Impact of DNOM photo-degradation on its bioavailability in freshwater lakes

The slow amelioration in the eutrophic status at lake Vansjø, south-eastern Norway, despite numerous abatement actions to reduce the nutrient input from agricultural sites in the catchment, has brought focus to the role of background phosphorus (P) nutrient input. This flux of P is mainly associated to dissolved natural organic matter (DNOM) which accounts for approx. 40% of the total P loading to the lake. With 85% of the catchment being forested area leaching high concentrations of DNOM, and with a doubling of the concentration of DNOM over the last 20 years, particular focus has been brought onto the bioavailability of DNOM derived P. For this reason much focus is directed toward photo-degradation in the lake, which alters the DNOM and can potentially release phosphate making it bioavailable. A variety of laboratory experiments using artificial sunlight (Q-Sun Xenon Test Chamber) to expose highly humic stream water and a lake study deploying passive samplers for orthophosphate have resulted in varying and contradictory results leading only to more uncertainty regarding the effects of photo-degradation on the bioavailability of P. In addition to these experiments regarding the fate of P, the effect of sunlight on DNOM bioavailability was analyzed through a combined photo-degradation incubation experiment. Changes in DNOM concentrations and characteristics during the photo- oxidation and pre- and post- incubation were measured by use of UV-VIS and fluorescence spectroscopy, TOC analyzer, and pH.