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Fungal infection of the filamentous cyanobacterium Anabaena smithii was observed in Lake Shumarinai in 2004–2006. Two fungal species were found to parasitize the specialized cells of A. smithii. These fungi might not correspond to the chytrid species that the previous studies reported as the parasites for Anabaena species. One fungus showed selective attachment to the akinete (akinete type). The filaments parasitized by this fungus increased in August 2004 and October 2006, when akinete and filament densities also increased. The maximum percentage of parasitized filaments was 3.2% of all filaments in October 2006. The other fungus was usually attached to the heterocyst (heterocyst type). The filaments parasitized by this fungus increased in October from 2004 to 2006. The maximum percentage of parasitized filaments was 20.6% in October 2004. The biomass of A. smithii was not suppressed by akinete-type fungus because of the low percentage of parasitized filaments. The heterocyst-type fungus might disturb the nitrogen fixation, but its effect was negligible due to a high concentration of available nitrogen for planktonic algae in Lake Shumarinai.

森林伐採のような人為的なインパクトに対する環境影響, 特に湖沼生態系への影響を評価するために, 湖沼の流動場と生物地球化学的な循環を記述するモデルを結合したモデルを開発する事が必要である。ここでは対象を朱鞠内湖とし, 湖水の流動モデルの適用の結果と, 今後開発する生物地球化学的な循環モデルの考え方について紹介する。

We examined the photosynthetic activity of the phytoplankton community collected from the surface to a depth of 1000 m in the south basin of Lake Baikal. Experiments were conducted in June (mixing period) and August (stratified period). The carbon fixation rate was measured by the use of a 13C tracer after the incubation of samples under light conditions in the upper water column. Photosynthetic fixation of 13C was detected for samples collected from a depth of 500 m, indicating the viability of phytoplankton in deep water. The community composition was dominated by Bacillariophyceae in deep water. The finding of lower activity at a depth of 200 m than that at a depth of 500 m in August suggests that the spring diatom bloom could be a significant source of viable populations at a depth of 500 m. Photosynthetic activity was not detected in samples collected at a depth of 1000 m.

The diatom biomass of Lake Barato, as measured from July to September, decreased simultaneously with an increase in filament density of Phormidium tenue after 1997. There was a high negative correlation between the diatom biomass and the densities of P. tenue (r 2 = 0.928). Although total nitrogen (TN) and total phosphorus (TP) concentrations decreased from 1996, TN : TP ratio increased from 1997 because the TP concentration became markedly lower. The decrease in diatom biomass might have been due to the loss in phosphorus available for algae. Because the increase in density of P. tenue might have been due to the decrease in diatom biomass, experiments using a growth inhibitor for diatoms were performed to examine whether the density of cyanobacteria increases without diatom growth. Samples of the lake water collected in three seasons (August and October 1998, May 1999) were incubated with and without germanium (Ge) as a growth inhibitor of diatoms. The increase in density of P. tenue was inhibited concurrent with the increase in diatom biomass in the first and middle stages of incubation without the addition of Ge in August 1998 and May 1999. In contrast, a higher density of P. tenue was observed in the incubation with diatom growth inhibited by Ge over the same period. These results suggest that diatoms have an effect in restraining the growth of P. tenue.

We measured primary production by phytoplankton in the south basin of Lake Baikal, Russia, by in situ 13C-bicarbonate incubations within the period March–October in two consecutive years (1999 and 2000). Primary production was highest in the subsurface layer, possibly due to near-surface photoinhibition of photosynthesis, even under 0.8 m of ice cover in March. Areal primary production varied from 79 mg C m−2 day−1 (March) to 424 mg C m−2 day−1 (August), and annual primary production was roughly estimated as 75 g C m−2 year−1, both of which are within the lower range of previous estimates. Size fractionation measurements revealed that phytoplankton in the

