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The trophic status and water quality of Lake Tilitso (4920 m above sea level) in a high altitude region in central Nepal were surveyed in September, 1984. The lake is rather large with a maximum depth of 95 m and a surface area of 10.2 km2. The lake water was turbid due to glacier silt and the euphotic layer was only 5 m deep. The nutrient concentration was very low with total phosphorus concentration 1 -6 pg l-i, and lYTN concentration 0.10-0.22 mg 1-l. The phytoplankton biomass and chlorophyll-a concentration were also low. Primary production was estimated to be about 12 mg C me2 d-i. The concentrations of particulate matter and most cations and bacterial number were higher in the epilimnion than in the hypolimnion. The trophic status of this lake was estimated as ultraoligotrophic.

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Aiming at estimating the effect of an amelioration scheme for water quality in a highly eutrophicated and strongly enclosed estuary, especially placing emphasis on the alleviation of a oxygen-depleted water body, a threedimensional (3-D) time-dependent coupled physical and ecological model was applied to Lakes Shinji and Nakaumi, a typical coastal brackish lagoon system in Japan. First, the model was run to reproduce the seasonal variation in water temperature, salinity, dissolved oxygen, and other ecological constituents during the period from April 1998 to March 1999. The daily change regimes of the observed sea surface winds, tidal elevation at the open boundary, river discharges, and meteorological parameters were incorporated into the forcing variables of the ecosystem model. Next, as a numerical experiment, a scenario simulation was carried out under four premises to alleviate the oxygen-depleted water body. The model results agreed fairly well with the fi eld measurements obtained through the regular environmental monitoring in the lagoon system. It was found that an oxygen-depleted water body appears in the eastern part of Lake Nakaumi in early May and develops to cover the whole lagoon in midsummer to leave it almost anoxic. The scenario simulation revealed that each amelioration scheme has a considerable effect in alleviating this hypoxic water body. It was found from the model results and the additional fl ux analyses that the oxygen concentration will improve as a result of (1) change in the fl ow fi eld by cutting the dikes built up during lake reclamation and (2) decrease in benthic oxygen consumption by fi lling up deep holes once created by dredging activities.

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The seasonal effects of nutrient loading from migratory waterfowl on water and the successive changes of the aquatic ecosystem based on each biomass of phytoplankton, zooplankton and submerged macrophytes were surveyed in Tsubasa Pond, Yonago Waterbird Sanctuary, Japan. The pond's water quality gradually deteriorated with the influx of migratory waterfowl. The concentration of total nitrogen corresponded rapidly with waterfowl biomass. Peak concentrations of total phosphorus and chlorophyll a (Chl.a) were observed about one month later when the migratory waterfowl had flown away. After one month, the peak concentration of Chemical Oxygen Demand (COD Mn ) appeared. Thus, the water quality of the pond had gotten worst after the waterfowl had flown away. Then, decrease of Chl.a concentration and increase of zooplankton density were observed in spring. In summer, the population of a submerged macrophyte increased temporarily and water quality recovered. These results indicated that the primary producer in the bird sanctuary pond alternated from phytoplankton to submerged macrophyte in one year.