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: Oils 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 lake.

Sedimentation is greatly accelerated, however, by human activities that leave the soil exposed without vegetation for extended periods. Land development, construction and agricultural activities near lakes and streams, or farming steep slopes leaves soils vulnerable to erosion. Sedimentation is best controlled through soil and water conservation practices, maintaining vegetation on soils, and use of best management practices (BMPs) during construction.

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--to oxygen depletion and fish kills, and causing taste and odor problems in water and fish. Some species of algae release toxins.

Excess algae can interfere with the simple pleasure of looking at a lake for its beauty. Unsightly scums are usually caused either by tangled masses of filamentous algae or by "blooms" of certain planktonic algae that float on the lake surface forming scums. The regular occurrence of visible algal blooms often indicates that nutrient levels, especially phosphorus, are too high.

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.

When plants die they can introduce significant quantities of nutrients and organic matter back into the water, stimulating algal blooms and raising dissolved oxygen consumption. Macrophytes are grouped into classes called emergents (cattails and bulrushes), floating-leaved (water lilies and duck weed), and submersed (milfoil and pondweeds).

Submersed plants grow profusely only where underwater light is sufficient. Steep-sided lakes have fewer common nuisance weeds because most of the sediments are too far below the surface. Thick sediments can create a favorable weed habitat unless the sediment loading also creates severe water turbidity (cloudiness).

Most macrophytes obtain their nutrients from lake sediments via roots. Therefore, they can grow abundantly even in lakes in which the nutrient concentration of the water has been reduced.

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.

Toxic contamination may cause dramatic impacts such as fish kills. Less obvious impacts include decreased reproduction or slower growth rates in fish and reductions in invertebrate diversity. One long-term danger of toxic contamination is the bioaccumulation or build-up of toxic substances--mercury, for example--in fish flesh. The toxic efforts may be passed on to humans eating the fish.