浮遊系一底生糸結合生態系モデルを用いた有明海での貧酸素水塊形成機構の支配要因の解析

摘要

大きな潮位·潮流と活発な生物生息環境としての広 大な干潟を有する有明海の物理·化学·生物的な特徴 を表現した「浮遊系一底生糸結合生態系モデル」を構築 し，長期間にわたる現況再現計算を実施した．構築した モデルでは，計算期間中同一のパラメータを用いて，有 明海の経年的な貧酸素水塊の発生状況を精度良く表現で きた．%Recently, the serious environmental problems are occurring in Ariake Bay which was the sea of the abundant harvest. In order to determine the effective policy about environmental and ecosystem restoration project, we developed a pelagic-benthic coupled ecosystem model for Ariake Bay and carried out hind-cast simulation during 7 years. Simulation results of hydrological features and water quality were consistent with those of observed one. Furthermore calculated concentration of dissolved oxygen indicated distinct variability in spring-neap tidal cycle and variation of hypoxia during recent 7 years from 2000 to 2006 in high accuracy. These features of calculated hypoxia water were consistent with results of many observational aspects. In order to clarify these hypoxia generation mechanisms, fluxes of dissolved oxygen due to diffusion, advection and many biochemical processed were calculated from simulation results. Calculated fluxes suggested that vertical diffusion was dominant process for oxygen supply to bottom water at Isahaya Bay and near the edge of tidal-flat at head of bay and it was controlled by river discharge through strength of stratification. On the other hand calculated oxygen consumption rate in pelagic system was larger than benthic system in hypoxia water except for near head of bay, where oxygen consumption in benthic system was not negligible. Furthermore flux analysis of biochemical processes indicated that about a half of oxygen consumption in pelagic system was caused by decomposition of POM originated by primary production. However POM production was occurred over relatively wide area, horizontal distribution of POM flux indicated the convergence area at head of bay and setting flux to sediment was dominant near this area at same time. It was considered that oxygen consumption rate in benthic system was maintained at larger value as a result of physical accumulation of POC and respiration of benthos in these area. Therefore, transport processes of POC through the estuarine circulation plays a crucial role about generation mechanism of hypoxia water in Ariake bay.