The trophic significance of epilithic algal production in a fertilized tundra river ecosystem

Fertilization of a pristine tundra river with phosphorus and nitrogen changed the nitrogen and carbon stable isotope ratios in epilithic algae. Determination of P5N and 613C values in consumers allowed this algal organic matter to be traced in the food web for several kilometers downstream of the fertilizer addition sites. Changes in the isotopic composition of consumers documented the tight coupling between algal production in response to fertilization and measured increases in the growth of insects and fish. Understanding the controls of biological productivity in flowing waters is of applied and fundamental interest because humans impact and use rivers heavily. However, the analysis of river ecosystem dynamics is made difficult by the complexity of material transfers in systems where water, dissolved nutrients, and suspended particles are continuously moving, insects are drifting, and fish are free to migrate. In our experimental river fertilization study, nutrient addition caused shifts in the carbon and nitrogen stable isotope ratios in the biota which provided a unique opportunity to trace organic matter flows and to help determine the effects of fertilization on a tundra river ecosystem. Phosphorus and nitrogen addition stimulated epilithic algal production which in turn promoted growth of insects and fish. Experimental design, the Kuparuk River, and isotope methods