was compensated by increasing the cross-sectional area of the river. A slower flow velocity causes up to a 3-h time lag for the water mass to pass through the Ohashi River, which in principle should affect the temporal variations of the water temperature and salinity. Fortunately, as the typical time scale for water temperature and salinity fluctuations in the Ohashi River is a few days, a 3-h time lag did not cause any problems.

中海・宍道湖ラムサール条約登録 15 周年イベント報告:広域かつ県境を越えたラムサール登録湿地の一体感を目指して

中海・宍道湖ラムサール条約登録 15 周年イベント報告:広域かつ県境を越えたラムサール登録湿地の一体感を目指して

Long-term changes in the number of diving ducks wintering in Lakes Nakaumi and Shinji, Japan

Long-term changes in the number of diving ducks wintering in Lakes Nakaumi and Shinji, Japan

Impacts of Moriyama Dike opening on the water quality in summer at Honjo area, Lake Nakaumi

Impacts of Moriyama Dike opening on the water quality in summer at Honjo area, Lake Nakaumi

森山堤防開削が中海本庄水域の水質及び水生動物相へ与えた影響

森山堤防開削が中海本庄水域の水質及び水生動物相へ与えた影響

Mechanism of silicate elution by hydrogen sulfide from bottom sediment in a brackish lake

Highly concentrated dissolved silicate was detected in pore water from anoxic-reducing sediment in Lake Nakaumi, a brackish lake. Silicate concentration also simultaneously increased with total hydrogen sulfide concentration during the summer. Generally, dissolved silicate is readily adsorbed onto ferric hydroxide and precipitates in an oxidative environment. In this study, we focused on the behavior of ferric hydroxide adsorbing silicate in sediment and determined that hydrogen sulfide was the main cause of dissolved silicate elution from ferric hydroxide adsorbing silicate because the hydrogen sulfide produced via microbiological processes in the anoxic-reducing environment was reducible for other metal oxides. According to laboratory experiments, silicate was released from ferric hydroxide by reacting with sodium sulfide, causing increasing elution of dissolved silicate from anoxic-reducing sediments with increasing concentration of sodium sulfide in the solutions. This result shows that hydrogen sulfide is very crucial for silicate release under a reducing environment. Therefore, in Lake Nakaumi, silicate would be released from the bottom after ferric hydroxide adsorbing silicate reacted with hydrogen sulfide in a summer reductive environment.

Effect of hypoxic water mass in summer on the distribution of ark shells, <i>Anadara kagoshimensis,</i> in Lake Nakaumi, Japan

Effect of hypoxic water mass in summer on the distribution of ark shells, <i>Anadara kagoshimensis,</i> in Lake Nakaumi, Japan

Dispersal potential of neritic benthic foraminifera in the coastal areas of the Japanese Islands and its application to paleoenvironmental studies

In this study, we evaluated the dispersal potential of both modern and fossil benthic foraminifera in lagoon and bay environments in the Japanese Islands. Our investigations of living (stained) benthic foraminifera in Maizuru Bay (central Japan) revealed complicated lateral distributions and high temporal variation among common benthic species. Benthic foraminiferal communities in bay environments are often characterized by strong lateral trends in species richness due to frequent fluctuations in the salinity and/or dissolved oxygen content of the hypolimnion. However, our results do not show the existence of similar trends in Maizuru Bay. In fact, the dispersal of opportunistic species during intense coastal winter mixing events may account for this complexity in species diversity and evenness. Sediment trap and plankton tow observations in Lake Nakaumi (southwestern Japan) imply that benthic foraminifera utilize suspended organic matter aggregates as mobile substrates for passive dispersal. Furthermore, we examined three horizons of high fossil benthic foraminiferal abundance in the Aso-kai Lagoon (central Japan) dating to ca. AD 1070, 1400, and 1570, which were nearly coincident with coastal mixing events that were likely caused by enhanced East Asian winter monsoons. Many of the neritic benthic foraminifera in our study region appear to possess high dispersal potential, suggesting that the distribution of opportunistic benthic foraminifera may be useful in inferring the past status of the East Asian winter monsoon.

Polycyclic aromatic hydrocarbons (PAHs) in sediment cores from lakes Shinji and Nakaumi, SW Japan: A proxy of recent fire events in the watershed

