Influence of the construction of Nagaragawa Barrage on habitats and ecosystems in the estuary

本课题组取得的主要成果如下：1）通过对5年的监测资料和现有的现场资料进行详尽的研究，明确了拦河坝上游为淡水区，下游为海水区，无黑色区的现状。2）1994 ~ 1999年，共记录鱼类94种，其中消失16种，新出现19种。鱼类种类的变化率达37%，尤其是黑色鱼类的消失受到很大影响。3）控制河口溶解氧浓度的一个主要因素是洪水扰动的潮汐循环。大洪水使溶解氧浓度恢复，但由于伊势湾缺氧水体的侵入，造成洪水后溶解氧浓度降低。4）包括底层水在内的完全混合并非每天都发生，夏季会维持几天的暂时性热分层。从水柱迅速耗氧的速度来看，这种暂时的热分层可能导致夏季拦河坝蓄水时经常出现缺氧。

Major results obtained by this research group are summarized as follows.1) Exhaustive study both on five-year monitoring data and other existing field data clarified a present situation that the upstream of the Barrage is fresh water zone and the downstream of the Barrage is sea water zone with no blackish zone. Salinity distribution downstream of the Barrage changed from mixed type to fully mixed type almost all days through a year.2) Recorded total number of fish species is 94 within which 16 species disappeared and 19 species appeared newly during 1994 through 1999. Altered ratio of fish species amounted to 37% and especially blackish species were affected greatly to disappear.3) A major component that governs the dissolved oxygen concentration in an estuary is a tidal cycle with disturbance of floods. Large floods recover DO concentration but bring about low DO conditions after floods by the intrusion of anoxic water body from Ise Bay.4) Complete mixing including the bottom water did not occur every day, maintaing temporary thermal stratification for a few days in summer. Judging from the rapid oxygen consumption rate by the water column, such temporary thermal stratifications could cause frequent oxygen deficits in the impoundment of the Barrage in summer.