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[AOS17-06] Interannual variation of bottom water DO in summer in Isahaya Bay, Ariake Sea
Keywords:hypoxic water mass, Ariake Sea, Isahaya Bay, interannual variation
In Ariake Sea, hypoxic water masses were formed every summer in the inner part of Ariake Sea and Isahaya Bay. Since massive kills of bivalves were generated by the hypoxia, they became serious problems. The interannual variation of hypoxia in the inner part of Ariake Sea was analyzed by Tokunaga et al (2013). They showed that there was high positive correlation between the bottom water dissolved oxygen (DO) and the tidal current. However there was no analysis about the interannual variation in the bottom DO in Isahaya Bay. Therefore, we analyzed the bottom DO data in Isahaya Bay for 17 years (2004-2020) and investigated the mechanism of the variation.
Temperature, salinity, DO, chlorophyll fluorescence and turbidity data monitored by the Kyushu Regional Agricultural Administration Office using vertical profiling systems of water quality at 6 stations in Isahaya Bay were analyzed. The sea level data at Oura monitored by the Japanese Meteorological Agency and the river discharge data of the Chikugo River monitored at Senoshita were also used.
There was a significant negative correlation between the bottom DO and the vertical stratification parameter at the 4 stations (B3-B6) located at central to the bay mouth area of Isahaya Bay in summer (July and August). There was a significant positive correlation between the bottom water DO and vertically averaged chlorophyll fluorescence. The stratification parameter was highly correlated with the river discharge of the Chikugo River. However, there was no correlation between the stratification and the tidal amplitude of the M2 tide. The M2 tidal amplitude varied 10 cm caused by the lunar nodal cycle in these 17 years.
These results shows that the interannual variation of bottom water DO in Isahaya Bay was generated by the variation in stratification caused by the variation in river discharge of the Chikugo River. The variation of the phytoplankton biomass also affected the interannual variation. The increase of the phytoplankton raised the bottom water DO. The influence of the tidal amplitude was small.
Temperature, salinity, DO, chlorophyll fluorescence and turbidity data monitored by the Kyushu Regional Agricultural Administration Office using vertical profiling systems of water quality at 6 stations in Isahaya Bay were analyzed. The sea level data at Oura monitored by the Japanese Meteorological Agency and the river discharge data of the Chikugo River monitored at Senoshita were also used.
There was a significant negative correlation between the bottom DO and the vertical stratification parameter at the 4 stations (B3-B6) located at central to the bay mouth area of Isahaya Bay in summer (July and August). There was a significant positive correlation between the bottom water DO and vertically averaged chlorophyll fluorescence. The stratification parameter was highly correlated with the river discharge of the Chikugo River. However, there was no correlation between the stratification and the tidal amplitude of the M2 tide. The M2 tidal amplitude varied 10 cm caused by the lunar nodal cycle in these 17 years.
These results shows that the interannual variation of bottom water DO in Isahaya Bay was generated by the variation in stratification caused by the variation in river discharge of the Chikugo River. The variation of the phytoplankton biomass also affected the interannual variation. The increase of the phytoplankton raised the bottom water DO. The influence of the tidal amplitude was small.