3:45 PM - 4:00 PM
[AOS22-08] Inter-annual variation in tidal current and its relationship between dissolved oxygen concentration in bottom layer in the Isahaya Bay, Ariake Sea
Keywords:Ariake Sea, hypoxic water mass, tidal current, luner nodal variation, inter-annual variation, dissolved oxygen
The Ariake Sea is the area with the largest tide in Japan. The predominant tidal constituent in the Ariake Sea is the M2 tide. The lunar nodal cycle with the period of 18.6 years is dominant in the inter-annual variation of M2 tide. On the other hand, hypoxic water masses occur every summer in the Isahaya Bay and the inner part of the Ariake Sea, which has a great impact on bivalve fisheries. These hypoxic water masses are strongly affected by density stratification and tidal mixing. The hypoxia progresses during neap tides when tidal mixing is weak, and is diminished during spring tides. Therefore, the lunar node movement would affect the dissolved oxygen concentration in the bottom layer, and if the tidal current weakens, the stratification may be strengthened and the dissolved oxygen concentration in the bottom layer may decrease.
In this study, to clarify the effect of inter-annual variation in tides on the dissolved oxygen concentration in the bottom layer, continuous measurements of currents were carried out in summer and winter from 2013 to 2021 in the Isahaya Bay. Vertical distributions of currents were observed with bottom mounted ADCPs located at 4 stations close to the water quality monitoring tower of the Kyushu Regional Agricultural Administration Office. The measured currents were converted into 10 sigma layers vertically and the 9 layers except for the first surface layer were analyzed. In addition, the tide level data at Oura and the water temperature, salinity, density, and dissolved oxygen concentration data measured by the Kyushu Regional Agricultural Administration Office with automatic water quality monitoring systems were analyzed for comparison with the current data.
Tide and tidal currents increased since 2013, peaked in 2016 and then decreased. The inter-annual variation of M2 component of barotropic flow had positive correlation with the M2 component of the tide level. On the other hand, the average dissolved oxygen concentration in the bottom layer during the summer observation period reached a minimum in 2016, which was negatively correlated with the M2 component of the tidal current. This means that, contrary to the assumption at the beginning of this study, the dissolved oxygen concentration in the bottom layer decreases as the tidal current becomes stronger. The correlation between the average stratification and the M2 tide component of the current during the observation period was weak, but a positive correlation was found between them at stations B5 and B6 (the stratification was strengthened when the tidal current became stronger). We would also like to show the mechanism of these unexpected results in our presentation in the meeting.
In this study, to clarify the effect of inter-annual variation in tides on the dissolved oxygen concentration in the bottom layer, continuous measurements of currents were carried out in summer and winter from 2013 to 2021 in the Isahaya Bay. Vertical distributions of currents were observed with bottom mounted ADCPs located at 4 stations close to the water quality monitoring tower of the Kyushu Regional Agricultural Administration Office. The measured currents were converted into 10 sigma layers vertically and the 9 layers except for the first surface layer were analyzed. In addition, the tide level data at Oura and the water temperature, salinity, density, and dissolved oxygen concentration data measured by the Kyushu Regional Agricultural Administration Office with automatic water quality monitoring systems were analyzed for comparison with the current data.
Tide and tidal currents increased since 2013, peaked in 2016 and then decreased. The inter-annual variation of M2 component of barotropic flow had positive correlation with the M2 component of the tide level. On the other hand, the average dissolved oxygen concentration in the bottom layer during the summer observation period reached a minimum in 2016, which was negatively correlated with the M2 component of the tidal current. This means that, contrary to the assumption at the beginning of this study, the dissolved oxygen concentration in the bottom layer decreases as the tidal current becomes stronger. The correlation between the average stratification and the M2 tide component of the current during the observation period was weak, but a positive correlation was found between them at stations B5 and B6 (the stratification was strengthened when the tidal current became stronger). We would also like to show the mechanism of these unexpected results in our presentation in the meeting.