Abstract
Downstream nutrient flux has been modelled in a second-order springfed stream in New Zealand where the stream banks have been protected by the establishment of fenced riparian strips. This has allowed the development of an aquatic stream bank vegetation which assists in the removal of dissolved nitrogen from the stream water. The model comprises a coupled system for first-order, non-linear partial differential equations to incorporate the feedback effects of nitrogen uptake and release as the water moves downstream. The most sensitive parameters in the model were shown to be the growth and mortality constants for the aquatic vegetation. Analysis of varied farming practices on nutrient uptake by the riparian vegetation shows that winter grazing of the system is disadvantageous to nutrient uptake but that grazing at rates 2 to 3 times the regional average can be sustained during summer months without adversely affecting uptake. Decreasing nutrient mass flows at the springs either by altering concentration or discharge would greatly reduce nutrient export when below defined threshold values, and increases in mass flows above these values would increase export proportionately. Nutrient uptake was found to be proportional to concentration only at low flows.
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