Eutrophication: The aging process
Over its lifetime, a lake progresses from a more oligotrophic to a more eutrophic state. When nutrients such as phosphorus and nitrogen wash into a lake with stormwater or by soil erosion, they fertilize the lake and encourage algae and larger plants to grow. As plants and the animals that feed on them die and decompose, they accumulate on the lake bottom as organic sediments. After hundreds or thousands of years of plant growth and decomposition, the character of a lake may more closely resemble a marsh or a bog. This natural transition process is called eutrophication.
Lakes also obtain nutrients from various human activities, which can literally make a lake old before its time. This accelerated transition is called cultural eutrophication. Nutrients from agricultural areas, stormwater runoff, urban development, fertilized yards and gardens, failing septic systems, land clearing, municipal and industrial wastewater, runoff from construction projects, and recreational activities contribute to accelerated enrichment or eutrophication.
Sedimentation: Soils wash into the lake
Sedimentation is closely associated with eutrophication. Wind and water move
soils from the watershed into a lake. The soils settle on the bottom of a lake,
and the lake becomes increasingly shallow as part of the natural filling of the
Algae: Microscopic aquatic plants
Algae are a source of food and energy for fish and other lake organisms and a
vital part of all lakes. Too many or nuisance types of algae, however, can
interfere with lake uses by clogging the filters in water treatment plants,
inhibiting the growth of other plants by clouding the water so that it shades
them, contributing--as they decay--tooxygen depletion and fish kills, and
causing taste and odor problems in water and fish. Some species of algae release
Aquatic Plants: Large rooted or floating plants
Aquatic plants also limit many uses of the lake. Like algae, aquatic plants (macrophytes)
are a vital part of the lake because they provide cover, habitat and sometimes
food for fish, the organisms that fish eat, and other wildlife. However, too
many rooted and floating plants can limit swimming, fishing, skiing, sailing,
boating, and aesthetic appreciation. Excessive plant growth can physically
prevent mixing of oxygen through the water.
Contamination: Pollution from toxic substances
Lakes can be contaminated by toxic substances including industrial chemicals
such as PCBs (polychlorinated biphenyls), metals, and solvents; pesticides from
agricultural runoff; urban stormwater runoff containing petroleum hydrocarbons,
metals and pesticides; and air-deposited chemicals.