Impact of Land-use and Climate on the Transport of TOC and Nutrients from Three Small Drainage Basins in North Europe

In this research we studied to what extent the concentrations and loads of carbon, nitrogen and phosphorus may vary seasonally and annually in three forested upstream rivers with slightly different land-use practices in the surrounding drainage area. The three drainage basins belong to the drainage area of Lake Pääjärvi with a total area of 223 km². The data set consists of continuous discharge measurements in all rivers since early 1970's, and chemical analyses based on weekly sampling (or during a few winters biweekly or once a month) since autumn 1994. In the rivers the concentration of TOC, N and P varied seasonally up to 5-10 -fold, and their annual loads up to 4 fold. Within such a small geographical area the rivers behaved in a very coherent way both in terms of hydrology and nutrient concentrations. The results strongly support the hypothesis that besides land-use, climate is the key driving force which regulates not only short-term variability in nutrient concentrations and loads in boreal rivers, but very much also the annual loads of the nutrients. As a consequence of warmer winter conditions nutrient transportation to downstream lakes and seas from boreal drainage basins may substantially increase.