Search

An increase in available nitrogen loading in intertidal ecosystems causes eutrophication and macroalgae blooms. Denitrification and anaerobic ammonium oxidation (anammox) lead to the removal of bioavailable nitrogen, but few studies have examined this in intertidal sediments. The sediment anammox and denitrification rates in September 2015 and November 2016 were measured using a 15N tracer technique at two sites, with and without macroalgae, in the hypereutrophic Yatsu tidal flat, eastern Japan. At both sites, the rate of N2 production via anammox was consistently low compared with that via denitrification, accounting for

The Fukushima Daiichi Nuclear Power Plant accident on March 11, 2011 released substantial amounts of radiocesium (134Cs and 137Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of 137Cs in the aquatic ecosystem of Lake Onuma, Mt. Akagi. We estimated the effective ecological half-life of 137Cs in fish and lake water, using 137Cs concentration survey data collected from 2011 to 2019, and single- and two-component decay function models (SDM and TDM, respectively). TDMs provided a good fit for 137Cs concentration decline processes in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), and lake water. We concluded that 137Cs concentrations approached a state of dynamic equilibrium between these aquatic organisms (wakasagi and pale chub) and the lake water. However, SDMs better predicted decline processes in 137Cs concentrations in charr (Salvelinus leucomaenis), Japanese dace (Tribolodon hakonensis), and western waterweed (Elodea nuttallii). The 137Cs mass balance in Lake Onuma suggests that 137Cs may be eluted from the lake bottom sediments. In summary, 137Cs contamination levels in Lake Onuma have significantly decreased, but monitoring surveys should continue to further our understanding of the processes underlying this reduction.

Kushiro Mire is the largest mire in Japan and in 1980 was the first wetland in Japan registered under the Ramsar Convention. Recent reports indicate an increase in nutrient loading into Kushiro Mire from changes in land use. We measured vertical profiles of dissolved inorganic nitrogen (DIN; NO3 –, NO2 –, NH4 +), dissolved organic carbon (DOC), and various types of microbial activity in soil samples collected to approximately 1.5 m deep at two sites in Kushiro Mire. We found an accumulation of NO3 – and DOC in the deeper soil. Denitrifying activity was highest in the shallower soils and decreased drastically with depth, whereas higher levels of fluoresceindiacetate hydrolysis, β-glucosidase, acid phosphatase, and xylosidase enzyme activity were found in the deeper layers. We also detected humic-like substances as components of the DOC. These results suggest that the DOC in the wetland soil cannot be used as a substrate for denitrification, causing denitrification to be suppressed in the deeper soil. In addition, denitrifying activity would be very low in the deeper layers due to low soil temperature. As a result, nitrogen input to the mire has resulted in a large accumulation of NO3 – in the deeper soil. This will eventually change the mire ecosystem through effects such as increased eutrophication and acidification.

尾瀬ヶ原では例年5月～6月の融雪時期にアカシボと呼ばれる彩雪現象がみられる。この着色の原因は, 雪中の赤褐色粒子に起因し, 融雪時期に積雪下の湿原表面から雪原表層にむけての水の動きに従い着色が進み, 融雪の後期には雪原表面全体に赤褐色化が拡大する。赤褐色粒子は緑藻類, Phacotaceae科Hemitoma sp. の休眠胞子で, 被殻には多量の酸化鉄が存在した。この赤褐色粒子数は雪の中の全鉄量と高い相関関係があった。尾瀬ヶ原のアカシボ現象はHemitoma 属の休眠胞子に由来すると推測した。

