Nitrogen Uptake, Retention and Cycling in Stream Ecosystems: An Intersite N-15 Tracer Experiement

II. BACKGROUND, RESEARCH OBJECTIVES AND SIGNIFICANCE A. Nitrogen cycling in streams Nitrogen is an element of considerable interest in lotic ecosystems. It limits productivity in some flowing waters (e. g. Grimm and Fisher 1986), while in others it is present to such excess that legislators have called for a 50% reduction by the year 2000 ( Haycock et al. 1993). In the U. S. nitrogen loading has increased 20-fold in the last 50 years, and loading varies by an order of magnitude in different parts of the country (Puckett 1995). Knowledge of factors regulating nitrogen cycling in streams is integral to understanding lotic ecosystem structure and function (Meyer et al. 1988). There is a rich literature of process-level studies in which temporal and spatial variation in rates of key nitrogen transformations have been documented (Stream Solute Workshop 1990). Rates of nitrogen transport in some rivers are well documented (e. g. Triska et al. 1993). However, a good understanding of the controls of nitrogen cycling in lotic ecosystems is lacking. Although the theory of nutrient spiraling in streams is well-developed and is being widely applied (Stream Solute Workshop 1990), it treats the processes responsible for nitrogen uptake and release as a black box. A robust model linking hydrodynamics, nitrogen transformations, and food web dynamics in streams is needed. Because streams serve as key hydrologic and biogeochemical links between uplands and downstream ecosystems, understanding how changes in climate, atmospheric deposition, or land use will affect entire landscapes requires working models of the structure and function of streams. Such models are needed to address important large-scale issues such as water quality, biotic diversity, and coastal eutrophication. The role of lotic ecosystems in the response of the biosphere to anthropogenic change has been largely ignored by the scientific community. In a recent assessment of the sources of water pollution, no consideration was given to in-stream transformations of nitrogen that could alter nitrogen transported by rivers (Puckett 1995). The reason for this is not that in-stream processes are unimportant, but rather that stream ecosystem functions have not been synthesized in robust models that are general and can speak to entire watershed, regional and continental-scale changes (Naiman et al. 1995). Such models are available for oceans and for terrestrial ecosystems but not for the networks of rivers that link land and ocean. Comparisons of streams in different biomes with respect to hydrologic and nitrogen transport processes are important and necessary steps in the development of general