Estuaries are productive ecosystems that provide important ecosystem functions such as the storage and cycling of dissolved organic matter (DOM) and nutrients. Intermittently closed/open lakes and lagoons (ICOLLs) can significantly impact biogeochemical processing and release of terrestrial nitrogen and carbon into the coastal environment due to longer residence times that can extend nutrient processing within the ICOLL. Pulses of nutrient release then occur when there is connectivity between the catchment and coastal waters. It remains unclear how modifications to estuaries and their catchments impact internal processes. To better understand the balance between autochthonous and allochthonous nutrients in a heavily modified ICOLL, multiple stable isotopes (δ¹³C, δ¹⁵N) of dissolved nutrients were used to evaluate seasonal and spatial changes to nitrogen sources and sinks in a southwest Australian ICOLL. The eutrophic status of water bodies has traditionally been based on concentrations of inorganic nitrogen (DIN) (particularly NH₄⁺ and NO_x) due to its presumed higher bioavailability and association with anthropogenic pollution. However, both NH₄⁺ and NO_x concentrations were low (0 – 12.5 µM) throughout the study area in both wet and dry seasons. Despite low surface water DIN concentrations, the system suffers from eutrophication issues such as algal blooms, low dissolved oxygen, and fish kills. The differences between the surface and porewater dissolved organic nitrogen (DON) and carbon (DOC) pools decreased in the wet season (high connectivity), suggesting that internal DOM turnover sustains eutrophication. This work demonstrates that including DON and its isotopic signature can be an effective way to study N in waterbodies with low DIN concentrations. It also highlights the need for DON, as a major constituent of total dissolved nitrogen, to be included in N studies and ultimately may have significant impact on the current understanding of the global N budget.

河口是多产的生态系统，提供重要的生态系统功能，例如溶解有机物 (DOM) 和养分的储存和循环。间歇性关闭/开放的湖泊和泻湖 (ICOLL) 会显着影响生物地球化学处理以及陆地氮和碳释放到沿海环境中，因为较长的停留时间可以延长 ICOLL 内的营养处理。当集水区和沿海水域连通时，就会出现养分释放脉冲。目前尚不清楚对河口及其集水区的修改如何影响内部过程。为了更好地了解经过大量修改的 ICOLL、多种稳定同位素（δ ¹³ C、δ ¹⁵N) 的溶解养分用于评估澳大利亚西南部 ICOLL 中氮源和汇的季节性和空间变化。水体的富营养状态传统上基于无机氮 (DIN)（特别是 NH ₄ ⁺和 NO _x）的浓度，因为它被认为具有更高的生物利用度并且与人为污染有关。然而， NH ₄ ⁺和NO _x整个研究区域在雨季和旱季的浓度都很低 (0 – 12.5 µM)。尽管地表水 DIN 浓度较低，但该系统仍存在富营养化问题，例如藻华、低溶解氧和鱼类死亡。地表和孔隙水溶解有机氮 (DON) 和碳 (DOC) 库之间的差异在雨季减少（高连通性），表明内部 DOM 周转维持了富营养化。这项工作表明，包括 DON 及其同位素特征可以成为研究低 DIN 浓度水体中 N 的有效方法。它还强调了将 DON 作为总溶解氮的主要成分纳入 N 研究的必要性，并最终可能对当前对全球 N 预算的理解产生重大影响。