Sapropel is natural aqueous lake-deposit possessing biological activities. Sapropel covers a great variety of bottom lacustrine deposit which contain not less than 15% of organic substances in the dry residue. The organic substances (humic acid and so on) are formed from plankton and macrophytes. The mineral parts come with surface river and ground water flows. Mineral compounds in lakes participate in biological process, and transform during oxdizing and reduction reactions.
Both survey and basic study on sapropel were conducted by the Institute for Problems of Exploitation of Natured Resources and Ecology of the Belarussian Academy of Science. Authors inspected Chervonoye lake and peat land in Belarus and discussed with the staff of the Institute. Many data about sapropel which is the most prospective for using as fertilizers, food additives for farm animals, and for applying for medical purpose were collected and reviewed. Sapropel would play an important part as fertilizers/water absorbents for poor land/desert.

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The free water method was applied to outdoor experimental ponds with and without fish using semi continuously measured dissolved oxygen (DO) and pH data for 39 days. The rates of primary production and respiration were determined by subtraction the oxygen and CO₂ fluxes due to air-water gas exchange and water inflow and outflow from the rates of change in the masses of DO and dissolved inorganic carbon in the ponds. The calculated DIC fluxes could explain the pond carbon budget./Gross primary production rates normalized to chlorophyll-a, particularly the daily rates, were highly correlated with depth-averaged light intensity./The oxygen consumption rates, i.e. respiration and/or decomposition rates were within the range of reported values./They changed little at night, fluctuated between days and varied with the settling rates of particulate matter./We suggest that the metabolic quotients (MQ) were more affected by nitrogen sources during productive periods and by sediment conditions during respiration dominated periods. Important information on carbon and oxygen cycles and the metabolic characteristics of water bodies can be easily obtained and evaluated with such a method.

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In this study, water quality prediction and verification were carried out at the newly-constructed Jozankei dam reservoir (Lake Sapporo), located in the southern part of Sapporo city, Hokkaido.
Field observations with the aim of estimating the pollutant inflow load to the Jozankei dam reservoir had been carried out since 1975, and the results of these observations were used to predict the water quality by the phosphorus-loading model. Phosphorus-loading model was also applied to Hoheikyo dam reservoir (Lake Jozan, an oligotrophic lake) and the model was shown to be reliable, thus confirming the usefulness of the phosphorus-loading model for water quality prediction. The water quality in the Jozankei dam reservoir was predicted to become oligotrophic or mesotrophic.
Field observations have been carried out at the Jozankei dam for period of 6 years since 1989, which was the first year of storage, for the purpose of verifying prediction results. After 3 years trial storage, full operations were commenced in 1992.
During the first 2 years after the beginning of storage, the TP concentrations were higher than the predicted values due to the nutrient release from submerged soils and plants. However, after the third year, TP concentrations decreased to almost the same level as the predicted values, which confirmed that a stable and clean water quality environment was being maintained.

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The distribution and biomass of Peridinium bipes f. occultatum were observed in autumn 1991, 1992 and 1994 at the head of the Shorenji Dam reservoir (Mie prefecture, Japan). Distributions show that Peridinium vertically migrated, and dispersed at the plunging point and horizontally moved to upper and lower stream in the study area. From these observations and informations in references, we supposed that the spatial biomass of Peridinium would be changed by “vertical migration” and “entrainment” ; horizontal “import” and “export” ; “outflow” ; and “growth” (including several biological processes). In the study area, biomass of Peridinium increased in rate similarly to growth day by day, and periodically varied about 2/3 of maximum biomass in a day. We estimated transport rates (cells·s^-1) by vertical profiles of the current velocity and the population density, and could explain this diurnal change of biomass. Thus, we concluded that the horizontal import and export will mainly change red tide's biomass at the head of the reservoir, and with the growth control, the flow control in the reservoir will be also important to suppress the fresh water red tide.

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Microcosm system was applied to assess effect of surfactants on aquatic ecosystem. Surfactants such as LAS and Soap were added to an aquatic flask-size microcosm consisting of four species of bacteria as decomposer, one species of ciliate protozoa (Cyclidium glaucoma), two rotifers (Philodina sp. and Lepadella sp.) and one aquatic oligochaete (Aeolosoma hemprichi) as predator, and a green alga (Chlorella sp.) and a filamentous blue-green alga (Tolypothrix sp.) as producer. In the system, NOEC (no observed effect concentration) of LAS was below 1.5mg·l^-1, whereas soap was below 30mg·l^-1. Microcosm test is a pertinent tool to assess the effect of surfactant on ecosystem because microcosm test makes it possible to evaluate the effect of surfactant from a viewpoint of the interaction of microorganisms, material cycle and energy flow. With these respects, microcosm test is useful environmental assessment method which can reflect aquatic ecosystem.

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Characteristics and performance of granular media filtration of eutrophicated moat water were studied in the bench-scale equipment to determine the design and operating parameters (AL/T, medium size, filtration rate) for optimum filter operation.
The reasonable aluminum/turbidity ratio (AL/T) was about 0.01 in both terms of achieving sufficient stability of the filter performance and lessening the influence of residual aluminum salt on fish and shellfish in the moat. The 1.2mm size medium had the highest SS removal ratio, but the weight of removed SS was the lowest due to clogging in the short term. The 2.4mm size medium was suited for the removal of the SS which were mostly particles larger than 20 μm and the VSS in influent, making long-term filtration possible. The filtration process of 2.4mm media was expressed in a filtrate equation by Mintz and a conservation equation. Higher filtration rates decreased the SS removal ratio more rapidly and reduced the ultimate deposit (σ_u). Judging from the calculation result, the filtration rate should be about 100m·d^-1 at the high SS concentration of influent in summer and should be about 100-175m·d^-1 at the low SS concentration of influent in winter.

