Introduction

Eutrophication refers to enrichment of aquatic systems by inorganic plant nutrients (Wetzel, 1983; Mason, 1991). 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 lake productivity. Causes of eutrophication include human (anthropogenic eutrophication) and non-human (natural eutrophication) disturbances. Marked natural eutrophication events are rare and may result from dramatic episodes, such as forest fire (e.g., Hickman et al., 1990) and tree die-off (Boucherle et al., 1986; Hall & Smol, 1993). Climatic shifts, such as droughts, may also concentrate lakewater nutrients or give rise to an increased contribution of nutrient-rich groundwater (e.g., Webster et al., 1996). In most cases, however, water-quality problems are caused by anthropogenic nutrient inputs from domestic and industrial sewage disposal, farming activities and soil erosion.

Eutrophication is the most widespread form of lake pollution on a global scale, and has many deleterious impacts on aquatic systems (Harper, 1992). In addition to increasing overall primary production, eutrophication causes considerable changes to biochemical cycles and biological communities. Marked changes occur at all levels in the food web and entire communities can change or die out. For example, changes in the ratio of N:P often results in primary production shifting from primarily diatoms and other smaller edible algae towards larger cyanobacteria that are better competitors for N (Tilman et al., 1986), and more resistant to grazing (Reynolds, 1984).