Seasonal changes in photochemical properties of dissolved organic matter in small boreal streams

Abstract. The fate of dissolved organic matter (DOM) in lakes and streams is significantly affected by photochemical transformation of DOM. A series of laboratory photochemical experiments was conducted to describe seasonal changes in photochemical properties of DOM. The stream samples used in this study originated from three different catchments in the southernmost part of the Boreal ecozone near Dorset, Ontario, Canada. A first-order kinetics equation was used to model photochemical degradation of DOM and the kinetic rate constant, K, was used as an indicator of photochemical properties of DOM. Kinetic rate constants from all three catchments showed a sinusoidal pattern during the hydrological year. K increased steadily during autumn and winter and decreased during spring and summer with a more than 3-fold range in each stream. The highest values were observed during spring melt events when DOM was flushed from terrestrial sources by high flows. The minimum rate constants were found in summer when discharge was lowest. K was strongly correlated with pH and iron. DOM molecular weight and specific absorbance at 254 nm also exhibited annual cycles corresponding to the seasonal cycles of terrestrial organic matter, but the relationships between these properties and K differed between seasons and may have been affected by previous exposure to solar radiation during transit from the catchment.