On the summer regulation of nitrogen and phosphorus transport in a small stream of southwestern British Columbia
Creators: name: Perrin, Christopher John
Publisher: name: University of British Columbia
Year: 2010

description: Patterns of in-stream summer transport of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in a high gradient second order stream in southwestern British Columbia were described. Some biological, physical, and chemical processes were also identified and evaluated with respect to their role in regulating in-stream transport of TDN and TDP. Fifteen water sampling stations were established along an alternating series of open clearcut and densely forest covered drainage areas, each several hundred metres in length. These drainage sections represented young and midsuccessional ecosystems. The furthest upstream section (which was forest covered) and an adjacent downstream open clearcut section each had weirs which were used for comparisons of TDN and TDP export between the successionally different systems. Export of TDN was consistently lower from the clearcut section than from the upstream forested section. This indicated that there was consistent in-stream absorption of TDN within the clearcut section. Nitrate flux dominated the absorption of TDN. A pairing analysis of precutting nitrate export data from 1972 with post-treatment export data from 1979 indicated that the absorption of nitrate was due to increased solar radiation reaching the stream channel. It was proposed that the increased light inputs which provided an additional energy input to the stream ecosystem, increased nitrate spiralling rates. Without disruptive stormflow events the net effect was a retention of nitrate within the stream. Concentrations of TDP were often less than detectable limits so comparisons of P export could not be made. The TDN data suggested that if nutrient absorption in small watershed streams is widespread, nutrient retention on land may have been overestimated in past small watershed nutrient budget studies. A further analysis of downstream flux in element concentrations was conducted for all forms of TDN and TDP using three-dimensional images (distance downstream x time x element concentration) from data collected in summer 1979. Nitrate concentrations consistently declined in open sections but increased in densely forest covered sections. Trends in ammonium concentrations were difficult to detect. Orthophosphate levels were rarely greater than the detectable limit of 3 ug/1 in all sections. Dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) flux was highly variable with no apparent trend. Nutrient spiralling processes were proposed to explain the flux of each of these forms of TDN and TDP. An enrichment experiment conducted in a clearcut section of the study stream in mid-summer indicated that rates of disappearance of N0₃⁻-N, NH₄⁺-N, and HP0₄⁻²-P from solution were directly related to element concentrations in solution and that relationship could be expressed as fourth, first, and third degree polynomials respectively. Disappearance rates of the three ions were in the order NH₄⁺-N&gt; HP0₄⁻²--P&gt; N0₃⁻-N. Reasons for the differences in disappearance rates between elements and element forms were proposed. The first end-of-summer storm event produced a 20-fold increase in stream discharge at peak flow over summer base flow. This resulted in a 10-fold increase in N0₃⁻-N concentrations in all stream sections. The increased loading was attributed to flushing of N0₃⁻-N from land. Stream power was not destructive, however, and N0₃⁻-N removed from solution increased over that during summer base flow. The increased uptake rates were attributed to an increase in active nutrient processing areas of the stream substrate. DON and DOP concentrations and export increased during the storm but NH₄⁺-N and HPO₄⁻²-P concentrations did not change. Again, detailed explanations for this element behaviour were proposed. Results of an enrichment experiment designed to evaluate the role of periphyton in regulation of nutrient transport were insufficient to fully assess the quantitative role of periphyton. Hypotheses were proposed, however, to further evaluate the role of periphyton using alternative methods. Concluding hypotheses were presented to aid in the development of future research plans.