1. Ariake Bay, which is located in western Japan, has a large tidal range (>6 m) and a vast tidal flat (200 km 2 ). In the early 1990s, the government-managed Isahaya Reclamation Project began in the western part of Ariake Bay. A 16-km 2 area of tidal flat in the inner part of Isahaya Bay was destroyed through reclamation and separated from the sea by a dyke, which created land and a freshwater reservoir.2. Since the initiation of the project, fishery yields around Isahaya Bay have dramatically decreased. The objective of this study was to clarify the relationship between the work associated with the Isahaya Reclamation Project and the recent environmental deterioration in Ariake Bay, with references to present sediment thickness and organic matter content, and hypoxic water distributions in Isahaya Bay.3. The organic matter load from the reservoir has increased since the initiation of the reclamation project and has been associated with a thick layer of fine sediment at the bottom of Isahaya Bay. The thickness of fine sediment and the total organic carbon content were higher in Isahaya Bay than in the freshwater reservoir.4. Based on measurements in August 2001, hypoxic water spread widely in and around Isahaya Bay; the lowest dissolved oxygen (DO) concentration (0.53 mg L À1) was observed just outside the dyke. An analysis based on a two-layered box model using data obtained in August 2001 showed that the DO consumption rate in the bottom layer was high (0.61 mg O 2 L À1 day À1 ), and that 22-41% of the total organic carbon load needed to induce the hypoxic water was derived from the reclamation area.5. Our findings strongly suggest that enclosed seas may suffer from eutrophic and hypoxic conditions because of their low seawater-exchange rate. A comprehensive conservation programme and environmental assessment including physical and material transport processes in the system is needed to manage the environment of the enclosed sea.

The chironomid fauna of the lower reaches of the Nagara River was investigated on the basis of male adults emerging from bottom samples and those collected by light sampling. As a result, a total of 547 males representing 72 species were collected. For bottom samples, one occurrence peak was observed from May to June. The number of species increased remarkably with the distance from the rivermouth. For light samples, two peaks of species richness were observed, one in June and another from September to November. Larval distribution ranges differed considerably with species. Cryptochironomus albofasciatus was recorded from all the sites. Fourteen out of 18 major species were collected only at the upper reaches of more than 20 km from the rivermouth. This suggests the existence of a salinity barrier for freshwater species between 15 km and 20 km. Clear differences in the seasonal occurrence of larvae were observed among species. For example, Polypedilum masudai and P. cultellatum occurred only from spring to summer and summer to autumn, respectively. Paratendipes albimanus was collected only by bottom sampling, suggesting the weak phototaxis of male adults of the species. In contrast, Smittia aterrima was collected only by light sampling, probably due to the terrestrial nature of the species.

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