Abstract The dramatic marine environmental change seen today can be difficult to fully document and interpret without adequate, spatially and temporally comprehensive, baseline datasets of hydrographic properties. Here we present isotope data measured in water samples collected during a nine-day research cruise in October 2016 throughout the Gulf of Maine, a rapidly changing region of the world's oceans. A comparison of the oxygen isotopes of the water (δ18Owater) and salinity data reveal that water samples fall on a tight, linear mixing line between fresher shelf water and saltier slope waters, with the freshwater endmember originating from much higher latitudes (the Gulf of St. Lawrence and the Labrador Sea). Some subtle differences in freshwater endmembers are observed between the three different deep basins in the Gulf of Maine. These differences are likely reflecting differences in freshwater input and vertical mixing between the different basins. Additionally, these water samples have lower δ18Owater values for a given salinity value than previously published values of marine water mass endmembers. This offset may be related to systematic changesin water mass endmember values or year to year variability, as well as differences in the proportions of water masses entering the Gulf of Maine. Nitrogen and oxygen isotopes of dissolved nitrate (NO3−; δ15NNO3- and δ18ONO3-, respectively) measured in the water samples suggest a strong influence of phytoplankton assimilation near the surface in both isotopic systems. Combining these two datasets using Δ(15, 18) to look at the rates of fractionation between the two isotope systems reveals potential water column nitrification above 100 m in most places in the Gulf of Maine. This finding provides support for previous hypotheses of water column nitrification in the Gulf of Maine based on nutrient distribution and nitrogen box modeling. However, these calculations rely on the assumption that all nitrate is sourced from deeper waters. It is possible these results are instead caused by NO3− from different sources at the surface and therefore do not necessarily indicate the presence of nitrification.

中文翻译：

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使用稳定同位素作为缅因湾水团和养分循环过程的示踪剂

摘要 如果没有足够的、空间和时间全面的水文特性基线数据集，今天所看到的剧烈海洋环境变化可能难以完全记录和解释。在这里，我们展示了 2016 年 10 月在缅因湾（世界海洋的快速变化区域）进行的为期 9 天的研究巡航期间收集的水样中测量的同位素数据。水的氧同位素 (δ18Owater) 和盐度数据的比较表明，水样落在较新鲜的陆架水和较咸的斜坡水之间的紧密线性混合线上，淡水端元来自高得多的纬度（圣加仑湾）。 .劳伦斯和拉布拉多海）。在缅因湾的三个不同深盆地之间观察到淡水端元的一些细微差异。这些差异可能反映了不同流域之间淡水输入和垂直混合的差异。此外，对于给定的盐度值，这些水样的 δ18Owater 值低于先前公布的海洋水团端元值。这种偏移可能与水团端元值的系统变化或逐年变化有关，以及进入缅因湾的水团比例的差异。在水样中测量的溶解硝酸盐（NO3-；分别为 δ15NNO3- 和 δ18ONO3-）的氮和氧同位素表明，在两个同位素系统中，表面附近的浮游植物同化作用有很大影响。使用 Δ(15, 18) 查看两个同位素系统之间的分馏速率，揭示了缅因湾大部分地区 100 m 以上的潜在水柱硝化作用。这一发现为先前基于养分分布和氮箱模型的缅因湾水柱硝化假设提供了支持。然而，这些计算依赖于所有硝酸盐均来自更深水域的假设。这些结果可能是由来自表面不同来源的 NO3- 引起的，因此不一定表明存在硝化作用。这些计算依赖于所有硝酸盐均来自更深水域的假设。这些结果可能是由来自表面不同来源的 NO3- 引起的，因此不一定表明存在硝化作用。这些计算依赖于所有硝酸盐均来自更深水域的假设。这些结果可能是由来自表面不同来源的 NO3- 引起的，因此不一定表明存在硝化作用。