Concentrations of nitrogen and phosphorus in streams draining the mixed land-use Dee Catchment, north-east Scotland

1. There is still some uncertainty about large-scale influences on nutrient budgets in rivers. In particular, reduced forms of nitrogen (N) in organic forms might represent a significant fraction of the soluble N present in headwater streams, but this is not well quantified. Nitrate increases in relative importance downstream within agriculturally dominated areas. Here we appraise variations in N dynamics for a representative temperate but upland river, the Dee. 2. In the Dee catchment, the source of organic N appears to vary seasonally. During summer under low flow conditions it originates primarily from in-stream biological production, while during the winter-spring period leaching from the plant-soil system would be the major contributor. 3. On any individual sampling day, a wide range of N:P ratios can occur in the catchment area. Generally the narrowest N:P ratios occur during the summer and early autumn, particularly for upland catchments dominated by semi-natural vegetation. It is possible that some of the tributaries and upper region of the main river may be limited by N during the summer. The interpretation of the N:P ratios depends greatly upon the potential biological availability of the organic N, which remains unknown. 4. Together, these data further illustrate that simple ideas about the relative limiting effects of N and P in temperate freshwaters may be misleading.