Growing human activities in recent decades have collectively resulted in large amounts of nutrients export into coastal oceans. As the most reactive nitrogen species, ammonium ( ) plays the critical role in biogeochemical cycles in estuaries and the coastal ocean. In the highly polluted Pearl River Estuary (PRE), predominates to be the energy source for nitrification and to be the material source for bacteria and phytoplankton to grow. Both above processes are affected by light, yet in opposite ways. Nevertheless, rare studies paid attention to dual transformation processes specifically during dark conditions. By using nitrogen isotope tracer technique, we quantitatively and simultaneously differentiated two distinctive consumption pathways, that is, oxidation (AO_D) and assimilation (AA_D) rates, especially under dark conditions along the PRE during the 2015 and 2017 summer cruises when biological activities were the highest. We found the transformations display a bilayer structure with AA_D > AO_D in almost all the surface waters and vice versa in all bottom waters, suggesting bacteria and phytoplankton (mainly bacteria) control consumption in surface during the night while nitrifiers are the major consumer in the bottom waters. Through redundancy analysis, we found that both processes are mainly driven by in the PRE during summer. In addition, in the downstream PRE during two cruises, AO_D at most contributed 27% and 8% of the water column community dissolved oxygen (DO) consumption assuming the Redfield stoichiometry has no direct effect on hypoxia formation in the PRE during summer.

中文翻译：

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夏季珠江口的暗铵转化

近几十年来，人类活动的增长共同导致大量养分出口到沿海海洋。作为最活泼的氮物种，铵（ ）在河口和沿海海洋的生物地球化学循环中起着至关重要的作用。在污染严重的珠江口（PRE），主要是硝化的能源，也是细菌和浮游植物生长的物质来源。以上两个过程都受到光线的影响，但是方式相反。然而，很少有研究特别是在黑暗条件下关注双重转化过程。通过使用氮同位素示踪技术，我们定量和同时区分了两种独特的消耗途径，即氧化（AO_D）和同化（AA _D）速率，尤其是在2015年和2017年夏季巡游期间沿PRE的黑暗条件下，此时生物活性最高。我们发现这些转化在几乎所有地表水中都显示出AA _D  > AO _D的双层结构，而在所有底水中反之亦然，这表明细菌和浮游植物（主要是细菌）在夜间控制了表层的消耗，而硝化器是该设备的主要消耗者。底层水域。通过冗余分析，我们发现这两个过程主要是在夏季由PRE驱动的。此外，在两次巡航期间的下游PRE中，AO _D 假设雷德菲尔德化学计量对夏季PRE中的低氧形成没有直接影响，那么最多最多贡献了27％和8％的水柱群落溶解氧（DO）消耗。