To clarify the variations in environmental factors and biological characteristics of eutrophic, brackish, meromictic Lake Abashiri, Japan, we studied long-term variations in concentrations of chloride and nutrients in lake water and characteristics of their supply. This lake has an anoxic layer containing much chloride, nutrients, and sulfide. Although a dense Anabaena bloom appeared in the lake every summer before 1986, the bloom did not appear after upwelling from the bottom layer (a blue tide) occurred in spring 1987. A dense layer of photosynthetic bacteria (Chlorobium sp.) formed at the top of the anoxic layer from 1989 to May 1992. The concentrations of nutrients and chloride in the oxygenic layer were determined by the balance between nutrients from influent rivers, including dissolved inorganic nitrogen, and matter supplied from the anoxic layer, including chloride and dissolved inorganic phosphorus. These influences controlled the dominant phytoplankton species and their biomass. The appearance of an Anabaena bloom in Lake Abashiri is promoted by nitrogen limitation caused by decreased loading from influent rivers and/or by a stable supply of phosphorus from the anoxic layer (no formation of photosynthetic bacterial layer and small upwelling), but the appearance is inhibited by a high chloride ion concentration with large upwelling. The main reason for the formation of a photosynthetic bacterial layer from 1989 to May 1992 was that sunlight reached the top of the anoxic layer because of the increase in transparency due to inhibition of the Anabaena bloom with upwelling. The dense layer of photosynthetic bacteria blocked nutrient diffusion from the anoxic layer to the oxygenic layer by uptake, and decreased the biomass of phytoplankton in the oxygenic layer.

The densities and growth rates of dominant phytoplankton in Lake Oshima-Ohnuma were determined during May to July 1996 to clarify the mechanism of change from spring-dominant to summer-dominant phytoplankton species, examining the influence of zooplankton grazing and nutrient limitation. The dominant phytoplankton in April and May were Asterionella gracillima and Nitzschia spp. Their growth rates were low in May and their densities fell thereafter. The bottle experiment suggests that the main reason for decreases in dominant species has been nutrient deficiency, and zooplankton (sized from 30 to 160 μm) have supported the growth of A. gracillima to recycle nutrients in May. The maximum growth rate was shown by Melosira (Aulacoseira) ambigua living at 2 m in May and June. Although the growth rate of the summer-dominant species Melosira (Aulacoseira) granulata was not as high as that of M. ambigua in May and June, the ambient density increased from late June. Survival in the lower zone of the unstratified euphotic layer might have contributed to the abundance of M. granulata, since the growth rate of this species was not so low as that of M. ambigua at 6 m in June and July. The bottle experiment suggests that the growth of M. granulata was also nutrient limited in June and July and that zooplankton grazing was partially responsible for repressing the biomass of M. granulata before it began growing rapidly in late June.

論文(Article), The first record of depth-time profiles for some physicochemical and biological parameters of Lake Bishamon-numa based on periodic observations during the ice-free season of 1996 are reported. Throughout the observed period, the thermocline was either absent or very weak. In accordance with this structure of water temperature, the dissolved oxygen was distributed uniformly in the water column at above a 90% saturation level. The average and standard deviation of transparency was 5.1 ± 1.1 m. On the assumption that the trophogenic zone is from the surface to a depth of twice the transparency, the whole water body of the east basin in which the sampling station is located, constitutes the trophogenic zone. pH was fairly constant from July to December with an average and standard deviation of 6.2 ± 0.1. Compared with past data, the pH of Lake Bishamon-numa has changed greatly within a relatively short time period in the past and has lately stabilized around pH 6. The concentrations of Na, K, Mg, and Ca were in the order of m mol 1~-1, while those of Fe and Mn were of μmol 1~-1. Ranked in descending order of concentrations, they are Na, Ca, Mg, K, Mn and Fe. The annual maximum of Chl. a was 1.5 μg 1~-1 in May. Though there were slight maxima and minima, the Chl. a concentration was below 1 μg 1~-1 in most seasons and averaged 0.51 μg 1~-1. This Chl. a level corresponds to those of other oligotrophic lakes. Nano-phytoplankton ranging from 2 to 20 μm predominated, and macro-phytoplankton greater than 20 μm were relatively scarce. Key words : Goshiki-numa Lake Group, depth-time profiles, metals seasonal trend

