Since 1993, systemic insecticides such as neonicotinoids have been extensively applied in Japanese rice farming, leading to contamination of surface waters. These chemicals may infiltrate groundwater, which serves as a source of tap water, yet data on their occurrence in Japan remained unclear. This study investigated the contamination of neonicotinoids and their metabolites, sulfoxaflor, and triflumezopyrim in surface water and groundwater of Hii River and tap water in Izumo City, Shimane Prefecture, Japan. Monthly sampling over one year followed by the liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis showed that all water types contained at least three compounds. Mean concentration ratios across surface water, groundwater, deep groundwater, and tap water were approximately 10:8:4:1, indicating that deeper depth and water treatment may reduce concentrations. Dinotefuran showed a 100% detection with concentrations far exceeding other compounds, particularly in summer months during the rice-growing season, reaching 225 ng/L in surface water. Its two metabolites were also present throughout all environmental water compartments. The findings emphasize that systemic insecticides could persist in deep groundwater up to 20 meters, leading to their occurrence in tap water.

山陰地方の宍道湖には，暖候期に下流部の中海から遡上する海産魚と汽水魚と甲殻類がおり，多くの淡水魚，汽水魚，通し回遊魚，海産魚と甲殻類が生息する。これらの魚介類を対象とする定置網漁業などが盛んに行われていたが，1993年の冷夏多雨と1994年の猛暑渇水の異常気象を境に，ワカサギなど主要な対象魚の漁獲量が減少し漁業が衰退した。ところが，ネオニコチノイド系殺虫剤の使用が始まった1993年以降，宍道湖の動物プランクトンやオオユスリカなどの水底動物が激減し，これらを餌とするワカサギとニホンウナギの減少に繋がったことを示す研究が2019年に公表された。しかしながら，この2種とシラウオを除くその他の魚介類の変動との関係は示されていないので，宍道湖漁業協同組合に保管されている1979～2008年度までの定置網の漁獲資料から魚介類8種の漁獲量の変動を検討した。その結果，年度毎の変動は見られるものの，1995年度以降，ワカサギとニホンウナギとハゼ類の漁獲量は減少していた。同じく1995年度以降，エビ類の漁獲量は9～12月に減少しアミ類は1～3月に増加していた。これら魚介類の変動を，年毎の塩分と水温の変動と1993年から始まったネオニコチノイド系殺虫剤の使用から考察した。

Many birds migrate during late autumn and stay until early spring at Lakes Nakaumi and Shinji in San-in district, western Honshu, Japan. The numbers of diving duck species Aythya ferina, Aythya fuligula, and Aythya marila, which constitute the majority of wintering bird populations, were determined monthly during the winter season from 1999 to 2015 in the main area of L. Nakaumi, Honjo Area, which is separated from L. Nakaumi by reclamation dikes, and L. Shinji. The mean value in December and January, obtained from changes in the monthly numbers of each diving duck in the three areas, was used as the wintering number. The long-term data showed a declining trend in the numbers of A. ferina and A. fuligula wintering in L. Nakaumi and Honjo Area. In L. Shinji, long-term data for A. fuligula also showed a decline trend, however, the data for A. marila indicated an upward trend. In L. Nakaumi and Honjo Area, the main diet for these diving duck species is the mussel Musclista senhousia, while in L. Shinji they feed on Corbicula japonica. The long-term changes in wintering bird numbers were investigated in relation to changes in food availability and habitat in the two lakes.

In May 2009, Moriyama Dike was opened in Honjo area, which lies near Lake Nakaumi along the coast of San-in District, Japan, and is isolated by two dikes for land reclamation. This study was conducted to examine the impact of the opening of the dike on water quality as well as fi sh and crustacean life. We carried out a statistical analysis using monthly water-quality observations and a small catch net survey through 2004 to 2014. The analysis results showed that seawater likely flowed into Honjo, and salinity stratification developed. Furthermore, the oxygen deficient layer became thicker and progressed with prolonged duration. From the catch net survey, we found that the number of species of marine fi sh greatly increased; however, no signi fi cant increase in the catch per net was observed. In terms of fi sh and crustacean types, the number of horse mackerel ( Trachurus japonicus ) and Taiwan blue crab ( Portunus pelagicus ) significantly increased. This was considered to be due to the easy access provided by the opening to the Sea of Japan. However, the catch of rock fi sh ( Sebastes schlegeli ) and spiny goby ( Acanthogobius flavimanus ) , decreased after the opening. Because these species prefer benthic life, it was considered to have been a ff ected by oxygen depletion in the lower layer of the Honjo area.

