Eutrophication remains a threat to coastal habitats and water quality worldwide. The U. S. Clean Water Act resulted in reductions of nutrient loading from point sources but management of nonpoint sources (NPS) of nutrients remains challenging despite efforts over at least three decades. The hydrological factors, best management practices (BMPs) and regulatory mechanisms that target nutrient NPS and improve coastal ecosystem function are poorly understood. We identified three case study sites in the U.S. with sufficient NPS management and monitoring history to quantify changes in estuarine habitat and water quality following BMP implementation and regulation targeting nutrient NPS. Utilizing publicly available data, we compared sites that are geographically distant and hydrologically distinct. We found that BMPs targeting NPS loads from surface waters into Roberts Bay (Florida) and Newport Bay (California) significantly reduced nutrient concentrations and harmful algal blooms within $\sim$20 years. Improvements occurred despite concurrent human population growth within both watersheds. Conversely, we found that the majority of BMPs implemented within the Peconic Estuary (New York) watershed targeted surface waters despite a dominance of nitrogen inputs (97%) from groundwater and atmospheric sources. Declines in habitat and water quality in Peconic Estuary may be due to a failure to control the dominant nutrient sources and the long residence time of nitrogen in groundwater. Compared to surface water, reducing groundwater and atmospheric nutrients face greater technical and financial challenges. Improvements to Peconic Estuary may occur with further reductions in surface water inputs and as nutrients leach out of the groundwater. Although the effectiveness of specific NPS BMPs has been examined at small spatial scales, our study is the first to quantify improvements at a watershed scale. We showed that successful NPS management pathways are those which targeted the dominant sources of nutrients to coastal ecosystems and applied multiple BMPs within watersheds.

富营养化仍然对全球沿海栖息地和水质构成威胁。美国《清洁水法》减少了点源的养分负荷，但尽管努力了至少 30 年，但对非点源 (NPS) 养分的管理仍然具有挑战性。针对营养 NPS 和改善沿海生态系统功能的水文因素、最佳管理实践 (BMP) 和监管机制知之甚少。我们在美国确定了三个具有足够 NPS 管理和监测历史的案例研究地点，以量化 BMP 实施和针对营养 NPS 的监管后河口栖息地和水质的变化。利用公开可用的数据，我们比较了地理上遥远且水文不同的站点。$～$20年。尽管两个流域内的人口同时增长，但情况还是有所改善。相反，我们发现在 Peconic 河口（纽约）流域内实施的大多数 BMPs 都针对地表水，尽管地下水和大气来源的氮输入（97%）占主导地位。Peconic Estuary 栖息地和水质的下降可能是由于未能控制主要营养源和地下水中氮的长时间停留。与地表水相比，减少地下水和大气养分面临更大的技术和财务挑战。随着地表水输入的进一步减少以及营养物质从地下水中渗出，Peconic Estuary 可能会得到改善。尽管特定 NPS BMP 的有效性已经在小空间尺度上进行了检验，我们的研究首次量化了流域范围内的改进。我们表明，成功的 NPS 管理途径是针对沿海生态系统的主要营养来源并在流域内应用多种 BMP 的途径。