The limnological characteristics and vertical distribution of phytoplankton in Lake Akan-Panke were surveyed to clarify the metabolism in an oligotrophic lake. Maximum water temperature was 19.6°C in August 1992, and the concentration of dissolved oxygen was always over 4.8 mg 1-1 in the hypolimnion. Transparency ranged from 7.1 to 17.6 m, and was relatively high in July and August. Standing stocks of nitrogen, phosphorus, and chlorophyll-a (chl-a) were 8.20, 0.33, and 0.114 g m-2, respectively. In particular, that of phosphorus was at a low level in July and August. Therefore, phosphorus may be the limiting factor in this lake. These data suggest the theory that Lake Akan-Panke is in an oligotrophic state. However, this lake had a relatively high concentration of chl-a at a deeper layer (30-45 m) during the early thawing season in May 1992 and July 1993, and large size phytoplankton (>20μm) dominated at those times. When thermal stratification was developing in the lake in July and August, the phytoplankton population and chl-a concentration decreased in the surface layer and formed a sub-surface maximum in the metalimnion, mainly dominated by large size phytoplankton (> 20 μm) . In contrast, relatively small size phytoplankton (< 20 μm) dominated the hypolimnion in August and October. Since most of the chl-a was trapped in the 45 m layer rather than in the 20 m layer by the sediment trap method in May 1992 and August 1993, the high chl-a concentration observed in the deeper layer might be due to the growth of phytoplankton in the layer and/or to sinking of the phytoplankton from the upper layer which increased in May (spring bloom). This suggests that the high biomass of phytoplankton in the deeper layer derived from sinking.

富栄養化の進んだ茨戸湖において微生物群集の生理活性の変化をアデニル酸の組成比を指標とし,ケイ藻およびラン藻が優占する6月および8月について調べた。>100μm,100-2μm,および2-0.2μmに分画された微生物群集において生物量を表すアデニレート総量と生理活性の指標となるAEC値は,ともに一日のうちで大きな変化を示したが,100μm以上と,100-2μmの両面分に含まれるプランクトン群集はアデニレート量の変化とAEC値の変化の間には対応が認められなかった。また,溶存無機態窒素は湖水中に多量に存在していること,およびリン酸塩の存在量にほとんど影響を受けないことから両群集の生物量や生理活性の変化はこれら栄養塩類以外の因子が関与していることが推定された。また,100-2μmと2-0.2μmの両画分に含まれる微生物群集のAEC値と湖水中の溶存蛋白質量や溶存糖類量の間には比較的高い相関が認められた。100-2μm画分中の微生物群集はAEC値が高くなると蛋白質や糖類といった有機物を体外に放出することにより,溶存有機物を増加させ,2-0.2μm画分に含まれる微生物群集を活性化したものと考えられた。

Dynamic behaviors of suspended sediment settling in volcanic Lake Kuttara, Hokkaido were examined by measuring the amount of trapped deposits and analyzing their particle size, biology, chemistry and mineralogy during a period from 14 September to 14 October 1989. The lake then was thermally stratified under conditions of the autumnal epilimnion cooling. The mean daily settling flux (g⋅m-2⋅d-1) of suspended sediment was numerically obtained from the amount of deposits trapped at five stations, and its horizontal distribution was successively described at depths of 15 m, 60 m and 100 m or 2 m above the bottom. About 50 to 60 vol.% of trapped sediment was occupied by endogenic phytoplankton (diatoms and dinoflagellates) and zooplankton (mainly, Bosmina longirostris), ranging over 11-125 μm in size. Its representative settling velocity is given as 1.26×10-5 m⋅sec-1, using the mean size of 32.6 μm and the mean particle density of 1, 034 kg⋅m-3. The spatial distributions of settling flux of sediment and a scale analysis of the sediment transport equation suggest that from the water surface to near the thermocline (15 m depth), the transport of settling sediment is controlled by vertically circulating flows due to a wind-forced surface wave, while below the thermocline (depths more than 15 m), it is decided by a secondary, vertically circulating flow due to rotating internal waves.

