DIFFUSE NUTRIENT LOSSES IN DENMARK

Since 1989, nutrient loading of the Danish aquatic environment has bee n monitored in 270 Danish streams draining catchment areas differing i n climate, physico-geographic and land usage. Diffuse nutrient loading from non-point sources (mainly agricultural) is now the main cause of eutrophication of the Danish aquatic environment; thus in 1993, diffu se sources accounted for 94% of riverine nitrogen loading and 52% of r iverine phosphorus loading. Annual riverine total nitrogen (total-N) l oading from diffuse sources during the period 1989-93 was on average 1 0 times greater in 66 small agricultural catchments (median 23.4 kg N ha(-1)) than in 9 natural catchments (median 2.2 kg N ha(-1)). Corresp ondingly, annual riverine total phosphorus (total-P) loading from diff use sources was on average 3.5 times greater in the agricultural catch ments (0.29 kg P ha(-1)) than in the natural catchments (0.07 kg P ha( -1)). The annual total-N and total-P load was found to increase with t he proportion of agricultural land in the catchments. In 1993, intensi ve measurements of phosphorus load in 8 agricultural catchments showed that normal point sampling (fortnightly) underestimates annual total- P loading by a median of 37% as compared to that estimated by frequent sampling. Moreover, estimates of monthly total-P loading are even mor e biased, especially in late summer and early autumn (-50% to -65%). C opyright (C) 1996 IAWQ.