In food webs between usual zooplankton and phytoplankton, an ecosystem model, which con tains a microbial loop between bacteria and zooplankton, was developed. Numerical simulations of the ecosystem in Tokyo Bay were carried out for 1 year using this model. The observation result for the time series fluctuation of 3 stations of the phytoplankton concen tration in the bay was compared with the calculation result. The fluctuating range of simulated values is smaller than that of observed values in winter. Simulated results are in good agreement with order of phytoplankton cycle throughout 1 year the observations. Primary production in Tokyo Bay from June 1995 to May 1996 is estimated at 1,600 tonC/y from the simulations. Dia toms are about 1.4 times dinoflagellate in terms of biomass. This indicates that diatom accounts for about 58% of phytoplankton standing stock in Tokyo Bay. Carbon flow fluxes to zooplankton from phytoplankton are 550 tonC/y, and carbon flow fluxes to zooplankton from bacteria through protozoa are 990 tonC/y, and microbial loop are about 1.8 times grazing food webs. In addition, nitrogen flow fluxes to phytoplankton from nitrogen pool are 156 tonN/y, and nitrogen flow fluxes to bacteria from nitrogen pool are 480 tonN/y,and microbial loop are about 3 times grazing food webs. Thus results suggest that the role was great and very important in carbon and nitrogen cycle. The horizontal distribution of phytoplankton in the summer season from the classical type of the ecosystem model showed the stabilized distribution. However, daily variations of horizon tal distributions of diatom concentration in the present model are revealed. It is considered that the effect by the advection is strong for the change of the horizontal distribution of phytoplankton.

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One of the effective measures to cope with eutrophication of lake waters is to remove nutrient substances that can cause to algal blooming, by taking advantage of the natural capability of water purification. Here the term 'purification' is defined, in a wide sense, as a potential role of the lake contributing to reduction of pollutants and thus restraint to eutrophication; in which various pro cesses concerning seasonal nutrient fixation such as uptake by aquatic plants, attachment to foli age substrates and feeding by organisms of higher-trophic level are regarded as biological purifi cation processes together with eternal losses or removals from waters. In order to evaluate the purification capacity, a numerical lake ecosystem model was developed highlighting on the role of macrophyte colonies that inhabit alongshore, and applied to Lakes Suwa, Kasumi, Biwa and five attached sublakes of Lake Biwa. The model takes account of the biological interactions between pelagic compartments (phytoand zooplankton, detritus, dissolved organic matter, pelagic fish and nutrients) and benthic com partments (macrophytes, attached algae, attached small animals, macro- and megalobenthos and demersal fish). Under the time-dependent conditions of meteorological and hydraulic factors, the model was run over a year to evaluate annual nutrient budget and purification capacity of each lake. The results revealed that the purification capacity is generally estimated, from the stand points of both stock and flux, higherin the shoreregion than in the offshoreregion becauseof a wide variety of organisms including aquatic plants. Moreover,it almost increases in proportion to vegetation densityof the shoreregion.Nutrientfluxes associatedwith purificationprocessesturned out closely related to surfacearea or equivalently to retentiontime of lake waters,suggestingthat the biological purificationdoes not becomedominantagainst physical turnoveruntil the surface area reaches around 103 ha or the retention time exceeds around 10^2 days.

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The latitudinal variation of phytoplankton community structures was estimated from phytoplankton pigment analysis in western Pacific Ocean (140°E-175°E, 30°N-25°S). Surface water was collected during the overseas educational cruise of Tokai University from February 15 to March 31, 2002. 8 pigments including chlorophyll a (Chla) were identified and quantified by reversed-phase HPLC. Chla concentration and the pigments composition changed distinctly across 25°N to 30°N where temperature fronts were found. On the north side of the front, Chla concentration was more than 0.2ug/L.The maximumvalue was 0.58ug/L in the late March, which was explainedas a spring bloom.The fucoxanthin and 19'-hexanoyloxyfucoxanthin, which stand for diatoms and prymnesiophyte, respectively, varied proportional to Chla concentration. On the other hand, zeaxanthin contained in cyanobacteriaand prochlorophyte dominated in the southern part of the front where Chla concentration was low (＜0.1ug/L). Zea : Chla ratio showed the highest value, 0.237 ± 0.139, in this area. Summarizing these results, we show that diatoms and prymnesiophyte increase inshore, whereas picophytoplankton including cyanobacteria and prochlorophytedominates more than 80% in oligotrophic waters of offshore.

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