Human activity has altered the global biogeochemical nitrogen cycle, most dramatically in the past century (Vitousek et al. 1997). The outlook for the growth and development of the world’s population reveals that the rise in available nitrogen will continue to be a problem for coming generations (Tilman et al. 2001). Various studies performed in the past two decades in fluvial ecosystems have demonstrated that these systems are highly efficient at retaining nitrogen, and that the amounts retained are proportional to the relative flow of nitrogen (Martí and Sabater 1996, Butturini and Sabater 1998, Webster et al. 2003). Rivers may therefore play a pivotal role in the flow of nitrogen from terrestrial ecosystems to marine ecosystems (Alexander et al. 2000). Nevertheless, identifying the factors that influence nutrient retention is not trivial (Webster et al. 2003). To date, the only clearly imputed factor is flow (Butturini and Sabater 1998); it is known that when flow increases, nutrient retention decreases. Rivers located in humanized basins, those that drain into agricultural basins have lower retention rates than those that drain into forested areas of urban activity, especially in conditions of low flow. The link between flow and land use (including disturbances such as fires) takes on heightened importance in the context of climate change. The stable nitrogen isotope ¹⁵N has been recently used (Mulholland et al. 2004) to trace river system reactivity under various conditions, leading to more accurate predictions towards river responses, including biofilm effects (Battin et al. 2003). In terms of the Iberian Peninsula, where rivers have low flow, surface territories are highly agriculturalized and fertilizer use is widespread, hence analyzing the response of rivers to climate change should have clear applications for the management of our continental and costal waters.