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Characteristics of water quality, seasonal variation of DOC and total nitrogen (T-N) concentrations, in the floating-leaved plant Trapa stand were studied in western part of Lake Imba-numa, Chiba Prefecture during three years from April 1990 to April 1993.
Water was sampled in inside and outside of the Trapa stand once a month during the experimental period. Each sample was filtered through a 0.45 μm membrane filter and used for the water quality analysis.
The results obtained in this study are as follows ; the annual change of DOC concentration is steady, 7.8 mg·l^-1, in the inside of Trapa stand. The concentration of T-N has seasonal variation, lower than annual mean value (2.2 mg·l^-1) of the quality in summer season and higher than the mean value in winter season respectively, in the inside of Trapa stand.
Similarly, the hourly variations of DOC and T-P concentrations have steady in the inside of Trapa stand.
It is suggested from the experimental study that the variation of DOC and T-N concentrations is strongly affected by the production of vigorous growing plants of Trapa in the western part of Lake Imba-numa.

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Usually irrigation reservoirs are shallow and closed water way systems of smaller volume of water compared with ponds or lakes. Irrigation reservoir water qualities in urban area are frequently affected by inflow of domestic wastewater containing higher concentration of TP and TN. Simple models are necessary to manage irrigation reservoir water qualities.
Vollenweider's type models for TP loading were examined to predict water qualities of 6 irrigation reservoirs in Nagoya City. The model applied for OECD's Inland Water Project on Shallow lakes and Reservoirs (1979),
[P]λ=[P]i·(1+7.239τ^0.608/Z^0.5)^-1,
provided satisfactory estimate of TP concentration. Since the ratio of predicted to observed TP concentrations is within the range of 1/1.5 to 1.5 for all irrigation reservoirs, this prediction model is adequate peculiar type of water like a very small and shallow.
Finally, the annual average concentration of TP gave good correlations to annual average concentrations of COD (γ=0.91), TN (γ=0.82) and Chl. a(γ=0.94), respectively, for 37 irrigation reservoirs in the city. These correlations support further application of the model for prediction of irrigation reservoir water qualities.

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Characteristics of algae and water quality were studied at a pond located in high latitudes. Domestic and industrial wastewaters discharged are treated through different type stabilization ponds made by enclosing a part of the pond to remove BOD₅, COD_cr and nutrients. Effects of the pond partition on the dominance species of algae and water quality were studied.
During the cold term (-20°C--5°C), Cryptomonas, Chlamydomonas and Euglena were dominance species in the pond. While Euglena, which may be grown by uptaking organic material was the dominance one in the stabilization pond, and its population increased in falling temperature. It plays an important role for removal of BOD₅ and COD_cr. On the other hand, during the warm term (15°C-30°C), dominance species were Cyclotella, Chlorella and Microcystis in the pond and Scenedesmus oblignus in facultative ponds of the stabilization ponds. Microcystis and Scenedesmus obliqnus would uptake NH₄⁺-N truly. Removal of NH₄⁺-N and PO₄^3--P depends on the concentration of Chl.a. The relationship between NH₄⁺-N and PO₄^3--P removal and the concentrations of Chl.a was obtained.

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Sewage treatment systems have been established for a countermeasure of eutrophication of the Lake Togo, located at the middle part of Tottori prefecture, and improvement of the living environment of inhabitants in this region. All of these systems has started the operation in 1995.
In this report, the transition of water qualities in the Lake Togo from 1971 to 1993 was analyzed. Moreover pollutant loads in this region were calculated by using the unit loading factors.
In this study, the following points are cleared :
(1) The water qualities tend to improve slightly. The main reason is because a lot of pollutant loads was transported to the regional sewage treatment system in the other region and treated. (2) The amount of loads, decreased by the rural sewage treatment systems in this region, is almost same as that of nonpoint source pollutant loads increased by changes of land utilities. (3) It is possible that the effect of establishment of the systems will be offset by increasing the nonpoint source pollutant loads.

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The Lake Katanuma in the Narugo Volcano, a crater lake with strong acid water influenced by volcanic activity, was surveyed to elucidate the heat flux in the lake and the formation mechanism of acid lake water. The results are to be made use of as basic data for the prediction of erruption and the prevention of disaster. The lake forms the ellipse of 462m E-W length and 326m N-S breadth, and forms 1,380m shore line. The surface area is 1.26×10⁵m², the maximum depth is 21.3m and the volume is 7.48×10⁵m³.
In the bottom of the Lake, there are some high temperature zones where warm water springs out. Those zones are situated in the deepest, the south and the northeast basins of the Lake. The ground temperature gradient in the south high temperature zone amounts to 0.245-0.391°C·cm^-1.
Volcanic gasses evolve out of the deepest portion, which forms some limited depression. The volcanic gas dissolves in under-ground water vein in the shallow part under the bottom and forms the acid water by means of auto redox reaction of H₂SO₃, and at the same time, supplies heat at a rate of 372×10^-6cal·cm^-2·s^-1.

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