Drivers of increasing iron concentrations in freshwaters

Abstract: Iron (Fe) concentrations are increasing in freshwaters (lakes and streams) in Sweden, as well as in other regions in northern Europe. Together with increasing concentrations of organic matter, Fe is contributing to the ongoing browning of freshwaters, which may have serious ecological consequences and implications for ecosystem services. The aim of this thesis was to investigated where, to what extent, and - primarily - why Fe concentrations are increasing in freshwaters.As catchment soils are the main source of Fe in freshwaters, higher catchment influx could explain increases in Fe concentrations, but the positive trends could also be the result of changes in processes controlling the distribution of Fe within freshwater systems. I investigated how both of these processes are affected by changes in climate, atmospheric sulfate deposition, and land-use. These three potential drivers were hypothesized to affect pH, redox conditions and OM availability, factors controlling Fe speciation and thus Fe mobility in the landscape.Analyses of temporal and spatial patterns in Fe concentrations from monitoring data of wide geographic distribution confirmed that positive Fe trends are widespread, and possibly more frequent in Northern Europe than in North America. Positive Fe trends were also more frequent in catchments with a high percentage of coniferous forest, and found in regions were precipitation had increased. Coniferous forest soils are particularly important sources of Fe due to high OM content, low pH, and consequently high weathering rates. A dramatic increase in spruce volume in the catchment of Lake Bolmen, illustrating a landscape wide shift from an agricultural to a forestry dominated land-use, was a strong predictor of long-term increases in Fe concentration in the lake. Moreover, increasing accumulation rates of Fe, Si and OM in more recent sediments of Lake Bolmen are in line with increased weathering and catchment loading in response to afforestation.Increases in precipitation could promote catchment export through increased runoff and water saturation of soils that favor reductive dissolution and mobilization of Fe. Accordingly, catchment loading of Fe was positively correlated to flow to Lake Bolmen. However, variation in precipitation seemed to explain short-term variability in Fe concentrations and loading, rather than long-term trends.Analyses of monitoring data further revealed a significant relationship between the increase in Fe concentrations and declining sulfate concentrations. However, when the role of S availability was investigated in the more mechanistic studies of this thesis, higher aqueous Fe concentrations were not found in response to lower S. In a microcosm experiment with soil slurries, Fe mobilization was in fact larger in treatments with high sulfate, probably due to increased acidity enhancing Fe solubility. Moreover, in sediment records of Lake Bolmen, Fe accumulation and the contribution of FeS were independent of elevated S accumulation during peak S deposition.Collectively, the results of this thesis point to the importance of increased loading in response to afforestation and enhanced weathering, as a driver of increasing Fe concentrations. Increasing precipitation may further enhance Fe mobilization from coniferous forest soils.