Effects of warming, eutrophication and climate variability on acidification of the seasonally stratified North Yellow Sea over the past 40 years.

The North Yellow Sea (NYS) is a productive marginal sea of the western North Pacific. In summer and autumn, CaCO saturation states beneath the seasonal thermocline in the NYS have frequently fallen below critical levels, indicating that marine calcifying organisms are under threat. To explore the long-term evolution of the acidification of the NYS, we reconstructed seasonal variations in subsurface aragonite saturation state (Ω) and pH during 1976-2017, using wintertime and summertime temperature, salinity, dissolved oxygen and pH data mainly from a quality-controlled oceanographic database. Over the past 40 years, the wintertime warming rate in the NYS was twice the rate of global ocean surface warming. Warming-induced decrease in CO solubility canceled out a part of the wintertime Ω decrease caused by atmospheric CO increase, and also had minor effect on pH changes in winter. Although the NYS is a semi-enclosed marginal sea, its interannual variations of wintertime temperature, salinity, pH and Ω were correlated to Pacific Decadal Oscillation with a lag of 2-3 years. Due to the eutrophication-induced enhancement of net community respiration beneath the seasonal thermocline, long-term declines of bottom-water Ω and pH in summer were substantially faster than the declines of assumed air-equilibrated Ω and pH in spring. Over the past 40 years, the amplitudes of seasonal variations of bottom-water Ω and pH from spring to summer/autumn have increased by 4-7 times. This amplification has pushed the NYS towards the critical threshold of net community CaCO dissolution at a pace faster than that forecast under scenarios of atmospheric CO increase. In summary, our results provide insights into the combined effects of ocean warming, eutrophication, atmospheric CO rise and climate variability on coastal hydrochemistry, explaining how the environmental stresses on local marine calcifying organisms and the benthic ecosystem increased over the past 40 years.