PAHs in sediment cores have been investigated as a useful proxy of historic fire events in the watershed of an estuary. Oxygen poor and anoxic lake bottom conditions in estuarine brackish lakes Shinji and Nakaumi could prevent the oxidization of polycyclic aromatic hydrocarbons (PAHs) and preserve a record of PAHs in sediments. PAHs emitted from wood and/or grass burning have been analyzed for composition and concentrations during the last ∼300 years. Carbonized plant material which remained after artificial agricultural fires and heating experiments have been used to estimate fire temperature. The equation relating maximum temperature and hydrogen to carbon (H/C) atomic ratio of the charcoal is as follows: Temp (°C) = 317 x (H/C)−0.708 (R2 = 0.924, 200–700 °C).These experiments and the gas chromatography - mass spectrometry analysis of PAHs on the charcoals suggest that PAHs were primarily produced at temperatures less than 500 °C in the burning of wild plants.The PAHs contents in the two sediment cores were not related to total organic carbon (TOC) contents and emission of industrial waste, and were directly due to fire events in this region. Although TOC contents of the sediments were constant at about 0.5% in Lake Shinji and 1.0% in Lake Nakaumi before ca. 1900 CE, and increased up to about 4% due to recent nutrient discharge, the profiles of PAHs contents were completely different. The sediment cores contained the following PAHs: phenanthrene (P), fluoranthene (Fla), pyrene (Py), benz[a]anthracene (BaAn), benzofluoranthenes (Bflas), benzo[e]pyrene (BePy), indeno[1,2,3-cd]pyrene (InPy) and benzo[ghi]perylene (BghiP). The contents of 6-ring InPy and BghiP, high temperature markers, are lower than those of the other low ring Py, BaAn, Bflas and BePy, showing low to medium intensity/temperature fire events. Relatively higher BaAn contents suggest that the PAHs were carried and deposited in the sediments for a short period after the fire. The PAHs peak groups B (1830–1870 CE) and C (1920 CE) may be attributed to the burning of wood buildings in Matsue City, and the PAHs peak group period D (1970–1980 and 2000–2007 CE) could be mainly due to forest fires. The general increase in PAHs after 1850 CE could be caused by climate change after the Little Ice Age. The PAHs with maximum total peak concentrations at 30–34 cm depth (1850 CE: 4890 ng g−1) in Lake Shinji and at 8–12 cm depth (1970–1980 CE: 2030 ng g−1) in Lake Nakaumi are not at a toxic level.

Long-term Assessment of the Environmental Conditions of Lake Nakaumi (Japan) Using Multivariate AZTI Marine Biotic Index (M-AMBI)

Long-term Assessment of the Environmental Conditions of Lake Nakaumi (Japan) Using Multivariate AZTI Marine Biotic Index (M-AMBI)

Evaluation of Unified Algorithms for Remote Sensing of Chlorophyll-a and Turbidity in Lake Shinji and Lake Nakaumi of Japan and the Vaal Dam Reservoir of South Africa under Eutrophic and Ultra-Turbid Conditions

We evaluated unified algorithms for remote sensing of chlorophyll-a (Chla) and turbidity in eutrophic and ultra-turbid waters such as Japan’s Lake Shinji and Lake Nakaumi (SJNU) and the Vaal Dam Reservoir (VDR) in South Africa. To realize this objective, we used 38 remote sensing reflectance (Rrs), Chla and turbidity datasets collected in these waters between July 2016 and March 2017. As a result, we clarified the following items. As a unified Chla model, we obtained strong correlation (R2 = 0.7, RMSE = 2 mg m−3) using a two-band model (2-BM) and three-band model (3-BM), with Rrs(687)/Rrs(672) and [Rrs−1(687) − Rrs−1(672)] × Rrs(832). As a unified turbidity model, we obtained strong correlation (R2 = 0.7, RMSE = 260 NTU) using 2-BM and 3-BM, with Rrs(763)/Rrs(821) and Rrs(810) − [Rrs(730) + Rrs(770)]/2. When targeting the Sentinel-2 Multispectral Imager (MSI) frequency band, we focused on MSI Bands 4 and 5 (Rrs(740) and Rrs(775)) for the Chla algorithm. When optically separating SJNU and VDR data, it is effective to use the slopes of MSI Bands 3 and 4 (Rrs(560) and Rrs(665)) and the slopes of MSI Bands 7 and 9 (Rrs(775) and Rrs(865)).

Spatiotemporal changes in distributions of the two dominant bivalve species in an estuarine river connecting two brackish lakes, Japan

ABSTRACTSpatiotemporal changes in the biomass and abundance of the two dominant bivalve species Corbicula japonica and Arcuatula senhousia were investigated at 14 stations positioned along the Ohashi River from November 2005 to August 2007. Corbicula japonica predominantly inhabited the upstream section of the primary branch and Kensaki stream of the Ohashi River, whereas A. senhousia predominantly inhabited the downstream section of the Ohashi River. However, the distributions were highly variable among stations and seasons. Intrusions of saline water from Lake Nakaumi varied with astronomical and climatological factors, and consequently, the halocline of the Ohashi River is subject to spatiotemporal changes. Changes in distributions of C. japonica and A. senhousia occurred in relation to a flood event, indicating that environmental disturbances affect the distributions of the two bivalve species. Responses of the two species to continual saline water intrusion were also observed during the period of normal flow conditions. We suggest that the distribution of the two species is closely related to the dynamics of the ambient saline water, which is the most important factor correlated to changes in the distributions of C. japonica and A. senhousia in the Ohashi River.

