Nitrogen cycling driven by organic matter export in the South Pacific oxygen minimum zone

Oxygen minimum zones are expanding globally, and at present account for around 20-40% of oceanic nitrogen loss. Heterotrophic denitrification and anammox-anaerobic ammonium oxidation with nitrite-are responsible for most nitrogen loss in these low-oxygen waters. Anammox is particularly significant in the eastern tropical South Pacific, one of the largest oxygen minimum zones globally. However, the factors that regulate anammox-driven nitrogen loss have remained unclear. Here, we present a comprehensive nitrogen budget for the eastern tropical South Pacific oxygen minimum zone, using measurements of nutrient concentrations, experimentally determined rates of nitrogen transformation and a numerical model of export production. Anammox was the dominant mode of nitrogen loss at the time of sampling. Rates of anammox, and related nitrogen transformations, were greatest in the productive shelf waters, and tailed off with distance from the coast. Within the shelf region, anammox activity peaked in both upper and bottom waters. Overall, rates of nitrogen transformation, including anammox, were strongly correlated with the export of organic matter. We suggest that the sinking of organic matter, and thus the release of ammonium into the water column, together with benthic ammonium release, fuel nitrogen loss from oxygen minimum zones.

全球海洋低氧区（Oxygen minimum zones）正持续扩张，目前约占海洋总氮流失量的20%至40%。异养反硝化作用与厌氧氨氧化（anaerobic ammonium oxidation with nitrite，anammox）是这类低氧水域中绝大多数氮流失的驱动过程。厌氧氨氧化在全球最大低氧区之一的东热带南太平洋中尤为关键。然而，调控厌氧氨氧化介导的氮流失的关键机制仍未明确。本研究通过营养盐浓度测定、氮转化速率实验测定以及生源有机质输出数值模型，构建了东热带南太平洋低氧区的完整氮收支。采样期间，厌氧氨氧化是占主导地位的氮流失途径。厌氧氨氧化及相关氮转化速率在高生产力的陆架水域中最高，并随离岸距离的增加而逐渐衰减。在陆架区域内，厌氧氨氧化活性在上层水体与底层水体中均出现峰值。总体而言，包括厌氧氨氧化在内的各类氮转化速率与生源有机质输出呈显著正相关。本研究认为，生源有机质的沉降及其引发的铵态氮向水体柱的释放，结合底栖铵态氮释放，共同支撑了低氧区的氮流失过程。