Chapter 9: Monitoring the Effects of Nutrient Enrichment on Freshwater Ecosystems
Peter M. Kiffney, Robert E. Bilby, and Beth L. Sanderson
Nutrient enrichment projects are manipulations of the natural environment that aim to increase the productive capacity of an ecosystem. Like other aquatic enhancement projects, they provide an opportunity to test hypotheses regarding physical, chemical, and biological responses to different habitat-enhancement actions. Unfortunately, evaluation of how nutrient additions affect aquatic ecosystems is rare (Roni et al. 2002). The lack of attention to food-web productivity in restoration efforts is due, in part, to a misperception that manipulating the nutrient or the productive status of a stream, river, or lake is not possible.
The nutrient status of lakes, streams, and rivers can be affected by a variety of human activities. Agriculture and urbanization typically increase nutrient delivery to aquatic systems, often so much so that the capacity of these systems to support desirable species is damaged by eutrophication (Vitousek et al. 1997). Under some circumstances, nutrient inputs have been artificially reduced by human activities, leading to reductions in productivity of naturally oligotrophic systems (Stockner et al. 2000). For example, dams, fishing, and habitat degradation have contributed to declines in salmon stocks and other anadromous fish, thereby reducing subsidies of nutrients and organic matter returning to watersheds along the west and east coasts of North America and much of northern Europe and eastern Russia. These reductions in organic matter and nutrients likely have had major impacts on the growth and survival of juvenile fish, as well as other species, accentuating the decline in anadromous fish populations.