Rapid reduction of pH and CaCO3 saturation state in the Tsugaru Strait by the intensified Tsugaru warm current during 2012‐2019

Abstract

To examine the ocean acidification of coastal water as the result of the oceanic uptake of anthropogenic atmospheric CO2, we initiated acidification monitoring in the eastern part of the Tsugaru Strait, through which the Tsugaru Warm Current flows eastward from the Sea of Japan to the North Pacific. Annual mean pH and CaCO3 saturation state during 2012–2019 decreased considerably throughout all depths at rates of 0.0030−0.0051 yr−1 and 0.017−0.036 yr−1, respectively. These rates of decrease are faster than those caused by increasing atmospheric CO2, and faster than those observed during previous research. These fast rates are attributed to an enhanced increase in dissolved inorganic carbon concurrently with increases in salinity and density caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western part of the strait. The elevated mixing is attributable to the strengthening of the Tsugaru Warm Current.

Plain Language Summary

Approximately 30% of the total amount of CO2 released to the atmosphere by human activities has accumulated in the global ocean. This oceanic uptake of CO2 has resulted in ocean acidification. In coastal waters the acidification affects marine organisms, thus coastal ecosystems may be more vulnerable to acidification than the open ocean. To examine the extent to which acidification has advanced in the eastern part of the Tsugaru Strait, through which the Tsugaru Warm Current passes from the Sea of Japan to the North Pacific, we initiated a time‐series observation of acidification. The pH reduction is found to have enhanced considerably across the whole depth during 2012‐2019 at a rate faster than that caused by increasing atmospheric CO2 and at the highest rates observed during previous research. The rapid pH reduction is found to be attributable to the enhanced rate of increase of dissolved inorganic carbon concurrently with increases in salinity and density caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western strait due to the strengthening of the Tsugaru Warm Current. In other straits that are connected to the open ocean, the strengthening of their throughflow may also accelerate acidification.

Wakita M., Sasaki K., Nagano A., Abe H., Tanaka T., Nagano K., Sugie K., Kaneko H., Kimoto K., Okunishi T., Takada M., Yoshino J. & Watanabe S., 2021. Rapid reduction of pH and CaCO3 saturation state in the Tsugaru Strait by the intensified Tsugaru warm current during 2012‐2019. Geophysical Research Letters 48: e2020GL091332. doi: 10.1029/2020GL091332. Article (subscription requied).

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