Abstract
The Nagara rivermouth barrage was constructed 5.4km upstream from the rivermouth and has been in operation since July 1995. In this study, an analysis using monitoring data was performed to establish the cause of the high concentration of phytoplankton at St. Ise (1.0km upstream from the barrage). The high concentration of phytoplankton appears under good conditions for reproduction (retention time, water temperature, solar radiation, and nutrients), therefore, self-replication of phytoplankton was one of the factors associated with this phenomenon. In addition, GR (the actual value of chlorophyll a (Chl.a) growth rate calculated from the monitoring data) was compared with GRmax (the maximum value of Chl.a growth rate determined from self-replication of phytoplankton based on its composition) to evaluate the effect of the accumulation of phytoplankton. GR was frequently greater than GRmax and it was verified that accumulation plays an important role in the occurrence of the high concentration of phytoplankton at St. Ise. The reasons for accumulation were thought to be that opposite flow induced by an oncoming wind horizontally and reversely transports the phytoplankton and that the settling velocity of the phytoplankton decreases in the presence of a thermal stratification.
