IMPLICATION OF LAKE WATER RESIDENCE TIME ON THE CLASSIFICATION OF NORWEGIAN SURFACE WATER SITES INTO PROGRESSIVE STAGES OF NITROGEN SATURATION

摘要

Seasonal behaviour of NO_3~- 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 NO_3~- 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 NO_3~- oscillations in the outlet streams. Under given runoff conditions (1.0-2.2 m yr~(-1)), the seasonal NO_3~- 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 NO_3~- 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 NO_3~- 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 characterises 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.