宍道湖では近年，沈水植物が広範囲に繁茂するようになった。2008年秋に漁網に水草が大量付着したと報告されていることから，本研究では前年の2007年に沈水植物の繁茂が始まったと仮定し，湖心部での水質（水温，電気伝導度，浮遊物質濃度，植物色素濃度，透明度）の長期変動（2001年1月から2012年12月）と，2002年から2012年までの水田除草剤年間販売量を検討した。また沈水植物が発芽する5月における湖内７地点での水質を，繁茂前（2001～2006年）と繁茂後（2007～2012年）とで比較した。水温と電気伝導度は全地点において繁茂後の方が高かった。光環境に関わる浮遊物質濃度と透明度は地点間で傾向が異なったが，植物色素濃度は全地点で繁茂後に増加していた。全水質項目において繁茂前後の6年間の平均値を比べたところ，植物色素濃度は繁茂後が有意に高かった。宍道湖周辺で販売された水田除草剤の有効成分量は，2006年度の4291kgから2007年度の3305kgと激減し，以後2000kg台にまで減少した。以上より宍道湖で沈水植物が繁茂するようになった原因は，2007年度以降に水田除草剤使用量が減少したためである可能性が高いと判断した。

対象水域の地形や構造などが大規模公共工事によって変化している場合，工事以前の状態に戻すことが自然再生にとって不可欠と主張されることが多い．しかし，当該工事から数十年経る間に生じた工事以外の要素も自然環境を改変していることから，地形や構造を戻すことで環境が戻るとは限らない．島根県と鳥取県に位置する中海は，干拓目的で本庄工区と呼ばれる水域が堤防で囲まれ，また中海本湖への海水の出入りは中浦水門を通じる工事が行われた．この堤防の開削は中海を元の状態に戻すことになり本庄工区と中海本湖の貧酸素化を緩和するとの見解と，開削は本庄工区の貧酸素化を強化するとの見解が対立したが，前者の見解を支持するシミュレーション結果が地域住民の気運に合致していたため，事業主体者は開削に踏み切った．本研究の結果により，開削によって本庄工区の貧酸素化は強化したことが確認された．また中海本湖の貧酸素化は工事以前から生じていたことが既報により確認できた．これにより，工事以前の状態に戻すことは必ずしも自然環境の再生につながらず，科学的な根拠に基づかない「分かりやすい主張」には慎重に対処すべきであることが示された．開削によって本庄工区や中海本湖の貧酸素化が緩和するとの自然科学の原理に反し，過去の記録とも矛盾したシミュレーション結果を提示するに至った過程の検証が望まれる．

低鹹汽水湖である宍道湖における酸素消費過程の現場観測と，宍道湖湖心部堆積物と底層水を用いた酸素消費実験を2005年の夏季と秋季に行った。実験においては酸素消費と共に，有機物指標（COD（Mn），COD（Cr），TOC）の濃度がどのように変化するかを検討した。現場観測では，隣接する中海から湧昇する貧酸素化した高塩分水が，宍道湖堆積物上を移動するにつれて溶存酸素濃度が上昇する状況が観測された。また成層内部の溶存酸素濃度は風によって引き起こされる湖水の流動に影響されており，底層の貧酸素化は風速の低下により成層内部の流動が弱くなった場合にのみ発生すると考えられた。底層水を用いた実験では，溶存酸素濃度の減少と有機物指標との間に対応が認められなかった。底層水と堆積物を用いた酸素消費実験では，溶存酸素濃度がゼロになるまでに2日以上を要した。これらの結果から，COD（Mn）であらわされている有機物量を削減するよりも成層状態が長期に渡らない対策を立てる方が，宍道湖での貧酸素化の緩和に有効であると考えられた。

In situ phosphorus release rates in three contiguous shallow brackish lakes were calculated by considering the amount of water inflow, changes in salinity and phosphorus stock, and loading from phosphorus inflow based on monthly data. The annual amount of sedimental phosphorus relative to that of phosphorus inflow was different for each of the three water bodies: 16% for Lake Shinji, 3% for the Honjo area, and −8% for Lake Nakaumi, as estimated in a 10-year period from January 1993 to December 2002. During the warm season, the quantity of phosphorus released surpassed sedimentation in these three water bodies. The low annual sedimentation ratio in Lake Nakaumi is related to a large seawater backflow resulting in phosphorus removal, in addition to a stable stratified structure promoting phosphorus release from sediment due to oxygen depletion in the lower layer. In Lake Nakaumi, field data shows that if dissolved oxygen at the sediment surface falls below 2.54 mg L−1, phosphorus release from the sediment begins to be accelerated.