春の融雪期に, 尾瀬ケ原の一部で, 例年「アカシボ」と呼ばれる彩雪現象がみられる。このアカシボ雪の中や表面に, 線虫類, ソコミジンコ類, ケンミジンコ類, ミズダニ類, クマムシ類, 貧毛類, ガガンボ科幼虫, ヌカカ科幼虫, ユスリカ科幼虫などが出現した。この中でも特に, ソコミジンコ類, 貧毛類, ガガンボ科幼虫, ヌカカ科幼虫, ユスリカ科幼虫の密度が高かった。見本園アカシボの総密度は雪中で2.4～3.6×104個体m-2, 雪表面で0.3～2.2×103個体m-2であった。見本園の根雪前, 融雪後のアカシボ残存物中にはアカシボ雪と共通の分類群が出現することより, アカシボ雪の無脊椎動物は湿原の土壌動物に由来するものと推定された。土壌動物がアカシボ雪中に移動する原因として, 融雪期に積雪下の地表面が湛水すること, 溶存酸素濃度が低下すること, さらにアカシボ雪の形成時にFe2+の酸化のために, 酸素が消費され, 雪中の酸素濃度が低下することが推定された。雪原食物網研究の重要性について考察した。

Estimation of the isotopic turnover rate in the tissues of organisms allows us to estimate the temporal relationship between the isotope ratio of an organism and its prey, and to analyze seasonal variations of food webs and migratory patterns of animals. We analyzed the isotopic turnover rate in the tissue of lacustrine sockeye salmon, Oncorhynchus nerka, of three age classes in a laboratory food alteration experiment to clarify the isotopic turnover rate. The contribution of growth to isotopic turnover was higher in young and fast-growing lacustrine sockeye salmon than in older and slow-growing fish. Maturation masks the relationship between the contributions of growth and metabolism to isotopic change. Our estimate of the isotopic time lag of lacustrine sockeye salmon can be used to research trophic relationships of anadromous sockeye salmon.

1998年3月から2003年5月の融雪期に東北地方に発生した47地点の赤雪についての原因調査を行った。赤雪中には赤褐色の粒子が多数存在し,高濃度のFeが含まれていた。この赤褐色の粒子の多くは緑藻Hemitoma sp.の休眠胞子であり,粒子数と全Fe含有量とのあいだには高い相関関係があった。赤雪の発生地域は融雪時期に積雪下層部と湿原もしくは土壌との境界面に流れた水が留まる窪地であった。休眠胞子形成の時期や胞子形成の機構は不明であるが,融雪期に融雪水の動きに伴って積雪中を移動し,雪の表層にあらわれ出てくるため,雪が赤褐色に着色するものと推定した。

底泥中の酵素活性は空間的,時間的に変動するため,干潟底泥の酵素活性から干潟生態系の機能を評価するには適切に試料採取する必要がある。本報告では,適切な試料採取法を検討するために,小櫃川河口の盤洲干潟において2001年6,9月に調査を行い,7個所の調査地点で5層位(0-75mm)から底泥試料を採取し,3種類の酵素活性(エステラーゼ,EST;β-グルコシダーゼ,GLU;β-アセチルグルコサミニダーゼ,AGA)を測定し,酵素活性の分布と変動および変動要因を調べた。計70点の試料についての平均値と標準偏差はそれぞれ0.022±0.012 unit g-1h -1 (EST),10.4±8.5μmol g-1h-1 (GLU),6.8±5,0μmol g-1h-1 (AGA)であった。調査時期,地点,層位の酵素活性への影響を3元分散分析法で解析した結果,調査地点,層位および両者の交互作用は有意に寄与していた。一方,表層(0-15mm)の14試料を除いた56試料について分散分析を行った結果,地点間差は酵素活性の変動に有意に寄与していたが,層位はGLUとAGAの変動に対しては有意でなかった。以上の結果から,干潟底泥の酵素活性を評価するには表層を除いたある層位から試料を採取することにより採取誤差が軽減できる事が示された。

