A TRACER INVESTIGATION OF NITROGEN CYCLING IN A PRISTINE TUNDRA RIVER

The fate of nitrogen (N) in drainages depends in part on N storage and transformation within the river network. The addition of an [N-15]NH4 + isotopic tracer to a tundra river provided information on the uptake , food web transfer, and ecosystem retention of N. The fourth-order Ku paruk River ecosystem took up 60% of the NH4+ within 1 h and 1 km of t he point of tracer addition. Long-lived biota and epilithon retained m easurable amounts of the added N-15 for UP to 2 years. These transform ations and storages within rivers exert control over the timing of N e xport, the chemical form of exported N, and, consequently, its ecologi cal impact on downstream aquatic ecosystems.