Data from: Importance of boreal rivers in providing iron to marine waters

This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters – the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

本研究报道了汇入波罗的海的河流中铁浓度呈上升趋势。鉴于铁对水生生态系统的结构与生物地球化学功能具有决定性作用，这一趋势很可能对该受纳系统产生深远影响，而其具体影响取决于铁在河口混合过程中的归趋。本研究通过将7条寒带河流的水样与人工海水盐混合，评估了河流水体中铁的稳定性。结果显示，随着盐度升高，悬浮态铁逐渐流失；不过不同河流水样维持铁悬浮态的能力差异显著：在盐度为30时，仅有1%至54%的铁处于悬浮状态。这种差异性可通过河流水体的铁:有机碳比值得到最佳解释，即比值越低，悬浮态铁的留存比例越高。通过铁添加实验证实，初始铁:有机碳比值较低的水体，可在整个盐度梯度下维持远超环境浓度的铁呈悬浮态。此外，河流水体有机质的分子尺寸与悬浮态铁的含量呈正相关关系。综上，研究结果表明寒带河流具有极高的铁输运能力，这意味着河口铁浓度的升高可能会导致海洋系统中具有潜在生物可利用性的铁浓度上升。