Differential Trends in Iron Concentrations of Boreal Streams Linked to Catchment Characteristics. Issue 3 (6th March 2023)

• Record Type: Journal Article
• Authors: Škerlep, M.; Nehzati, S.; Sponseller, R. A.; Persson, P.; Laudon, H.; Kritzberg, E. S.
• Is Part Of: Global biogeochemical cycles. Volume 37:Issue 3(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-03-06
• Subjects: iron -- catchment -- stream -- riparian zone -- mire -- browning; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2022GB007484
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Increasing iron (Fe) concentrations have been reported for freshwaters across northern Europe over the last decades. This increase, together with elevated concentrations of dissolved organic carbon (DOC), leads to browning of freshwaters, which affects aquatic organisms, ecosystem functioning, biogeochemical cycles, and brings challenges to drinking water production. However, how such increasing trends in stream Fe concentrations reflect the contribution of different catchment sources remains poorly resolved. Here, we explored how catchment characteristics, that is, mires and coniferous soils, regulate spatial and temporal patterns of Fe in a boreal stream network. For this, we determined Fe speciation in riparian and mire soils, and studied temporal Fe dynamics in soil‐water and stream‐water over a span of 18 years. Positive Fe trends were found in the solution of the riparian soil, while no long‐term trend was observed in the mire. These differences were reflected in stream‐water, where three headwater streams dominated by coniferous cover also displayed positive Fe trends, whereas the mire dominated stream showed no trend. Surprisingly, the majority of higher order streams showed declining Fe trends, despite long‐term increases in DOC. In addition, we found that an extreme drought event led to a prolonged release of Fe and DOC from the riparian soils, that could have long‐term effects on stream Fe concentrations. Our results show that riparian forest soils can be major contributors to ongoing increases in freshwater Fe concentrations and that drought can further promote the release of Fe from organic soils. Plain Language Summary: Iron (Fe) concentrations have been increasing in many northern streams, rivers and lakes. Together with increasing organic matter concentrations this leads to water becoming browner, which has consequences for aquatic organisms and interferes with drinking water production. To better understand what causes the increase in Fe, we studied Fe trends in boreal streams and organic rich mire and forest soils, which are the main sources of Fe to aquatic systems. We found that where coniferous forest soils were the main catchment source, Fe concentrations in streams were increasing, whereas Fe concentrations did not change over time when mires were the main sources. Our results also suggest that extreme events such as drought can lead to increased release of Fe from organic forest soils to streams. This research improves our understanding of the catchment Fe sources and how they contribute to past and future Fe trends in streams. Key Points: Increasing Fe trends observed in headwater streams with coniferous forest catchments, but not in streams draining a mire‐dominated catchment Surprisingly, the majority of higher order streams showed declining Fe trends despite long‐term increases in dissolved organic carbon (DOC) Extreme drought event led to prolonged release of Fe and DOC from riparian soils and could affect stream Fe concentrations in the long‐term