Excessive nutrient enrichment also has many additional eects on the biology, chemistry, and human use of lakes and reservoirs, however. These secondary eects are frequently deleterious, and may be of great concern to users of the resource (Table 3). For example, eutrophic lakes are typically characterized by shifts towards dominance of the phytoplankton by blue-green algae (cyanobacteria), some of which produce compounds that are more toxic than cobra venom (Skulberg et al., 1984; Carmichael, 1991).