Changing land cover as a driver of surface water browning

Abstract: Streams, rivers and lakes across wide areas of the Northern hemisphere have been subject to an increase in water color over the last decades. This increase, also known as browning, is a result of rising dissolved organic matter (DOM) and iron (Fe) concentrations in the water. Over the years, browning has received ample attention due to its impact on the structure and function of freshwater ecosystems, biogeochemical cycles and drinking water production. Several hypotheses have been proposed to explain what causes browning, with climate change and recovery from atmospheric S deposition receiving the most attention as potential drivers. Recently, it has been suggested that land cover changes, specifically increases in coniferous forest cover which promote accumulation of organic soils, is an important and overlooked factor behind browning.The main aim of this thesis was to evaluate the contribution of changing land cover on long-term water color trends. Since afforestation represents a major shift in land cover in the last century, my aim was to explore how this may have affected processes in the catchment that could contribute to the observed increase in Fe and DOM exports to surface waters.By combining uniquely long water color time series, with parallel records of climate variables, S deposition, and land cover, it was clear that all drivers significantly influence water color, but that including afforestation was integral to explain long-term browning. Low water color in the decades before peak S deposition, showed that observed browning is not representing a return to pre-industrial conditions. Rather, an earlier water color increase may have been suppressed by the reduced mobility of OM during peak acidification and has led to an accelerated water color increase in recent decades. The importance of spruce forest to the mobilization of Fe and DOM from soils was also supported by results from a field study based on a forest chronosequence. Both Fe and DOM concentrations were considerably higher in soil solution under old spruce forest stands than under young spruce forest and arable land. This supports a previously suggested notion that impacts of afforestation on soil processes that influence Fe and DOM mobilization can only be seen after several decades, and explain why changes in land cover and water color are temporarily mismatched. Characterization of Fe speciation by X-ray absorption spectroscopy (XAS) supported that mononuclear Fe(III)-OM complexes were the dominant Fe phase in soil solution, a finding that highlights the strong link between Fe and OM mobilization and the importance of organic soils in providing Fe to surface waters. The role of spruce forests was further indicated by the fact that increasing trends in stream Fe concentrations were observed in catchments with high proportion of old spruce forest, but not in catchments with more mire, open land, and deciduous forest cover. Long-term increase of Fe concentrations in soil solution of a riparian forest soil, and the lack of a trend in soil solution of a mire, also point to spruce forest soils as an important source to support positive Fe trends in streams.Collectively, these findings show that land cover sets the baseline for the amount of DOM and Fe that is available, and that changes in land cover, S deposition and climate drivers contribute to observed browning trends.