Sunlit surface waters : exploring the photochemical reactivity of dissolved organic carbon

In surface waters, solar radiation can photochemically mineralise the dissolved organic carbon (DOC, a measure of dissolved organic matter, DOM) to dissolved inorganic carbon (DIC). This DIC photoproduction constitutes an essential yet vague flux in the aquatic carbon cycling. The present thesis is based on the empirical assessment of the DOC photochemical reactivity, which was determined as the spectral apparent quantum yields (AQY) for DIC photoproduction. First, AQYs were determined in DOM solutions to quantify the impact of pH and DOM-associated iron. Then boreal lake waters were used for assessing the alteration of DOC photoreactivity due to water quality and catchment property. By simulating DIC production, further, AQYs were used to approximate the photomineralisation of terrigenous DOC (tDOC) in coastal waters. Finally, the experimental protocols determining AQY were compared by four laboratories. The results demonstrated the variation of AQYs triggered by the laboratory-specific procedures was less than that across the examined inland waters. Up to 86 % of the DIC photoproduction in DOM solutions can be justified by iron-stimulated photoreactions with acidic pH, while the effect was negligible at pH > 7. This interaction between iron and acidity was similarly influential on DOC photoreactivity in boreal lake waters. Across lakes, the DOC photoreactivity was varied relevant to the water quality and catchment land use patterns. A high DOC photoreactivity can be expected when the contents of DOC and chromophoric DOM are high, more so in small lakes enriched by peaty soils. Although DOC photoreactivity in lakes was higher, the estimates revealed that solar radiation mineralised far more tDOC in marine waters, which may be attributed to the extensive spreading of tDOC during mixing over the coastal ocean.