Eutrophication

Nitrates, phosphates, and certain other chemical compounds promote the growth of algae and plants in a lake. These chemical compounds increase in a lake over time—in a process called eutrophication. Under natural conditions, the increase is slow and gradual. However, pollution (as from city sewage or runoff from fields that have been fertilized) greatly speeds up the process. As the quantity of the chemical compounds in the lake increases, the population of algae and plants also increases.

One consequence of eutrophication is an increase in the accumulation of decaying algal and plant matter in a lake. In a lake in the advanced stages of eutrophication (such as a heavily polluted lake), the bacteria that feed on this matter use up much of the oxygen dissolved in the water. The reduction in the amount of dissolved oxygen leads to changes in the types of species in the lake. For example, commercially and recreationally desirable fish such as trout are replaced by undesirable fish such as carp.