Past changes in the input/output, and internal cycling, of bioavailable nitrogen (N) in marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (δ15N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ15N (δ15Nbulk) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom‐bound N isotope (δ15Ndb) paleo‐proxy to lake sediments. By comparing δ15Nbulk and δ15Ndb in a sedimentary record from eutrophic Lake Lugano (Switzerland), we demonstrate that changing redox conditions influence the degree of N‐isotopic alteration of the bulk sediment, emphasizing the need for caution when interpreting δ15Nbulk in paleolimnological studies. Furthermore, in combining δ15Ndb measurements with X‐ray fluorescence scanning and state‐of‐the‐art molecular biomarker analyses, we reconstruct nutrient cycling and paleoenvironmental conditions in the lake over the past ~ 125 yr. Coeval with the period of severe eutrophication in Lake Lugano in the 1960s, our proxy data indicate that export production, δ15Ndb, and the concentration of heterocyst glycolipids (a biomarker for N2‐fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ15Ndb is likely the result of enhanced water‐column denitrification in response to increased phytoplankton productivity. We hypothesize that greater export production during eutrophication led to anoxic conditions in the hypolimnion as a result of enhanced organic matter remineralization, raising water‐column denitrification. Enhanced N loss and remobilization of phosphorous (P) from the sediments under anoxic conditions lowered the N : P ratio in the lake, fostering cyanobacterial N2 fixation in surface waters.

中文翻译：

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硅藻结合氮同位素古代理的首次湖泊应用揭示了超富营养化湖泊中反硝化和固氮的耦合


通过分析沉积记录中有机质的 N 同位素组成 (δ15N)，可以重建海洋和湖泊环境中生物可利用氮 (N) 的输入/输出和内部循环的过去变化。为了验证并消除因成岩蚀变和外部氮输入造成的大量沉积 δ15N (δ15Nbulk) 特征的潜在偏差，我们首次将硅藻结合的氮同位素 (δ15Ndb) 古代理应用于湖泊沉积物。通过比较富营养化卢加诺湖（瑞士）沉积记录中的 δ15Nbulk 和 δ15Ndb，我们证明氧化还原条件的变化会影响大量沉积物的 N 同位素改变程度，强调在古湖泊学研究中解释 δ15Nbulk 时需要谨慎。此外，通过将 δ15Ndb 测量与 X 射线荧光扫描和最先进的分子生物标志物分析相结合，我们重建了过去约 125 年湖中的养分循环和古环境条件。与 20 世纪 60 年代卢加诺湖严重富营养化时期同时期，我们的代理数据表明出口产量、δ15Ndb 和异囊糖脂（固氮蓝细菌的生物标志物）浓度同时增加。总之，这些数据表明 δ15Ndb 的增加可能是浮游植物生产力增加导致水体反硝化增强的结果。我们假设，富营养化期间更大的出口产量导致了低层缺氧条件，这是由于有机物再矿化增强而导致的，从而提高了水柱反硝化作用。 在缺氧条件下，沉积物中氮损失的增加和磷（P）的再利用降低了湖中的氮：磷比率，促进了地表水中蓝细菌的氮固定。