Kaste, Ø., J.L. Stoddard, and A. Henricksen. 2003. Implication of lake water residence time on the classification of Norwegian surface water sites into progressive stages of nitrogen saturation. Water Air Soil Pollut. 142:409-424. WED-03-182
Seasonal behaviour of NO3 in surface water is often used as an indicator on a catchment's ability to retain N from atmospheric deposition. In this paper, we classify 12 pristine sites (five streams and seven lakes) in southernmost Norway according to the N saturation stage concept. All examined sites were acid-sensitive and had annual mean NO3 concentrations in the range 6-36 µeq L-1. At lake sites with relatively long water residence times, hydrology will have a damping effect on the seasonal NO3 oscillations in the outlet streams. Under given runoff conditions (1.0-2.2 m yr-1), the seasonal NO3 pattern was clearly affected when lake:catchment area ratios exceeded 0.15 and the total residence time of lakes in the catchments exceeded 0.3 yr. In such cases, annual mean NO3 concentration rather than seasonal variations may be the better indicator of N saturation. To account for this we propose a set of supplementary criteria for such lake systems, defining limit values for annual mean NO3 concentrations within each stage class. Before being applied on a broader scale, however, we recommend an examination of additional lake-dominated catchments representing a larger gradient in ecosystem types, N deposition levels and hydrologic regimes. When applying the supplementary criteria on the Norwegian sites, two were classified as stage 0 (no saturation), five as stage 1 (early stage of saturation) and five as stage 2 (saturated - moderate N loss). No sites were showing symptoms of stage 3, which characterizes watersheds that are net sources of N, rather than sinks. The results indicate a great variability in N retention capacity in the study region, despite the fact that many of the catchments experience relatively uniform N deposition amounts and climatic conditions. This suggests that much of this variability must be due to specific catchment characteristics as e.g. soil type, soil depth, and vegetation cover.