青森県南西部に位置する津軽十二湖湖沼群の全30湖沼を対象とした水質調査を実施し,湖沼群の水質特性,水質形成因子について検討した。1)本湖沼群の水質はキーダイヤグラムにより領域I(Ca・HCO3型)と領域IV(Na・Cl型)に属するものが多く,領域Iには18湖沼,領域IVには7湖沼が含まれた。2)キーダイヤグラムとパターンダイヤグラムから,本湖沼群は,典型的なCa・HCO3型の水質で同一水脈の湧水により涵養されるグループ(11湖沼),海水の希釈により派生するグループ(6湖沼),それらの中間型のグループ(12湖沼)の3タイプに分類された。3)本湖沼群の水質を支配する要因としては,海岸に近いことと湧水に涵養される湖沼が多いことから,風送塩と地質の影響が大きいことが推測された。4)本湖沼群のCl-濃度は,海からの風送塩に由来し,最高値と最低値は海岸からの距離を反映していた。また,隣接した湖沼のCl-度には,谷筋,山蔭という地形を反映した違いが見られた。5)地質にかかわる成分のうち,Ca2++Mg2+はアルカリ度との間に正の相関関係がみられたことから,地質と炭酸を含んだ水との反応により生じると考えられた。Ca2+,Mg2+濃度は崩山の崩壊面に起源する湧水に涵養される湖沼群で高く,沸壺の池のCa2+濃度はわが国の調和型湖沼では最も高い部類に属していた。本湖沼群中には,K+も特異的に高い湖沼が存在し,そのような湖沼は同時に溶存ケイ酸も高い傾向が認められた。6)落葉堆積物で被われた小湖沼群の水質は,色度,PPC,総鉄が高かった。これらの項目の間には正の相関関係がみられることから,鉄は有機態で存在していると推測された。

Variations in physiological state and biomass of the phytoplankton community were examined in three different types of lakes, namely Lake Barato, Lake Akan, and Lake Shikaribetu. When the physiological state of the phytoplankton community was estimated by its adenylate energy charge (AEC), low biomass and low physiological state co-appeared gradually in the metalimnion and hypolimnion during stratification. The physiological state of the phytoplankton as estimated by its AEC value did not always correspond to its biomass, estimated by chlorophyll-a and ATP in these three lakes. A high physiological state of the community was usually observed in the euphotic zone, but the low AEC value observed in the euphotic zone of Lake Barato was not identified in the euphotic zones of the other lakes. Thus, the relationship between the value of AEC, and biomass of phytoplankton is a complex variable, which is further discussed in this paper.

To clarify the changes in Si content of diatoms, the particulate silicon (PSi) concentration and total diatom volume (TDV) were determined in Lake Barato, Japan, from April to July 1998–2000. The soluble reactive silicon (SRSi) concentration decreased markedly with the rapid increase in TDV in May and June in all three years, although the value did not fall below that at which diatom growth might be limited. The proliferation of small discoid diatoms contributed to the decrease in SRSi concentration each year. The Si content of diatoms may not be constant as indicated by the changes in PSi : TDV ratio. The low PSi : TDV ratio and the fact that PSi concentration was lower than diatom PSi concentration (calculated from the volume of diatom species) accompanying the decrease in TDV suggests the possibility of a disturbance in the silicification in May and June 1999. These parameter changes accompanying the increase in TDV suggest that the silicification did not catch up with the cell division in early April 1998, early May 1999, and mid-June 2000. In addition, the PSi : TDV ratio increased rapidly and showed large fluctuations in July 1998 and 1999. This may have been caused by a change in dominant species from small discoid diatoms to Aulacoseira granulata because of the differences in Si content per unit cell volume.

Sources of sediment organic matter from the strongly acidic Lake Katanuma (0.14 km2; average pH 2.2), in Japan, were determined from an analysis of carbon stable isotope ratios (δ13C). Organic carbon was derived mainly from the benthic diatom, Pinnularia braunii, and particulate organic matter (POM) primarily from the phytoplankton, Chlamydomonas acidophila, and not from emergent or terrestrial C3 plants such as Phragmites australis, Sasa kurilensis or Fagus crenata. Although the sediment organic matter of most small lakes, especially strongly acidic lakes, is commonly thought to be of allochthonous origin, the sources of sediment organic matter in the small and acidic Lake Katanuma were clearly autochthonous.