Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706;
Center for Limnology, University of Wisconsin-Madison, Madison, WI 53706.
Proc Natl Acad Sci U S A. 2020 Nov 10;117(45):28175-28182. doi: 10.1073/pnas.2001376117. Epub 2020 Oct 26.
Excessive nitrogen (N) and phosphorus (P) loading is one of the greatest threats to aquatic ecosystems in the Anthropocene, causing eutrophication of rivers, lakes, and marine coastlines worldwide. For lakes across the United States, eutrophication is driven largely by nonpoint nutrient sources from tributaries that drain surrounding watersheds. Decades of monitoring and regulatory efforts have paid little attention to small tributaries of large water bodies, despite their ubiquity and potential local importance. We used a snapshot of nutrient inputs from nearly all tributaries of Lake Michigan-the world's fifth largest freshwater lake by volume-to determine how land cover and dams alter nutrient inputs across watershed sizes. Loads, concentrations, stoichiometry (N:P), and bioavailability (percentage dissolved inorganic nutrients) varied by orders of magnitude among tributaries, creating a mosaic of coastal nutrient inputs. The 6 largest of 235 tributaries accounted for ∼70% of the daily N and P delivered to Lake Michigan. However, small tributaries exhibited nutrient loads that were high for their size and biased toward dissolved inorganic forms. Higher bioavailability of nutrients from small watersheds suggests greater potential to fuel algal blooms in coastal areas, especially given the likelihood that their plumes become trapped and then overlap in the nearshore zone. Our findings reveal an underappreciated role that small streams may play in driving coastal eutrophication in large water bodies. Although they represent only a modest proportion of lake-wide loads, expanding nutrient management efforts to address smaller watersheds could reduce the ecological impacts of nutrient loading on valuable nearshore ecosystems.
过度的氮（N）和磷（P）负荷是人类世对水生生态系统的最大威胁之一，导致全球河流、湖泊和沿海水域富营养化。对于美国的湖泊来说，富营养化主要是由支流的非点源营养物质引起的，这些支流从周围流域排水。尽管小支流在数量上普遍存在且具有潜在的局部重要性，但数十年来的监测和监管工作几乎没有关注大型水体的小支流。我们利用密歇根湖几乎所有支流的养分输入快照——按体积计算是世界第五大淡水湖——来确定土地覆盖和水坝如何改变流域大小的养分输入。负荷、浓度、化学计量（N:P）和生物可利用性（溶解无机养分的百分比）在支流之间存在数量级差异，形成了沿海养分输入的马赛克。235 条支流中的 6 条最大的支流占每日输入密歇根湖的 N 和 P 的约 70%。然而，小支流的养分负荷与其规模相比很高，且偏向于溶解无机形式。来自小流域的养分具有更高的生物可利用性，这表明它们在沿海地区引发藻类大量繁殖的潜力更大，尤其是因为它们的羽流很可能被困在近岸区域并相互重叠。我们的研究结果揭示了小溪流在驱动大型水体沿海富营养化方面可能被低估的作用。尽管它们仅占湖泊总负荷的一小部分，但扩大养分管理工作以解决较小的流域可能会减少养分负荷对有价值的近岸生态系统的生态影响。
