Data from: Widespread increases in iron concentration in European and North American freshwaters

Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and geographical distribution of Fe trends, or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Hence, Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional-scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land-use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in non-forested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.

近期有研究报告显示，淡水水体中铁（Fe）浓度呈上升趋势。鉴于铁在生物地球化学过程中发挥着核心作用，该现象已引发学界广泛关注。然而目前学界对铁浓度变化趋势的发生频次、地理分布特征及其潜在驱动机制仍知之甚少。为更清晰地明晰铁浓度上升的发生区域、影响程度及驱动原因，本研究针对北欧与北美10个国家的340个水体开展分析，探究铁浓度的时间变化趋势。研究结果显示，28%的监测点位铁浓度显著上升，4%的点位出现显著下降，其中绝大多数浓度上升的点位集中于北欧地区。铁浓度上升的区域往往与有机碳（OC）浓度上升的区域高度重合。由此可见，铁与有机碳浓度的上升可能并非存在直接的机制性关联，但二者均可能对区域尺度的共同驱动因子做出响应，例如硫沉降减少或水文条件改变。铁与溶解态二氧化硅的同步变化趋势进一步佐证了水文因子的作用：这两类元素通常源自相似深度的土壤圈层。铁浓度上升与针叶林覆盖度呈正相关，这表明土地利用方式转变与林业扩张可能促进了铁的土壤输出通量；不过即便在非森林区域，同样观测到了铁浓度上升的现象。本研究得出结论：淡水水体铁浓度上升的现象已广泛分布，尤以北欧地区为甚，该现象可能对全域生态系统的生物地球化学循环以及当前淡水水体的褐变过程产生重大潜在影响。