How the partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N₂ production responds to hypoxia in seasonally hypoxic estuaries has not been well understood. In this study, ¹⁵N tracer incubations were used to investigate benthic denitrification, anammox and DNRA rates for three years (2016–2018) across seasonally hypoxic areas off the Changjiang Estuary. Dissolved oxygen contents in bottom water were significantly lower in July than in February/March. Concentrations of NH₄⁺ and NO_x^– in bottom water throughout the sampling sites did not vary between seasons. Rates of benthic denitrification (0.45–6.55 nmol N g⁻¹ h⁻¹), anammox (0.02–0.30 nmol N g⁻¹ h⁻¹) and DNRA (0.06–0.59 nmol N g⁻¹ h⁻¹) varied largely along the estuary. Denitrification, anammox and DNRA rates did not vary significantly between inshore and offshore sites. However, denitrification rates were significantly lower and DNRA rates were significantly higher in consistently than transiently hypoxic sites. Anammox rates were slightly lower in consistently than transiently hypoxic sites. Ratios of DNRA to N₂ production were in a range of 0.015–0.53, and were significantly higher in consistently than transiently hypoxic sites, but did not vary significantly between inshore and offshore sites. The lower temperature sensitivity of denitrification and anammox compared to their Q₁₀ values further indicated that hypoxia may inhibit N₂ production. These results suggest that high incidence hypoxia can decrease denitrification and anammox rates but increase DNRA rates with important implications on N retention versus removal, thereby exacerbating eutrophication and hypoxia in estuarine and coastal environments.

中文翻译：

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长江口附近季节性缺氧地区的高发生率缺氧增加底栖氮保留但减少氮去除

在季节性缺氧的河口，异化硝酸盐还原成铵 (DNRA) 和 N ₂生产之间的分配如何对缺氧做出反应尚不清楚。在这项研究中，¹⁵ N 示踪剂孵化用于调查长江口外季节性缺氧地区三年（2016-2018 年）的底栖反硝化、厌氧氨氧化和 DNRA 率。7 月份底水中溶解氧含量明显低于 2/3 月份。整个采样点底水中NH ₄ ⁺和NO _x ^{- 的}浓度在季节之间没有变化。底栖反硝化速率 (0.45–6.55 nmol N g ^-1  h ^-1)、厌氧氨氧化 (0.02–0.30 nmol N g ^-1  h ^-1 ) 和 DNRA (0.06–0.59 nmol N g ^-1  h ^-1 ) 沿河口变化很大。近海和近海场地的反硝化、厌氧氨氧化和 DNRA 速率没有显着差异。然而，反硝化率显着较低，DNRA 率显着高于持续性低氧位点。厌氧氨氧化率在持续比暂时缺氧部位略低。DNRA 与 N _{2 的}比率产量在 0.015-0.53 的范围内，并且在持续性缺氧地点显着高于暂时缺氧地点，但在近海地点和离岸地点之间没有显着差异。与它们的Q ₁₀值相比，反硝化和厌氧氨氧化的较低温度敏感性进一步表明缺氧可能会抑制 N _{2 的}产生。这些结果表明，高发生率缺氧可以降低反硝化和厌氧氨氧化率，但会增加 DNRA 率，对 N 保留与去除有重要影响，从而加剧河口和沿海环境中的富营养化和缺氧。