The biomass and flora of planktonic communities before and after construction of a river mouth barrage in the lower Nagara River, (Central Japan) are described in relation to long-term trends in some environmental factors. Despite a lack of environmental trends which might accelerate algal production (except for the prolonged retention time of river water), the algal occurrence modes, such as maximal biomass or occurrence period, changed remarkably after construction. Cyclotella atomus and C. meneghiniana (diatom) remained dominant in summer as they were pre-construction. Longitudinal increases or decreases in algal biomass along the stream resulting from a balance between production and sedimentation, which were dependent on the discharge, were recorded. Additionally, grazing by rotifers seemed to be widespread in midsummer. The lower Nagara is regarded as a river-lake hybrid based on its role as a primary producer of algal biomass and its manner of controlling it. However, unlike a lake, it varies in its extent both spatially and temporally.

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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.

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The temporal variations in C, N and P concentrations of the periphyton on the natural substratum (Echinochloa polystachya) and its relationships to the water's climatological (air temperature and precipitation), hydrodynamic (water discharge and water level), physical (temperature, transparency) and chemical variables (conductivity, total suspended solids, nitrate, nitrite, ammonia, silica, total phosphorus and orthophosphate) were evaluated in a lagoon at the Par-anapanema River mouth into the Jurumirim Reservoir (São Paulo, Brazil) from August 1993 to July 1994. An amplitude variation of 2.0 m in water level was observed. Depending on to the hydrologic conditions, the lagoon presents intermediate conditions of lentic and lotic environments. The amplitude variations in C, N and P were 14.5-52.1% C DW^-1, 0.5-3.2% N DW^-1, and 0.03-0.4% P DW^-1, respectively. The periphyton presented a decreased protein content. The environmental trophic degree appears to be decisive in chemical periphyton composition.

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The technique of judging cells dead or alive with a fluorescence microscope has been adapted to studying the growth and death rates of a dinoflagellate Peridinium bipes which periodically formed freshwater red tides in the Asahi reservoir, Nara Prefecture. Clonal axenic population and synthetic medium (AFH) were used. The controlling factors of the cultivation were water temperature (5-25°C) and light intensity (0-114 μE m^-2 sec^-1, 14L: 10D).
Effects of water temperature and light intensity on the growth rate of P. bipes were expressed by the influential functions of water temperature and light intensity as follows : fT={(T/19.7) exp (1-T/19.7)}^4.08 and fI=I/(I+3.52). The growth rate of P. bipes was given by μm=μ_max fT fI. The death rate of P. bipes was also written in the form dM=dT₌₂₀, I₌₀ f'T f'I in which the influential functions of water temperature and light intensity were expressed as follows : f'T=1.11T^-20 and f'I=1-0.88I/(I+5.6). As a result of calculation by the functions mentioned above and the average values of dissolved nitrogen and dissolved phosphorus in the Asahi reservoir during 7 years, P. bipes was estimated to be capable of multiplying there as the water temperature was more than 5°C and the light intensity was more than 5μE m^-2 sec^-1.
No studies have ever tried to evaluate the relation among the death rate, water temperature and light intensity. These results were considered to yield useful information for developing red tide models.

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Differences in the species composition of a benthic diatom community among habitats were studied in a fourth order stream in Central Japan. Samples were collected from four different habitats (cobbles, pebbles, sands, and stolons of reed Phragmites japonica) within a morphological unit twice a month from May 1993 to May 1994. In each habitat, especially on cobbles, the temporal variation in an algal standing crop was large. The standing. crop on pebbles was significantly smaller than that on sands and reeds. Among habitats, the diatom species composition was different : the differences among habitats were almost always larger than those within a single habitat. The species compositions on cobbles, on sands and on reeds were clearly distinct, but that on pebbles, was somehow similar to the others. Motile diatoms such as Navicula spp. were abundant on sands. In summer and autumn, Cocconeis placentula was dominant on reeds. When rosetteforming filamentous cyanobacteria, mainly Homoeothrix janthina, were dominant on cobbles, Achnanthidium japonicum and Nitzschia hantzschiana often became dominant among diatoms.

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The seasonal succession of a planktonic algal community was investigated in a small pond of Naka-ikemi marsh from April to November 1996. Predominant algal classes seasonally predominate as follows ; Bacillariophyceae, Cryptophyceae and Euglenophyceae in April and May ; Dinophyceae in July ; Cryptophyceae from August to November. The most definite characteristic of the planktonic algal community was that the rise and fall of Peridinium inconspicuum (Dinophyceae) and Cryptomonas ovata (Cryptophyceae) seemed to be the main cause of a fluctuation in the total algal biomass from July to October. The results of this study significantly differed in predominant species from the results of previous studies on Japanese moors. One reason was presumed to be the difference in water quality between the marsh and the moor. The dominance of P. inconspicuum and C. ovata was accounted for by underwater light conditions and swimming ability. Photon flux density in the water rapidly decreased from July to November (light attenuation coefficient : k=2.44-3.96). It was suggested that such a light condition was advantageous for motile algae, because they had the swimming ability to remain in the euphotic zone.

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This lake was mainly stocked with Carassius cuvieri and Hypomesus by the Kawaguchiko Fishermen's Association until the 1980s. However, since Micropterus salmoides salmoides were confirmed to exist in the lake in 1973, by early 1985 Hypomesus were no longer caught. This condition continued for a short period ; then, in the 1990s, the fish species composition showed a great change in the lake. Micropterus salmoides salmoides and Salmo (Oncorhynchus) mykiss became the main catch. As a result, 370, 000 anglers visited Lake Kawaguchi in 1996, against 35, 000 in 1978. The total number of anglers who visit the lake is increasing rapidly. At present, Lake Kawaguchi has become the most famous lake for fly fishing in Japan. In order to respond to the demand of so many anglers, the Kawaguchiko Fishermen's Association stocks the lake with many species of carnivorous fish. Moreover, they annually stock an abundance of small plankton-feeding fish which in turn become the food of carnivorous fish. Owing to the above circumstances, Lake Kawaguchi has become like an artificial fishing pond, with disturbing effects on the lake ecosystem. The ecological impact on other aquatic organisms has become a cause for concern.

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