The sediment oxygen demand (SOD) and the efflux rates of nutrients and hydrogen sulfide (H₂S) were determined by the benthic chamber method to estimate the effect of the efflux of N·P·S from sediment in a dredging hollow. The sand capping experiment was carried out with granulated coal ash in a part of the dredging hollow located off-shore of Lake Nakaumi in Hosoi. The SOD maximum was 712 g·m^-2·day^-1, and the efflux rate maxima of NH₄-N, PO₄-P and H₂S were 640 mg·m^-2·day^-1, 103 mg·m^-2·day^-1 and about 12600 mg·m^-2·day^-1, respectively. The dredging hollow was shown to have a large effect on the internal load flowing into Lake Nakaumi. The rates of load reduction by sand capping with granulated coal ash during examination were as follows. SOD decreased by 23%. The efflux rates of H₂S, NH₄-N and PO₄-P decreased by 69%, 87% and 52%, respectively. Since the effect of sand capping was shown sufficient to improve the oxygen-poor state in the dredging hollow over a long period, it was revealed that sand capping with granulated coal ash could control efflux from sediment from the dredging hollow.

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Dissolved organic matter (DOM) in water samples obtained from raw water, reservoir, mountain stream and treated water in purification plants in the Ogasawara Islands, was fractionated into hydrophilic fractions, hydrophobic acids and hydrophobic neutrals. Hydrophilic fractions and hydrophobic acids of DOM were the main components of water samples from mountain streams, reservoirs and raw water in the Chichijima Island and Hahajima Island. The total trihalomethane formation potential (TTHMFP) and the contribution of each fraction were mostly in agreement with the total DOM and the rate of each fraction, respectively. The contribution of the hydrophobic acids fraction to the total haloacetic acid formation potential (THAAFP) was higher than that to TTHMFP. The rates or contributions of the hydrophilic fractions in DOM or TTHMFP/THAAFP were the highest, respectively, in activated carbon- and coagulation-sedimentation-treated water of the Chichijima Island, and coagulation-sedimentation-treated water of the Hahajima Island. It is thus suggested that a water treatment method for removing hydrophilic matter of DOM in raw water can effectively decrease the amount of DOM thus resulting in a decrease in the amount of disinfection by-products in treated water. It is presumed that the weight-average molecular weights of DOM in mountain stream water and raw water are both 1220∼1390 g·mol^-1 , and it is indicated that DOM in raw water originates from such mountain streams flowing into a reservoir. It is also suggested that the weight-average molecular weights of DOM in treated water are reduced to 630∼970 g·mol^-1 after water purification treatment.

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We investigated the annual nitrate nitrogen (NO₃^--N) load for seven years, taking stormy flood events into consideration, in a forested catchment in the headwaters of the Shimanto River, Shikoku, Japan. This area frequently receives heavy rain exceeding 100 mm. The vegetation of the experimental catchment (18.7 ha), underlying sedimentary rock, is a natural forest comprising predominant conifers (Abies firma and Tsuga sieboldii), evergreen and deciduous trees. Streamwater was collected twice a month, adding streamwater of 17 flood events (total rainfall of 18 - 289 mm). It was revealed that NO₃^- concentration before rainfall was an effective predictor except runoff for evaluating the cumulative specific nitrate load. Based on the modified ΣL - ΣQ equation added this predictor, the annual nitrate load was evaluated 3 to 5 kg·ha^-1·yr^-1 and did not significantly correlate to the annual rainfall or annual runoff. This is due to the remarkable decline in NO₃^- concentration during stormy flood events, suggesting that cumulative specific nitrate load tends to plateau. The remarkable decrease in NO₃^- concentration would be due to that subsurface water on hillslope rather than near stream, which has an extremely low NO₃^- concentration, contributes predominantly to streamwater during stormy flood events, excluding the early stage.

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The biodegradability of the green alga Staurastrum arctiscon was studied, using a natural bacterial community sampled from Lake Biwa, to understand the generation characteristics of dissolved organic carbon (DOC) from the cytoplasm and extracellular polysaccharides (EPS). The biodegradation test revealed that 31% of the particulate organic carbon (POC) of the cytoplasm remained after 200 days of biodegradation and 2-4% of the initial cytoplasm POC was transformed into DOC. Moreover, it was clearly shown that 29-35% of the initial POC of the EPS was transformed into DOC and stably remained after 77 days of biodegradation. The morphology changes of S. arctiscon observed in the biodegradation experiments by staining EPS with fluorescently labeled wheat germ agglutinin (WGA) revealed that the fluorescence intensity gradually decreased over time and disappeared after 21 days of biodegradation, implying the partial degradation of the EPS. However, the EPS were still present even at 100 days, suggesting that some of them were comparatively recalcitrant to biodegradation. Consequently, our results suggest that phytoplankton-derived organic matter may play an important role as a source of DOC and POC in Lake Biwa.

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We developed a population model considering the biological characteristics of Aetobatus flagellum, for the conservation of bivalves and the reduction of ecological damage to Ariake Bay. On the basis of the numerical model developed, we studied an effective method of A. flagellum extermination and calculated the impact of carrying it out in the bay to reduce predation damage to bivalves. The populations of A. flagellum calculated using the model were in agreement with those observed by field study. The calculation result of current A. flagellum extermination projects being performed from June to October revealed that more than 40% of the damaged bivalves were reduced as compared with the amount when extermination had not been performed. In addition, our model revealed that the reduction in the population of damaged bivalves could be more effective if the extermination is carried out in the northern head area of Ariake Bay, within Fukuoka and Saga Prefectures, during the spawning season in June and July.

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