干潟生態系の有機物分解機能を底泥の酵素活性から評価する事を目的として,日本国内5地域(北海道東部,東京湾,伊勢湾,有明海,八重山諸島),13箇所の干潟において夏季干潮時に底泥を採取し,セルロース分解酵素(セルラーゼ,CEL)及びキチン分解酵素(β-アセチルグルコサミニダーゼ,AGA)活性を,それぞれカルボキシルメチルセルロースとp-ニトロフェニル-b-D-グルコサミドを基質として測定した。その結果,CEL活性は2-30nmolg-1h-1,AGA活性は10-140nmolg-1h-1の範囲で干潟問で大きな差が認められた。これらの酵素活性の間には高い相関が見られた。また、これらの酵素活性と底泥の有機物量,シルト粘土含量の間には概ね正の相関が認められ,泥質で有機物含量の高い干潟で酵素活性は高く,分解機能も高いことが示唆された。また,甲殻類の多い干潟ではAGA活性が相対的に高い傾向が見られた。これらの結果から,干潟底泥の酵素活性は干潟の理化学性,生物特性によって大きく変動し,分解機能の指標の一つになると考えられた。

The restoration of the wetland ecosystems is an essential problem in the world, however, the evaluation technique for the functions of ecosystems, especially in the intertidal wetlands, has not been established still in spite of the ecosystems adjoining with the human life. The Functions of a wetland tend to roughly correspond with characteristics of both the water and sediments of the wetland. In this study, the 13 intertidal sandy and muddy flat wetlands were selected from Hokkaido to Okinawa, and the indicators for water and sediments were investigated as a first step of the functional ecosystem assessment in Japan. The classification for traditional landform subclass, i. e. fore-beach, estuary and lagoon, was unclearly changeable, and it often did not agree with the functional classification. In the intertidal sandy and muddy flat wetlands in Japan, the subclass fo HGM Approach should be set corresponding to the characteristics of influent water and sediments, and it was considered that the ecosystem evaluation should be carried out in each Hydrogeomorphic Unit classified at smaller-scale than the original HGM Approach in U.S.

The migration of Siamese mud carp (Henicorhynchus siamensis and H. lobatus), two of the most economically important fish species in the Mekong River, was studied using an otolith microchemistry technique. Fish and river water samples were collected in seven regions throughout the whole basin in Thailand, Laos and Cambodia over a 4 year study period. There was coherence between the elements in the ambient water and on the surface of the otoliths, with strontium (Sr) and barium (Ba) showing the strongest correlation. The partition coefficients were 0.409-0.496 for Sr and 0.055 for Ba. Otolith Sr-Ba profiles indicated extensive synchronized migrations with similar natal origins among individuals within the same region. H. siamensis movement has been severely suppressed in a tributary system where a series of irrigation dams has blocked their migration. H. lobatus collected both below and above the Khone Falls in the mainstream Mekong exhibited statistically different otolith surface elemental signatures but similar core elemental signatures. This result suggests a population originating from a single natal origin but bypassing the waterfalls through a passable side channel where a major hydroelectric dam is planned. The potential effects of damming in the Mekong River are discussed.

Pesticide residues in rainwater were investigated for the estimation of pesticide flux into a lake in Ibaraki Prefecture during the 1990 rainy season. The maximum concentration of oxiadiazon in the rainwater was 4.2μg l-1. The maximum concentrations of fenobucarb, fenitrothion and edifenphos in the rainwater were 0.58, 0.53 and 0.36μg l-1, respectively. The annual ratio of fenitrothion redeposition by rainfall was estimated at 1.5% of the total shipping amount in Ibaraki Prefecture. There was no significant difference between redepositions of pesticides by rainfall and shipping amounts in the prefecture (α= 0.05). Pesticide residues in lake water were investigated in Lake Nakanuma, Ibaraki Prefecture, from March to December 1990. The concentrations of molinate, simetryn, iprobenfos, and thiobencarb in irrigation canal water were in the high range (>1.2μg l-1). The concentrations of diazinon, malathion, and fenthion in the water were at a lower level (

