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[SCG53-P01] Spring water from the deep lake bottom in Lake Biwa
Keywords:Lake Biwa, Spring water from the lake bottom, Environment
1. Introduction
It is said that around 10-20% of the water flowing into Lake Biwa is spring water from lake bottom in Lake Biwa(Shiga Prefec, 2018). The spring water is considered to have a non-negligible effect on the environment of Lake Biwa. However spring water from the deep lake bottom has not been well clarified. In Lake Biwa water generally forms a stratified structure from spring to autumn. Then the whole circulation occurs and the structure is lost in winter. If there is spring water in the deep lake bottom, it must have much effect on the deep lake bottom environment from spring to autumn. In addition the whole circulation did not occur in winter in 2019 and 2020 because of warm winter. To protect the lake bottom environment, considering global warming, it is important to study the spring water from the deep lake bottom in Lake Biwa.
Kumagai et al.(2021) firstly found the spring water with gas from the deep lake bottom in Lake Biwa in 2009 using AUV(Fig.1). They also wrote that the number of benthic vents, which are outlets of the spring water and gas, was increasing from 2009 to 2012. They were concerned about the effect of the spring water on the environment of Lake Biwa. Besides they claimed that the deep bottom spring water was not usual groundwater and the increase in the vent number was related to tectonic activity in Japan. However the evidences for the claim are not considered to be sufficient. The deep bottom spring water of Lake Biwa has not been sufficiently investigated since 2013 (Kumagai et al., 2021). Therefore we conducted this research to know current conditions of the deep bottom spring water. It is supported by the Grant-in-Aid for Scientific Research "20H01974: Elucidation of the relationship between the underground structure and deep bottom spring in Lake Biwa and evaluation of its effect on the lake bottom environment".
2.Methods
We carried out sonic exploration, measurement of temperature gradient in lake bottom sediments, observation of the lake bottom with underwater cameras, CTD measurements, and hydrogen and oxygen isotope ratio measurements of the lake water around Y1 and T1 several times in 2020 and 2021. Y1 is the point where the acoustic anomalies by the gas bubbles from the deep bottom spring water is usually detected. T1 is the reference point, which is a periodical observation point of the University of the Shiga Prefecture. The water depth of T1 and Y1 is about 90m.
3.Result
At Y1 the sonic exploration usually detected the acoustic anomalies caused by the gas bubbles which are considered to be derived from the deep bottom spring water. On the other hand, in the area where we explored (Fig.1), no acoustic anomaly was detected except for Y1. We detected several high temperature gradients at the lake bottom in the vicinity of Y1 although we hardly detected such high temperature gradients in the other place. The underwater camera detected many white discoloration areas with a size of several tens of centimeters at the lake bottom of Y1. The camera also detected the white discoloration areas in the other place where Kumagai et al.(2021) had detected the acoustic anomalies. However only the white discoloration areas at Y1 produced gas bubbles. At a water depth of 20 m or deeper, the CTD measurement detected little difference in the vertical distribution of water temperature, electrical conductivity, dissolved oxygen concentration, and chlorophyll concentration between Y1 and T1. There was no significant difference in the oxygen and hydrogen isotope ratios of the lake water between T1 and Y1.
In our presentation, we will also consider the reason why the number of benthic vents was increasing from 2009 to 2012.
Acknowledgments
We are grateful to Dr. M. Kumagai, a professor at Ritsumeikan University, because he provided valuable data on the topography of Lake Biwa and the location of benthic vents.
Fig.1 Benthic vents location(X) by Kumagai et al. (2021), Y1 where deep bottom spring water is usually detected and the reference point (T1). The gray line is the contour line (unit: m) with the lake surface as 0 m. The area surrounded by the red line is where we mainly conducted the sonic exploration in 2021.
It is said that around 10-20% of the water flowing into Lake Biwa is spring water from lake bottom in Lake Biwa(Shiga Prefec, 2018). The spring water is considered to have a non-negligible effect on the environment of Lake Biwa. However spring water from the deep lake bottom has not been well clarified. In Lake Biwa water generally forms a stratified structure from spring to autumn. Then the whole circulation occurs and the structure is lost in winter. If there is spring water in the deep lake bottom, it must have much effect on the deep lake bottom environment from spring to autumn. In addition the whole circulation did not occur in winter in 2019 and 2020 because of warm winter. To protect the lake bottom environment, considering global warming, it is important to study the spring water from the deep lake bottom in Lake Biwa.
Kumagai et al.(2021) firstly found the spring water with gas from the deep lake bottom in Lake Biwa in 2009 using AUV(Fig.1). They also wrote that the number of benthic vents, which are outlets of the spring water and gas, was increasing from 2009 to 2012. They were concerned about the effect of the spring water on the environment of Lake Biwa. Besides they claimed that the deep bottom spring water was not usual groundwater and the increase in the vent number was related to tectonic activity in Japan. However the evidences for the claim are not considered to be sufficient. The deep bottom spring water of Lake Biwa has not been sufficiently investigated since 2013 (Kumagai et al., 2021). Therefore we conducted this research to know current conditions of the deep bottom spring water. It is supported by the Grant-in-Aid for Scientific Research "20H01974: Elucidation of the relationship between the underground structure and deep bottom spring in Lake Biwa and evaluation of its effect on the lake bottom environment".
2.Methods
We carried out sonic exploration, measurement of temperature gradient in lake bottom sediments, observation of the lake bottom with underwater cameras, CTD measurements, and hydrogen and oxygen isotope ratio measurements of the lake water around Y1 and T1 several times in 2020 and 2021. Y1 is the point where the acoustic anomalies by the gas bubbles from the deep bottom spring water is usually detected. T1 is the reference point, which is a periodical observation point of the University of the Shiga Prefecture. The water depth of T1 and Y1 is about 90m.
3.Result
At Y1 the sonic exploration usually detected the acoustic anomalies caused by the gas bubbles which are considered to be derived from the deep bottom spring water. On the other hand, in the area where we explored (Fig.1), no acoustic anomaly was detected except for Y1. We detected several high temperature gradients at the lake bottom in the vicinity of Y1 although we hardly detected such high temperature gradients in the other place. The underwater camera detected many white discoloration areas with a size of several tens of centimeters at the lake bottom of Y1. The camera also detected the white discoloration areas in the other place where Kumagai et al.(2021) had detected the acoustic anomalies. However only the white discoloration areas at Y1 produced gas bubbles. At a water depth of 20 m or deeper, the CTD measurement detected little difference in the vertical distribution of water temperature, electrical conductivity, dissolved oxygen concentration, and chlorophyll concentration between Y1 and T1. There was no significant difference in the oxygen and hydrogen isotope ratios of the lake water between T1 and Y1.
In our presentation, we will also consider the reason why the number of benthic vents was increasing from 2009 to 2012.
Acknowledgments
We are grateful to Dr. M. Kumagai, a professor at Ritsumeikan University, because he provided valuable data on the topography of Lake Biwa and the location of benthic vents.
Fig.1 Benthic vents location(X) by Kumagai et al. (2021), Y1 where deep bottom spring water is usually detected and the reference point (T1). The gray line is the contour line (unit: m) with the lake surface as 0 m. The area surrounded by the red line is where we mainly conducted the sonic exploration in 2021.