To identify the geosmin-producing organisms in Lake Shinji, Japan, we conducted preliminary chemical and biological investigations from April to July 2008, during which odor occurrence was observed. Geosmin was mainly detected in the suspended substance (SS) of lake water, and its concentration corresponded to the cell numbers of Coelosphaerium kuetzingianum (Cyanophyceae) and Monoraphidium contortum. C. kuetzingianum and Pseudodictyosphaerium minusculum (Chlorophyceae; which occurred only in April) were separated from the lake water of 30 April to the lighter, interlayers in a step density gradient medium. The highest concentration of geosmin was detected in the portion of the lightest layer separated from the water sample by gas chromatography mass spectrometry (GC/MS). Observation under optical microscope confirmed that abundant C. kuetzingianum with some P. minusculum and a very rare unknown material was retained in this portion. Similarly C. kuetzingianum was separated by the step density gradient medium from the sediment of 7 May. Abundant C. kuetzingianum with rare bacteria was recognized in the portion of the lightest layer, which contained the highest concentration of geosmin. The cell numbers of C. kuetzingianum correlated with the concentration of geosmin in the separated portions of water samples. These results suggest that C. kuetzingianum probably produced geosmin, although there has been no report about this to now.

Musculista senhousia is a marine mytilid mussel native to Japan that constructs byssal thread mats (hereafter “mats”) that destroy local fisheries, and so it is regarded as a biofouling species. To elucidate the factors that prevent the formation of mats of M. senhousia, we first observed the seasonal changes in the populations of M. senhousia in two rivers with similar salinities. Then we compared the condition of Lake Nakaumi, which is covered with dense mats at present, with the condition of the same lake before the decline of the eelgrass beds. The results showed that a particular set of conditions (in terms of salinity, dissolved oxygen concentration and sediment type) did not lead to the dense habitation of M. senhousia in one river, while mat construction was observed in another river with a similar set of conditions. Together with the past description of Lake Nakaumi, we conclude that larvae of M. senhousia head upstream all year round, and can settle as far upstream as the salinity remains within its range of tolerance. Whether it constructs mats depends on the presence of its predators.

We collected rainwater samples from every rainfall in Matsue, Japan in order to study variations of nitrogen and phosphorus concentrations over time. The seasonal average concentration by magnitude order of Total Nitrogen (here after T-N) was highest in winter, then in spring, fall, and summer and that of Total Phosphorus (here after T-P) was highest in spring, then in winter, fall, and summer. These seasonal variations were examined in relation to the transportation paths of arrived air masses by using a backward trajectory and rainfall patterns from a surface synoptic weather chart. In winter, continental air masses frequently flow from China or Siberia and the resultant winter rainfall is on many occasions of a continental type. In summer, maritime air masses frequently arrive from the Pacific Ocean and this resultant rainfall therefore was often of maritime type. Looking at average concentrations of T-N and T-P for each rainfall type, continental types were high range and maritime types were low. It was therefore concluded that the monthly average concentration of T-N was affected by continental air masses from northern China in winter and by maritime ones from the Pacific Ocean in summer. The maximum deposition of T-N was caused by this concentration in winter and rainfall depth in summer. Seasonal variation of T-P showed a different fluctuation tendency from T-N, with a maximum concentration in spring, and minimum in summer and fall. T-P was susceptible to the yellow sand phenomenon which maximised T-P deposition in spring.

To study the long-term change in nutrient loads from the Hii River to Lake Shinji, water samples were taken repeatedly over a year in 1983/1984 and again in 2001/2002. Annual total nitrogen (TN) loads, estimated from observations of water quality and river flow, increased from 860 to 920 t with a corresponding increase in NO3–N concentration during the cool season. In contrast, total phosphorus (TP) loads decreased from 96 to 62 t. Annual TN and TP loads, calculated using emission factors and annual statistics for the catchments, showed a tendency to decline from 1986 to 2002. No source could be identified which would result in the increase in TN in the catchments, therefore, the increase in observed TN loads was considered to originate in other areas. Atmospheric nitrogen deposition transported from long distances has elevated the sum of NH4–N and NO3–N concentration in rainwater in the cool season. Therefore, it was considered that this resulted in the increase in TN loads in the Hii River.

Long-term changes in Secchi disk transparency in Lake Nakaumi, Japan, from 1932 to the present, which includes the periods before and after the loss of eelgrass ( Zostera marina L.) beds, were compiled from previous reports. During the first (July 1932–February 1934) and the second (January 1949–November 1950) periods, the mean transparency was greater than 3 m at all observed stations. Data during the third period (July 1954–March 1956) are only available for the station at the center of the lake. The mean transparency during the third period was significantly lower than that of the second period but higher than the mean during the fourth period. This observation suggests that the decrease of transparency occurred during the third period. Because the decline of eelgrass beds in Lake Nakaumi occurred in the mid-1950s, the decrease of transparency most likely resulted from the shift in primary producers from submerged macrophytes to phytoplankton. Although the maximum transparency sometimes exceeded 3 m, mean transparencies during the fourth period (May 1975–December 2003) were less than 2 m at all stations, significantly lower than those during the first and second periods. The shift in the chief primary producer, from benthic macrophytes to phytoplankton, caused a subsequent shift in secondary producers. The opportunistic filter-feeding bivalve Musculista senhousia , regarded as a biofouling species of local fisheries, increased in Lake Nakaumi. The long-term monitoring data of transparency suggested that restoration of submerged aquatic vegetation may be indispensable for the remediation of the lake environment in this shallow eutrophic lagoon.

