Contrasting effects of temperature and biological processes on aragonite saturation state at the A4HDYD station in the North Yellow Sea

Abstract

To explore the effects of seawater temperature and biological processes on aragonite saturation state (Ω_arag), eight field surveys were conducted from December 2014 to November 2015 at A4HDYD station in the North Yellow Sea (NYS). Low regional yearly initial values of seawater Ω_arag (1.26-1.76) were obtained in winter due to low seawater temperature. During spring to early autumn, surface-water Ω_arag increased from 2.02 to 3.47 for the increasing seawater temperature and biological production, whereas bottom-water Ω_arag remained as low as 1.44-1.79 due to low seawater temperature, net community respiration and the influence of the North Yellow Sea Cold Water Mass (NYSCWM). Following the collapse of stratification in late autumn, the seawater was well-mixed with an average Ω_arag of 2.17. The annual cycle of seawater temperature dominated the seasonal variations of Ω_arag, counteracted by biological production increasing Ω_arag of 0.49 in surface water in March and strengthened by net community respiration decreasing Ω_arag of 0.11-0.41 below the pycnocline during summer and autumn. It is, therefore, mid-to-high latitude coastal areas impacted by human activities are likely subject to ocean acidification due to the combined effects of low seawater temperature and net community respiration.

Keywords: Aragonite saturation state; Coastal acidification; Dynamic mechanism; Net community respiration; North Yellow Sea.