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

Abstract

It is known that surface water eutrophication enhances bottom water ocean acidification via respiration in coastal oceans. However, the role of benthic processes in influencing bottom water acidification has not been sufficiently explored. We examined this issue by analyzing a 10-year summer carbonate chemistry dataset in bottom water together with recent benthic flux measurements and literature benthic flux data in the northern Gulf of Mexico. The difference between the observed and estimated pH (Ω) values calculated from anthropogenic CO 2 increase and water column aerobic respiration were defined as ΔpH (ΔΩ). We found that ΔpH and ΔΩ values in hypoxic condition were −0.03 ± 0.04 (mean ± standard deviation) and −0.15 ± 0.39, respectively. Both ΔpH and ΔΩ values in hypoxic conditions were significantly lower than zero (p < 0.05). The net results of anaerobic respiration, oxidation of reduced chemcials, burial of iron sulfide minerals, and possible CaCO 3 dissolution may have led to an alkalinity to DIC production ratio of less than 1 in porewater. This caused the ratio of alkalinity to dissolved inorganic carbon fluxes from sediment to bottom water to be less than 1, which led to additional bottom water acidification. Our analysis and model simulations demonstrate that severe hypoxic and anoxic conditions, which correspond to less water movement, favor the accumulation of benthic respiration products, leading to additional pH and Ω reductions. The findings on sediment processes contributing to acidification in bottom waters provide new insights into the sensitivity of coastal ocean acidification to low-oxygen conditions under current and future climates and anthropogenic nutrient loading scenarios. Plain Language Summary The ongoing decrease in seawater pH as a result of uptake of anthropogenic carbon dioxide (CO 2) from the atmosphere is known as ocean acidification, which can be enhanced by oxygen-consuming respiration in the water column. Meanwhile, regions of coastal hypoxia (dissolved oxygen <2 mg L −1 or 63 μmol L −1) have increased in size and number during the last several decades because of water column eutrophication. Previous studies take only the anthropogenic CO 2 intrusion and aerobic respiration (respiration that consumes oxygen) into consideration when predicting the water column pH and carbonate mineral saturation. However, we found that anaerobic respiration (respiration that does not consumes dissolved oxygen) and the subsequent alkalinity removal via metal-sulfide burials in sediments can further decrease pH and carbonate mineral saturation. Therefore, the bottom water acidification states are more aggravated in hypoxic conditions than previous estimations in the northern Gulf of Mexico. To our knowledge, this is the first study that uses data from multiple years and systematically examines the role of benthic fluxes on ocean acidification in eutrophic coastal bottom waters. The finding has profound implications for similar coastal systems where eutrophication-induced bottom ocean acidificati...

Seasonal Variability and Future Projection of Ocean Acidification on the East China Sea Shelf off the Changjiang Estuary

“…Ar decreased as AOU increased (Figure 12D). Undeniably, the benthic release of DIC produced in the sediment also contributes to bottom water acidification (Hu et al, 2017;Wang et al, 2020). As the benthic fluxes of DIC and TA in the hypoxic zone are similar (Berelson et al, 2019), the influence of benthic fluxes on the seasonal acidification in bottom water is limited, although it may eventually enhance acidification on decadal or longer time scales (Wang et al, 2020).…”

Multi‐Decadal Coastal Acidification in the Northern Gulf of Mexico Driven by Climate Change and Eutrophication

“…Non‐aerobic processes could result in additional pH drawdown in hypoxic/anoxic bottom water in the nGOM (−0.04 ± 0.05 at DO < 30 μmol L −1 ) while this effect is minor under high oxygen conditions (Wang et al., 2020). Thus, ΔpH nonaerobic was estimated as $$${\Delta }{\text{pH}}_{\text{nonaerobic}}=0.0004\times {\text{DO}}_{\text{obs}}\,-\,0.04\,\left({\text{DO}}_{\text{obs}}\le 100\,{\upmu }\text{mol}\,{\mathrm{L}}^{-1}\right)$$$ …”

“…Non-aerobic processes could result in additional pH drawdown in hypoxic/anoxic bottom water in the nGOM ( 0.04 ± 0.05 at DO < 30 μmol L 1 ) while this effect is minor under high oxygen conditions (Wang et al, 2020). Thus, ΔpH nonaerobic was estimated as…”

“…Respiration‐driven CO 2 addition and oceanic uptake of anthropogenic CO 2 have acted synergistically, diminishing the chemical buffering capacity of bottom waters and enhancing coastal acidification (Cai et al., 2011; Feely et al., 2018). Additionally, acidification in the nGOM is also modulated by other physical and biogeochemical factors, including variations in alkalinity and inorganic carbon inputs from the MARS, and various non‐aerobic processes at the sediment‐water interface and within sediments (Cai et al., 2011; Gomez et al., 2021; Jiang et al., 2019; Wang et al., 2020).…”

Enhanced Carbonate Dissolution Associated With Deglacial Dysoxic Events in the Subpolar North Pacific

“…OMZs also correspond with maxima in dissolved inorganic carbon (DIC), forming corresponding carbon maximum zones (CMZs), due to high‐organic carbon respiration and, potentially, enhanced carbonate dissolution driven by that respiration (Paulmier et al., 2011; Wyrtki, 1962). In addition, prolonged hypoxic to anoxic conditions may amplify the effects of benthic respiration on bottom water pH and carbonate saturation (Ω), further enhancing carbonate dissolution in OMZ sediments as they do in coastal environments (Hu et al., 2017; Wang et al., 2020). As a result of these processes, OMZs are presently the primary carbon reservoir of the subsurface ocean and can be sites of CO 2 transfer from the ocean to the atmosphere with significant consequences for oceanic carbon storage and global climates (Naik et al., 2014; Paulmier et al., 2011).…”