In this study, we characterized the impact of regulatory water releases relative to watershed inputs on the quality of receiving waters to identify if and how managed releases could be scheduled to mitigate nutrient export and downstream water quality impairment. We specifically investigated freshwater flow partitioning to the Caloosahatchee River and Estuary (CRE) from a large managed lake, Lake Okeechobee, and the CRE's upstream watershed, the C-43 basin, in southwest Florida (USA). A water balance was developed to identify dominant freshwater inflow sources (i.e., Lake Okeechobee vs. watershed inputs) over time. From the water balance, analyses of historical trends were performed to detect changes in freshwater inflow contributions to the CRE. Further, seasonal and annual concentration variations and long-term concentration-discharge (C-Q) relationships were analyzed to better understand biogeochemical and hydrological processes in the system in relation to freshwater source. Since 1966, we found the duration and magnitude of flows from the C-43 basin were higher than those from Lake Okeechobee releases. However, recent increases in the annual water volume and proportion of inflow coming from Lake Okeechobee to the CRE were observed. The C-Q analysis revealed that nitrate and ammonium concentrations in the CRE were responsive to changes in discharge, while total phosphorus and orthophosphate concentrations were chemostatic. While modifications to the Lake Okeechobee operation schedule could potentially mitigate downstream inorganic nitrogen loading, this potential is limited by complex, seasonal C-Q relationships and confounding effects from surrounding watersheds.

中文翻译：

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在高度工程化的流域中，下游养分浓度更多地取决于流域输入而不是水库释放

在这项研究中，我们描述了相对于流域输入的监管水排放对受纳水域质量的影响，以确定是否以及如何安排有管理的排放来减轻养分输出和下游水质损害。我们专门调查了从大型管理湖泊奥基乔比湖到卡卢萨哈奇河和河口 (CRE) 的淡水流分配，以及 CRE 的上游流域（位于美国佛罗里达州西南部的 C-43 盆地）。建立了水平衡，以确定随着时间的推移主要淡水流入源（即奥基​​乔比湖与流域输入）。根据水平衡，对历史趋势进行分析，以检测淡水流入对 CRE 贡献的变化。此外，还分析了季节性和年度浓度变化以及长期浓度-流量（CQ）关系，以更好地了解系统中与淡水源相关的生物地球化学和水文过程。自 1966 年以来，我们发现 C-43 盆地的流量持续时间和强度均高于奥基乔比湖排放的流量。然而，最近观察到从奥基乔比湖流入 CRE 的年水量和流入比例有所增加。CQ 分析表明，CRE 中的硝酸盐和铵浓度对排放变化敏感，而总磷和正磷酸盐浓度具有化学稳定性。虽然修改奥基乔比湖运营计划可能会减轻下游无机氮负荷，但这种潜力受到复杂的季节性 CQ 关系和周围流域的混杂影响的限制。