Influence of stream size on ammonium and suspended particulate nitrogen processing

We used (NH4)-N-15 tracer additions to determine travel distances of ammoni um (NH4) and suspended particulate organic nitrogen (SPON) in six streams r anging from second to fifth order located within a single watershed on the North Slope of Alaska. Based on the distribution of N-15 Stored in stream b ottom compartments (primary producers or grazers), we estimated NH4 travel lengths. We used a two-compartment model to estimate the travel length of S PON based on the distribution of source N-15 On the stream bottom and (SPON )-N-15 in the water column. Both NH4 and SPON travel lengths (S-w and S-p, respectively) increased with discharge primarily due to changes in depth an d velocity. Variation in the vertical mass transfer coefficient (v(f)) of b oth NH4 and SPON did occur among the streams but was not related to stream size and was relatively small compared to the change in physical characteri stics. Thus, in the Kuparuk watershed, physical gradients outweighed biolog ical or chemical changes as controls on NH4 and SPON travel length. The one exception was the Kuparuk fertilized reach, where phosphorus fertilization greatly increased biological activity and NH4 processing compared to unalt ered streams. Longitudinal gradients in major biological driving variables such as litter inputs, debris dams, and shading are absent in the Arctic, p erhaps explaining the relatively uniform NH4-v(f). Watersheds in other biom es may show differing degrees of physical versus biological/chemical contro ls. A conceptual model is presented for comparing the relative strength of these controls among different watersheds. Strong relationships between dis charge and travel length should greatly aid development of watershed models of nutrient dynamics.