The growth of Nelumbo nucifera GAERTN. was investigated at an artificial stand in outdoor concrete ponds. The ratio of above- to below-ground biomass (T/R ratio) ranged from 0.3 to 3. Although N. nucifera is a perennial, the reproductive organ (tuber) begins to form later in the growing period than in annual plants; in addition, the life span of the tuber is less than one year. Maximum relative growth rate was 0.037 g⋅g-1⋅day-1 in July.The leaf area index of floating leaves did not exceed 1 m2⋅m-2.The influence of initial tuber density on the foliage structure and below-ground biomass of N. nucifera were investigated. The initial death rate at the time of lamina rolling increased with leaf density. Control of the density of floating leaves was done twice ; once at the early stage of lamina rolling, and again at the late stage of the fully expanded leaf. The below-ground biomass in the following spring did not correlate with initial tuber size. When the tuber biomass was under about 400 g dry weight⋅m-2, net production of below-ground biomass of the N. nucifera stand seems to have attained a plus value. N. nucifera exhibited typical pioneer characteristics in an environment of frequent disturbances such as flooding, i.e., a high T/R ratio, high vegetative propagation by stolons and a short life span of the tuber.

霞ヶ浦高浜入に生育する浮葉植物ヒシの野外調査を1985年から1986年に行った。また,航空写真を用いて1972年から1990年の植生の変化を調べた。1985年8月に最大現存量340g乾重・m-2になったが,8月下旬の台風による激しい風によってよく発達したヒシ植分は崩壊巨した。1986年には攪乱前に形成された種子と埋土種子によって植分は回復した。1986年8月上旬の1mの増水にも拘わらず長い茎によって浮くため,流れの少ない所にあるヒシ植分は減少しなかった。生活史の中で起こる台風による激しい流れと風の攪乱は重要な環境因子である。攪乱にもかかわらず埋土種子と長い茎によってヒシが霞ヶ浦で生育することができると考えられた。

The chironomid fauna was studied in the emergent plant zone of Lake Kasumigaura in May, August and November of 1986. A total of 2, 181 adults were reared and emerged from plant samples and sediments in the emergent plant zone. We identified 1957 specimens belonging to 31 species of 18 genera. Ten species were abundant, of which five were recovered from plant samples, two from sediments and others from both. Species composition did not differ much from May to August, which might have corresponded with the change of in situ larval density. The abundance of the chironomids was highest in May and low in August for plant samples, while it was highest in August for sediments. On plant samples, it correlated with the chlorphyll a amount on the reed stems, which was higher in May than in August. In November, only two species predominated. Species composition differed more markedly among substratum types than among localities in the lake. Some chironomid species seemed to select specific sediments. Chironomid fauna in littoral hydrophyte zones was more diverse than in pelagic zones in Lake Kasumigaura.

Zonal distribution of aquatic macrophytes was investigated in the littoral zone of Edosaki-iri Bay in Lake Kasumigaura. Muddy sediment with litter was deposited on the sandy bed near the shore line. In the emergent plant zone, Phragmites australis (Cav.) Trin. ex Steud. tended to dominate near the shore line, but was successively replaced by Zizania latifolia Turcz. and Typha angustifolia L. toward the deeper sites. The emergent plants were distributed on the shelf to a depth of 80 cm, while a floatingleaved plant, Nymphoides peltata (Gmel.) O. Kuntze, occupied lakewards on the steeper slope from depths of 60 to 200 cm. Distribution of each species along the water depth gradient overlapped. The zonation of emergent plants was more associated with depth to sandy sediment surface than that with water depth and that with the thickness of muddy sediment.

Seasonal succession and vertical distribution of zooplankton in the central region of Lake Ozenuma were studied during icefree seasons in 1987 and 1988. Rotifers were more abundant in density than cladocerans. Copepods were rare. The most dominant species were Polyarthra trigla in Rotifera and Bosmina longirostris in Cladocera. In biomass, however, cladocerans usually surpassed rotifers, and the cladoceran Holopedium gibberum took the highest percentage of the total zooplankton biomass. Some zooplankton species were found commonly throughout the study period, whereas others appeared irregularly. It seems that, in recent years, rotifers have become more abundant in the lake.

The Japanese Journal of Limnology is another official publication of the Japanese Society of Limnology. The original papers in the journal were peer-reviewed by a few authorized referees, and appeared in Japanese with English abstracts.