Phosphorus transfer from agricultural areas and its impact on the eutrophication of lakes - two long-term integrated studies from Norway

Eutrophication of most fresh water systems is limited by phosphorus (P) concentration. High P concentrations originate from external and internal sources. In most Norwegian lakes, agriculture is a main external contributor of P. Two long-term, integrated studies of the relationship between agricultural management, transfer of P and suspended sediments (SS) from agricultural areas and the total P (TP) and chlorophyll-a (Chl-a) concentrations of the receiving lake were carried out in Norway. The Grimestad subcatchment/Aker Lake system (1993-2000) represents a cereal-growing area with mixed livestock production, while the Time subcatchment/Froyland Lake (1986-2000) system represents a grass and dairy cow production system. A comparison of the two systems showed that the mean annual concentration of SS in the Grimestad Stream was 20 times the corresponding concentration in the Time stream. The difference in transparency (secchi depth) of the two lakes reflected this difference. The losses of TP and SS from the Grimestad subcatchment increased significantly during the monitoring period. In the Time stream, there was a significant downward trend in concentrations of TP. Corresponding to the measured inputs, the TP concentration of the Aker Lake (recipient of Grimestad Stream) increased slightly during the monitoring period, while the TP concentration of the Froyland Lake (recipient of Time Stream) showed a slightly decreasing trend. Loads of TP from the Grimestad subcatchment during spring (March-April) described 70% of the variation in TP concentration of the Aker Lake the following summer. The TP concentration in the Time stream in November-December also were correlated (r(2) = 0.6) to the TP concentration in the Froyland Lake the following summer. The annual TP concentrations of the lakes were not very well correlated to the measured Chl-a in the lakes, partly because of bio-manipulation, which was performed in both lakes during the monitoring period. (c) 2005 Elsevier B.V. All rights reserved.