As the soil in southern Sweden recovers from acidification during the 20th Century, iron (Fe) and dissolved organic carbon (DOC) increasingly enter freshwater systems, resulting in "brownification." This process, characterised by light absorption and scattering of waterborne particles and dissolved molecules, causes a brown-yellow colouration that disrupts freshwater ecosystems. Although extensive research has shown that DOC and Fe strongly associate and transport together, their combined effect on light absorption remains poorly understood. This study assessed water quality from Lake Bolmen to investigate how DOC influences Fe light-absorbing properties. Using the data obtained from field samples, a laboratory study was designed and conducted where known concentrations of DOC and Fe were mixed in artificial river water and analysed using UV-Vis spectroscopy. The results revealed that Fe light absorption is markedly enhanced at very low C ratios, beyond which the impact of this enhancement on overall brownification is minimal. These findings provide new insights into the light-absorbing behaviours of Fe and DOC and suggest priorities for future brownification mitigation strategies.

This thesis investigates brownification in Lake Bolmen, Sweden, focusing on the precipitation's effect on water colouration in 3 tributaries to the lake. This study uses long-term meteorological data from the Swedish Meteorological and Hydrological Institute (SMHI) and the water samples collected over more than one year from Lake Bolmen and its three major tributaries Storån, Lillån, and Önnekvarn. Water colour was measured using spectrophotometric techniques. The key findings show a significant positive link between the amount of precipitation per month and water colour, used as a proxy for dissolved organic carbon. The study found that monthly precipitation did not show an immediate effect, but rather that the effect of precipitation on colour was visible with one-month delay. The study finds significant geographical variability in brownification intensity across different streams, indicating that localised effects are also important. The study highlights the crucial role of environmental factors in the organic matter movement and the necessity for specific conservation measures to reduce the negative effects of brownification on water quality. To develop effective Lake Bolmen management techniques, recommendations include continual monitoring and a multidisciplinary approach. This study advances our understanding of the dynamic interplay between freshwater ecosystems hydrology, meteorological, and organic matter processes.