Zostera marina L. was intensively harvested until the early 1950s in Lake Nakaumi, a eutrophic estuarine lagoon. We have estimated the amount of nitrogen (N) and phosphorus (P) removed from the lagoon through Z. marina harvesting. Lake Nakaumi lies in Tottori and Shimane prefectures, and the annual harvest of Z. marina in the late 1940s in Tottori was recorded as at least 56,250 t wet weight. The nutrient content of 56,250 t of Z. marina was calculated to be 61.9 t of N and 12.9 t of P, which is equivalent to 5.3% and 11%, respectively, of present annual nutrient loads to the lake. The nutrients formerly used by Z. marina were likely used by phytoplankton after the Z. marina started to decline in the mid-1950s at Lake Nakaumi. This shift in the chief primary producer, from benthic macrophytes to phytoplankton, caused a subsequent shift in secondary producers. Benthic fish and crustacean populations decreased and the non-commercial filter-feeding bivalve, Musculus senhausia, increased in Lake Nakaumi after the decline of seagrass beds. This affected the local economy, inducing not only eutrophication but also the collapse of local fisheries. On the other hand, at adjacent Lake Shinji, loss of submerged aquatic vegetation induced an increase of the commercial filter-feeding bivalve, Corbicula japonica, which doubled the fishery yield in the lake.

Lake Shinji from 1635 up to the time of the widening of the Ohashi River, which was carried out from 1924 to 1933 (from the late Taisyo era to the early Syowa era) has been believed as a fresh water lake. This belief is based on previous reports on the damage from salt after the widening, or a diatom analysis of the core sediments of Lake Shinji. To confirm the validity of this belief, the salinity of Lake Shinji before the widening was investigated from various viewpoints from reports on fishery operation, fishery statistics, sediment analysis and a hydraulic study. From this investigation, we propose that Lake Shinji before the widening was an oligohaline lake, with intermittent saline water intrusion from the adjacent Lake Nakaumi depending on meteorological conditions. The intrusion of saline water was considered to increase in summer with a decrease in fresh water inflow, and made brackish water fishes migrate and inhabit Lake Shinji. In the case of a dry year, marine dinophyceae predominated, which was recorded in the core sediments of the lake. Our results strongly suggest that a single approach to reconstructing past environments can be easily misinterpreted, and that an interdisciplinary approach is indispensable for such purpose.

二層に成層した潟湖の中海において,渦鞭毛藻類Prorocerztrum minimumによる赤潮の動態を明らかにするため,高頻度観測による水質状況の把握,植物プランクトン種の同定,および,懸濁態物質のC:N:P比分析を行った。季節風による上下混合の始まる1996年秋季,上層に過剰のリン酸態リンが存在する中で,硝酸態窒素が降雨によって供給された後にP. minimumによる赤潮が発生した。赤潮は冬季も継続し,1997年春季に再び増殖し,上層への無機態の窒素とリンの供給が減少した5月中旬に消滅した。1996年秋季から翌年春季まで,懸濁態のC:N:P比は,130:14:1から300:30:1へと連続的に変化した。1980年代初頭と比較すると,暖候期における湖心下層のリン酸態リン濃度が上昇し,このため,赤潮の発生する秋季の上層水質はリン制限から窒素制限に変化した。リン過剰の中で硝酸態窒素が供給された場合, P. minimumは硝酸態窒素の高い利用能を示す。水質環境が窒素制限に変わったことが,近年,珪藻類Skeletonema costatumによる赤潮が見られなくなり,ほとんどがP. minimumによる赤潮となった原因と考えられた。

A response in movement of a two-layered water body in Lake Nakaumi to a strong wind, which suddenly rose and continued for 15 h with nearly constant speed and direction, was observed using the current and salinity meters situated at two points in the lake. At the initial stage of water movement, large fluctuations in current velocity in reverse directions were observed in the upper and lower layers, accompanied by mixing between the two layers. At the stationary stage following the initial stage, current velocities in the upper and lower layers were kept constant, as was salinity. It was considered that a slab-type flow, which enhanced mixing between the two layers, occurred at the initial stage in water movement, then changed into a steady circulation flow at the stationary stage with completion of the set up in the two-layered structure.