Excess nutrients transported by the Mississippi River (MR) contribute to hypoxia in the Gulf of Mexico. Nutrient balances are key drivers to river nutrient loads and represent inputs (fertilizer, manure, deposition, wastewater, N-fixation, and weathering) minus outputs (nutrient uptake and removal in harvest, and N emissions). Here, we quantified annual changes in nitrogen (N) and phosphorus (P) river loads and nutrient balances at the MR Outlet and documented that the river load response to watershed nutrient balances shifted between 1975 and 2017. Annual nutrient balances and river loads were positively correlated between 1975 and 1985, but after, a disconnect between both the N and P balances and river loads emerged, and the subsequent river load patterns were different for N versus P. We evaluated the relative impacts of legacy nutrients and other latent factors, for which data were not available, on river nutrient load trends. Our analysis showed that in the case of N, latent factors were potentially just as important in explaining changes in river nutrient loads over time as N balances, and in the case of P, they were even more important. We hypothesized that these factors included implementation of best management practices, changes in watershed buffering capacity, the effects of tile drainage, or increased precipitation. Our analytical approach shows promise for the investigation of drivers of water quality trends that are not well-represented in typical national scale geospatial datasets.

中文翻译：

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密西西比河的长期趋势揭示了 1975 年至 2017 年间河流负荷对流域养分平衡的响应变化


密西西比河（MR）输送的过量营养物质导致墨西哥湾缺氧。养分平衡是河流养分负荷的关键驱动因素，代表输入（化肥、粪肥、沉积、废水、固氮和风化）减去输出（收获时养分吸收和去除以及氮排放）。在这里，我们量化了 MR 出水口氮 (N) 和磷 (P) 河流负荷和养分平衡的年度变化，并记录了河流负荷对流域养分平衡的响应在 1975 年至 2017 年间发生了变化。年度养分平衡和河流负荷呈正向变化。 1975 年至 1985 年间，N 和 P 平衡与河流负荷之间出现了脱节，随后 N 与 P 的河流负荷模式有所不同。我们评估了遗留养分和其他潜在因素的相对影响，没有关于河流养分负荷趋势的数据。我们的分析表明，就氮而言，潜在因素在解释河流养分负荷随时间变化的过程中可能与氮平衡一样重要，而就磷而言，它们甚至更为重要。我们假设这些因素包括最佳管理实践的实施、流域缓冲能力的变化、瓷砖排水的影响或降水增加。我们的分析方法显示了调查水质趋势驱动因素的前景，这些趋势在典型的国家尺度地理空间数据集中没有得到很好的体现。