Methodological issues

Estimation of nutrient uptake rates in streams requires the release of a measurable pulse of a nutrient. This can be accomplished by adding a large quantity of nutrients so that the pulse is detectable above ambient concentrations or through the release of trace amounts of stable or radioactive isotopes (Mulholland et al. 2002). Short-term nutrient additions, typically with orthophosphate-P, ammonium-N, or nitrate-N, are relatively simple and inexpensive. The passage of the nutrient pulse, corrected for dilution from the simultaneous release of a conservative solute, is the basis for estimating SW and vf. However, nutrient addition is likely to saturate biological uptake, and so this method may overestimate SW and underestimate vf. The release of isotopes avoids this problem because only small quantities are needed. In a comparison of methods in a small woodland stream, using an isotope-labeled nitrate release with and without simultaneous nitrate enrichment, transport distance was estimated to be 36 m using the tracer alone and 100 m when the stream was enriched (Mulholland et al. 2004).

Experimental releases of ammonium and nitrate labeled with the stable isotope 15N are becoming widely used, but because only the radioactive isotope of P (32P) is readily available and its release into the environment is heavily restricted, nutrient tracer studies are presently conducted almost exclusively with N. Although tracers have obvious advantages, they are expensive, especially for larger streams that would require greater quantities of stable isotope. Short-term nutrient releases are a less costly alternative, especially when multiple comparisons are needed across sites or over time (Tank et al. 2006), and can be improved upon by adding the smallest amount that can be detected and by conducting a series of additions using different concentrations (Mulholland et al. 2002, Payn et al. 2005).

In addition to providing more reliable estimates of uptake lengths and rates, tracers provide the opportunity to quantify transformations among nutrient forms and the role of different biotic compartments in nutrient uptake and retention (Mulholland et al. 2000, Webster et al. 2003). For example, by following the movement of 15N-la-belled ammonium in a Kansas prairie stream, Dodds et al. (2000) were able to measure not only uptake of ammonium but also its rate of transformation to nitrate, and trace the amount of N assimilated by primary producers and microbes and subsequently into primary consumers and predators. Although nutrient addition studies provide some insight into transformations by comparing the relative amounts of different forms of a nutrient entering and exiting an enriched stream reach, tracer studies are ideal for quantifying nutrient turnover within compartments.

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