Analysis of Historical Trend of Eutrophication in Lake Nakaumi, Japan, Using Concentrations of Several Index Elements in Sediment Cores

Analysis of Historical Trend of Eutrophication in Lake Nakaumi, Japan, Using Concentrations of Several Index Elements in Sediment Cores

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

ABSTRACTManagement of flood risk can benefit from a knowledge‐based approach to flooding history. However, the data on such events are often insufficient for an objective evaluation of flooding characteristics such as their intensity. Therefore, we analyzed a 14C‐dated sediment core recovered from near the Iinashi River mouth in Lake Nakaumi, western Japan, and reconstructed flooding records over the past 700 years from rock magnetism, grain‐size distribution and sediment geochemistry. The sediment core contained three black layers, characterized by high magnetic susceptibility, low anhysteretic remanent magnetization to saturation isothermal remanent magnetization ratio, high total organic carbon (TOC) to total nitrogen ratio, and coarse mean grain size, which we identified as possible flooding event deposits. Moreover, the chemical index of alteration was low and the TOC to total sulfur ratio was high in these layers, indicating intensified erosion associated with more freshwater inflow due to the flood events. We tentatively correlated the three event deposits with historical floods along the Iinashi River in AD 1596, 1666 and 1826. The variations in rock magnetic and geochemical properties suggest that the flooding in AD 1666 may have been the least extreme of the events detected in the core.

中海, 宍道湖地域における酸性硫酸塩土壌の分布とその特性 : 酸性硫酸塩土壌の特性と改良法 (第1報)

中海, 宍道湖地域における酸性硫酸塩土壌の分布とその特性 : 酸性硫酸塩土壌の特性と改良法 (第1報)

Sensitive Method for the Oxidation-determination of Trace Hydroxylamine in Environmental Water Using Hypochlorite Followed by Gas Chromatography.

We developed a method for quantifying trace NH2OH in brackish- and sea-water samples. Previously reported methods applicable to fresh water cannot be applied to such samples. We determined that interference in seawater owing to the bromide ion can be removed by the addition of phenol. In our procedure, phenol and hypochlorite solutions were added to a sample solution to oxidize NH2OH to N2O. N2O in the sample was then quantified by headspace analysis. The method is not affected by the salt content or ammonia, nitrate, or nitrite at concentrations of 300 μgN L-1 or less. It has a limit of detection of 0.2 μgN L-1, and can quantify NH2OH in natural water samples with a wide range of salinity. It was applied to samples from Lake Nakaumi, a brackish lake located in the eastern part of Shimane Prefecture, Japan.

環境保全運動の展開過程における地域性 : 中海・宍道湖の開拓・淡水化反対運動を事例として

中海・宍道湖の干拓・淡水化事業は, 1988年に地元を中心とする反対運動によって, 淡水化について事実上の中止に追い込まれた。干拓事業については1990年代半ばになって再び事業を進めるか否かが問い直されている。本稿では前者の淡水化事業が凍結されるまでの反対運動に注目し, その地域性及びその意味を明らかにする。反対運動は, 宍道湖でシジミ漁を営む漁民や生活環境・観光資源を重視する松江市民らを中心として, 広範に展開された。この一連の反対運動は, 関係する地域で一様な展開をみせたわけではなく, その発生や展開過程において顕著な地域差をみせた。それは, 鳥取県と島根県という行政領域の違い, 中海と宍道湖という直接の関心事となる対象の違い, 都市部と農村部という社会経済環境の違いという3つの視点からとらえることができる。環境保全運動は, 対象が, その存在する地域と不可分なことや, 運動の担い手がその地域の住民であること等のために, 地域性を反映したものとなる。今回の例では, 松江中心で運動が進んだために, 「宍道湖を守ろう」という性格の強い運動となった。運動を成功させるためにも, また, 当事者間の主張や利害関係を調整し, 問題を解決するためにも, 表面的な争点にとどまらず, 地域構造上の問題点に踏み込んだ環境問題の理解が求められる。

Determination of Trace Hydrazine in Environmental Water Samples by in situ Solid Phase Extraction.

A simple and rapid in situ method for the determination of hydrazine based on the concentration of aldazine compound formed by the reaction of hydrazine with p-dimethylaminobenzaldehyde was developed. This method was based on solid-phase extraction using a Sep-Pak C18 cartridge, followed by the quantification of hydrazine using a spectrophotometric method. To a sample solution of environmental water, p-dimethylaminobenzaldehyde solution was added to form aldazine by the reaction with hydrazine. The solution was passed through a Sep-Pak C18 cartridge for the adsorption of aldazine. In the laboratory, the aldazine adsorbed on the Sep-Pak C18 cartridge was eluted by passing a hydrochloric acid-ethanol (1:10) solution through the cartridge, and the color intensity of the solution was measured at 457 nm. The limit of detection for the new method was 0.2 mgN L-1 of hydrazine. The determination of hydrazine in solution was not influenced even by hydrogen sulfide and organic matter. This method was then applied to the brackish water of Lake Nakaumi in the eastern area of Shimane Prefecture, Japan. This method was used to determine hydrazine in freshwater, seawater and wastwater.