Benthic Respiration in Hypoxic Waters Enhances Bottom Water Acidification in the Northern Gulf of Mexico
Benthic Respiration in Hypoxic Waters Enhances Bottom Water Acidification in the Northern Gulf of Mexico
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DOI：
10.1029/2020jc016152
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发表时间：
2020-09
影响因子：
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通讯作者：
Hongjie Wang;J. Lehrter;K. Maiti;K. Fennel;A. Laurent;N. Rabalais;N. Hussain;Qian Li;Baoshan Chen;K. M. Scaboo;W. Cai
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文献类型：
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作者：
Hongjie Wang;J. Lehrter;K. Maiti;K. Fennel;A. Laurent;N. Rabalais;N. Hussain;Qian Li;Baoshan Chen;K. M. Scaboo;W. Cai
众所周知，地表水富营养化通过沿海海洋的呼吸作用加强了海底海洋的酸化。然而，底栖生物过程在影响底水酸化中的作用尚未得到充分的探讨。我们通过分析墨西哥湾北部海底10年夏季碳酸盐化学数据集，以及最近的底栖生物通量测量和文献底栖生物通量数据，研究了这个问题。通过人为CO2增加和水柱有氧呼吸计算得出的pH值（Ω）与观测值之间的差值定义为ΔpH （ΔΩ）。我们发现，在缺氧条件下，ΔpH和ΔΩ的值分别为- 0.03±0.04（平均值±标准差）和- 0.15±0.39。低氧条件下ΔpH、ΔΩ值均显著低于零（p < 0.05）。厌氧呼吸、还原性化学物质的氧化、硫化铁矿物的埋藏以及可能的CaCO3溶解的净结果可能导致孔隙水中碱度与DIC的生成比小于1。这导致碱度与从沉积物到底水的溶解无机碳通量之比小于1，从而导致底水进一步酸化。我们的分析和模型模拟表明，严重的缺氧和缺氧条件，对应较少的水运动，有利于底栖生物呼吸产物的积累，导致额外的pH值和Ω降低。这些关于海底酸化的沉积物过程的发现，为了解当前和未来气候以及人为营养物负荷情景下沿海海洋酸化对低氧条件的敏感性提供了新的见解。由于从大气中吸收人为二氧化碳（CO2），海水pH值持续下降，这被称为海洋酸化，这可以通过水柱中的耗氧呼吸来增强。近几十年来，由于水体富营养化，沿海缺氧区（溶解氧<2 mg L或63 μmol L）的面积和数量都有所增加。以往的研究在预测水柱pH值和碳酸盐矿物饱和度时，只考虑了人为CO2入侵和有氧呼吸（消耗氧气的呼吸）。然而，我们发现厌氧呼吸（不消耗溶解氧的呼吸）和随后通过沉积物中的金属硫化物埋葬去除碱度可以进一步降低pH值和碳酸盐矿物饱和度。因此，在低氧条件下，底层水酸化状态比之前在墨西哥湾北部的估计更为严重。据我们所知，这是第一个使用多年数据并系统地检查底栖生物通量对富营养化沿海底部水域海洋酸化作用的研究。这一发现对类似的沿海系统具有深远的影响，在这些系统中，富营养化引起的海底酸化可能比我们过去认为的更严重。
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 CO2 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 CaCO3 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 (CO2) 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 or 63 μmol L) have increased in size and number during the last several decades because of water column eutrophication. Previous studies take only the anthropogenic CO2 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 acidification is likely more severe than we used to think.