Abstract
Plausible causes of summer oxygen deficit in the lower reaches of the Nagara Rivermouth Barrage were examined using the continuous data of dissolved oxygen, chlorophyll a and salinity recorded by the Japanese Ministry of Construction. The following three causes were detected : i.e., 1) increase in a night oxygen consumption by a large biomass of algal plankton, 2)rise of hypoxic water from Ise Bay at spring tide, and 3) increase in magnitude of stratification at neap tide which hinders vertical mixing of water. The last cause accounted for the severe oxygen deficit frequently occurred after the barrage construction since it began a few days after neap tide and was maintained until the next spring tide or flood. Both annual changes in a total duration of oxygen deficit in the Nagara River and a result of comparison with the Kiso River indicated that the neap tide effect on the oxygen deficit was enhanced by operation of the Rivermouth Barrage.
