Export of DOM from boreal catchments: impacts of land use cover and climate

Dissolved organic matter (DOM) is an important fraction in carbon (C) and nutrient budgets for aquatic ecosystems and can have broad effects on food webs and nutrient cycling. To look at the role land use cover and climate might play in DOM transport from the boreal region, the export of total organic carbon (TOC), total organic nitrogen (TON) and dissolved organic phosphorus (DOP) was estimated for Finnish main rivers and their sub-catchments, altogether 86 catchments, situated between latitudes 60 degrees N and 69 degrees N and covering 297,322 km(2), 88% of the total area of Finland. On an average, 94% of the TOC, 90% of the total nitrogen (TN) and 40% of the total phosphorus (TP) in Finnish rivers was in a dissolved form. The majority of the DOM export from Finnish catchments consists of organic C. The TOC export increased with increasing peatland proportion (r = 0.39, p = 0.003), while TON export increased with the increasing percentage of agricultural land (r = 0.60, p < 0.001). Although upstream lakes covered only on average 9% of the catchment area, they were the most important predictor for TOC, TON and DOP export (r = -0.83, r = -0.82 and r = -0.61, respectively). The higher the upstream lake percentage, the lower the export indicating organic matter retention in lakes.