EPA ranks the pollutants and processes by the extent of their impacts on aquatic life and human activities rather than actual pollutant loads in lakes. This approach targets the pollutants and processes causing the most harm to aquatic life and public use of our waters rather than the most abundant pollutants in our lakes, reservoirs, and ponds. When temperature and light conditions are favorable, excessive nutrients stimulate population explosions of undesirable algae and aquatic weeds. The algae sink to the lake bottom after they die, where bacteria consume the available dissolved oxygen as the bacteria decompose the algae. Fish kills and foul odors may result if dissolved oxygen is depleted.
States consistently report metals as a major cause of impairment to lakes.
This is mainly due to the widespread detection of mercury in fish tissue samples. It is difficult to measure mercury in ambient water so most States rely on fish samples to indicate mercury contamination, since mercury bioaccumulates in tissue. States are actively studying the extent of the mercury problem, which is complex because it involves atmospheric transport from power-generating facilities and other sources. Often, several pollutants and processes impact a single lake. For example, a process such as removal of shoreline vegetation may accelerate erosion of sediment and nutrients into a lake.