The term eutrophication broadly refers to the enrichment of aquatic systems by inorganic plant nutrients (Mason, 1991; Wetzel, 2001). Lake eutrophication occurs when nutrient supplies, usually phosphorus (P) and nitrogen (N), are elevated over rates that occur in the absence of any system perturbation, and results in increased production. Causes of eutrophication include human (anthropogenic eutrophication) and non-human (natural eutrophication) disturbances. Marked natural eutrophication events are relatively rare and may result from dramatic episodes, such as forest fire (e.g. Hickman et al., 1990), tree die-off (Boucherle et al., 1986; Hall & Smol, 1993; St. Jacques et al., 2000) and prolific returns of spawning salmon to nursery lakes (Gregory-Eaves & Keatley, this volume), to name a few mechanisms. Climatic episodes, such as droughts, may also concentrate lake-water nutrients by increasing contributions of nutrient-rich groundwater (e.g. Webster et al., 1996), or reducing flushing rates and increasing deepwater anoxia leading to elevated internal P loading from sediments to the illuminated surface waters (Brüchmann & Negendank, 2004). Some lakes lie in naturally fertile catchments or receive high natural loads of nutrients from groundwater and are naturally eutrophic (e.g. Hall et al., 1999). In most cases, however, eutrophication is caused by anthropogenic nutrient inputs from domestic and industrial sewage disposal, farming activities, soil erosion, and numerous other activities.
Eutrophication is the most widespread form of lake pollution on a global scale, and has many deleterious effects on aquatic systems (Harper, 1992; Smith et al., 2006). In addition to increasing overall primary production, eutrophication causes considerable changes to biochemical cycles and biological communities (Schelske, 1999). Marked changes occur at all levels in the food web and entire communities can change or die out (Carpenter et al., 1995).