The Effect of Increased Dissolved Natural Organic Matter on Eutrophication

The concentration of dissolved natural organic matter (DNOM) has increased in many streams previously suffering the effects of acid rain. DNOM plays an important role in governing the flux of phosphorous (P) in natural acid-forested environments. Phosphorous fractions, including P bound to DNOM in nine streams draining watershed with land use varying from only agriculture to completely forested land, were studied to assess the role of land use on the effect of increased DNOM on eutrophication. The study area was the forest and agricultural-dominated watershed to western Vansjø. This is a lake in south-western Norway that has been suffering severe eutrophication with extensive blooming of blue-green algae. A clear spatial variation in the water chemistry in the streams was found which was mainly explained by the different land use in their watersheds: The acid forest streams were rich in DNOM, and their main P fraction was bound to this organic material (DNOM-P); Agricultural-influenced streams were more alkaline, had higher ionic strength and higher particle loading, and their phosphorous loading was mainly bound to particles. Much of the DNOM-P in streams draining forests is lost where the stream passes through low-lying agricultural land. This is likely mainly due to sorption to particles and precipitation with calcium. The DNOM-P that reaches the lake will partly be mineralized through photo-oxidation, releasing bioavailable orthophosphate (free PO₄ ³⁻) to the water. The DNOM contains significant amount of Fe(III) and Al(III) which are also mineralized. These ions will precipitate in the more pH neutral lake water co-precipitating the released PO₄ ³⁻. The fate of DNOM-P and its impact on the eutrophication needs therefore further studies in order to be assessed.