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.近期有研究报道淡水水体中铁（Fe）浓度持续上升，鉴于铁在生物地球化学过程中的核心功能，该现象已引起学界高度关注。但目前学界对铁浓度变化趋势的发生频次、地理分布格局及其潜在驱动机制仍缺乏深入认知。为明确淡水铁浓度上升的发生区域、影响幅度与驱动成因，我们对分布于北欧与北美10个国家的340个水体的铁浓度时间变化趋势进行了分析。研究结果显示，28%的监测点位铁浓度显著上升，4%的点位显著下降，其中多数浓度上升趋势集中于北欧区域。铁浓度上升的区域往往与有机碳（OC）浓度上升的区域重合，这表明尽管铁与有机碳的上升未必存在直接的机制关联，但二者可能共同响应区域尺度上的共性驱动因子，例如硫沉降减少或水文条件改变。铁与溶解硅的同步变化趋势佐证了水文因素的作用——这两类元素通常源自相似的土壤深度层级。铁浓度上升与针叶林覆盖度呈正相关，提示土地利用方式改变与林业扩张可能推动了铁输出量的提升，但在无林区域同样观测到了铁浓度上升的现象。综上，淡水铁浓度上升的现象已广泛分布，尤其在北欧地区更为显著，该趋势可能对生态系统整体生物地球化学循环以及当前的淡水褐化现象产生重要影响。