Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

Abstract

High‐accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017, https://doi.org/10.1016/j.marchem.2017.10.001; Müller & Rehder, 2018, https://doi.org/10.3389/fmars.2018.00177), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005 ± 0.016, n = 550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The nonconservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC, that is, per 100‐μmol/kg biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO2 invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.

Effects of Wind-Driven Lateral Upwelling on Estuarine Carbonate Chemistry

“…Our result for the dominance of upwelling over mixing in driving pH fluctuations is consistent with a related hypoxia modeling study in which Scully (2010) showed that wind-driven lateral upwelling and ventilation of deep hypoxic water onto shallow shoals are more important than wind mixing in supplying dissolved oxygen to the bottom water. This is probably more so for CO 2 than for O 2 as the air-sea gas exchange flux is generally much smaller for CO 2 than for O 2 (Jiang et al, 2019).…”

Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

“…In productive coastal zones, algae and other biogenic particles decompose in subsurface waters. Their respiration and/or remineralization processes consume dissolved oxygen (DO) and release a great deal of CO 2 into subsurface waters, leading to more rapid seawater acidification in coastal seas, compared with the open ocean (Feely et al, 2010;Cai et al, 2011;Melzner et al, 2013;Jiang et al, 2019). Moreover, the respirationinduced seasonal acidification is subject to uneven distributions of seawater temperature, salinity and alkalinity in coastal seas.…”

Benthic Respiration in Hypoxic Waters Enhances Bottom Water Acidification in the Northern Gulf of Mexico

“…The DIC and TA data from 2017 are reported herein. Data previously analyzed and reported include pH and DO data from 2006–2010 and 2017 (Cai et al, 2011; Hu et al, 2017; Jiang et al, 2019), DIC and TA data from 2006 to 2010 (Guo et al, 2012; Hu et al, 2014; Huang et al, 2015), and DIC and DO data from 2011 to 2016 (Wang et al, 2018).…”