Biogeochemical controls on the co-occurrence of mid-depth pH and DO minima in the inner shelf of the East China Sea

Highlights

Mid-depth pH minima co-occur with DO minima and nitrate maxima near the thermocline.
Mid-depth pH minima are driven by both organic matter respiration and upwelling.
Low carbonate buffering capacity amplifies mid-depth pH and pCO₂ signals.

Abstract

While ocean acidification in coastal oceans is well documented, mid-depth pH dynamics remains largely understudied. In August 2017, we conducted high-resolution vertical profiling of temperature, salinity, pH, dissolved oxygen (DO), and nitrate using in situ biogeochemical sensors in the inner East China Sea shelf. Additionally, vertical distributions of dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO₂ (pCO₂), and aragonite saturation state (Ω_a) were also calculated. Our observations revealed that mid-depth pH minima (<7.85) co-occurred with DO minima (<60 μmol L⁻¹) and nitrate maxima within or just below the seasonal thermocline. The DO–pH relationships at these stations followed Redfield stoichiometry, indicating organic matter respiration as a primary driver of mid-depth pH minima. High chlorophyll a concentrations (>5.0 μg L⁻¹) at these sites suggested recent phytoplankton blooms fueling the mid-depth oxygen and pH decrease. Although temperature-salinity relationships indicated upwelled water masses contribute to mid-depth pH minima at some stations, their low-pH signature is fundamentally caused by aerobic respiration. A synthesis of five years of cruise data showed that mid-depth pH and DO minima, as well as nitrate maxima, were consistently located along the margins of upwelling zones or salinity fronts—regions of high biological productivity. These patterns underscore the coupled effects of physical transport and biogeochemical processes on mid-depth pH dynamics. Additionally, waters at mid-depth exhibited the lowest carbonate buffer capacity and highest DIC/TA ratios in vertical profiles, amplifying pH declines and pCO₂ elevations. Such mid-depth pH minima may negatively affect upper-layer coastal ecosystems, including shellfish aquaculture.

Li D., Ji Z., Wang B., Zeng C., Jin H., Jiang Z., Ma X., Liu Q., Tao Y., Tang Y., Li H., Zhou F. & Chen J., 2026. Biogeochemical controls on the co-occurrence of mid-depth pH and DO minima in the inner shelf of the East China Sea. Marine Pollution Bulletin 222(Part 3): 118925. doi: 10.1016/j.marpolbul.2025.118925. Article.

Ocean Acidification