Nutrients in polluted runoff: effects on lakes

Fertilizers, leaves, grass clippings, eroded soil, and animal waste are all sources of nutrients, including phosphorus. Phosphorus is main nutrient that drives eutrophication (premature aging) in most lakes. Relatively small amounts of phosphorus can cause water quality declines. A concentration of 25 parts per billion of phosphorus in water can promote excessive algae growth in lakes.

Phosphorus—an essential nutrient for plant growth—is a common ingredient in many lawn and garden fertilizers. However, the same phosphorus that helps keep lawns green is also the primary nutrient that turns lakes green with algae. Algae blooms can turn lakes into a thick, smelly green soup that is undesirable for swimming and other recreation. Too much algae clouds water and blocks sunlight from reaching aquatic plants; it also lowers the oxygen levels in the water which can cause fish kills.

Soil can retain only so much phosphorus and plants cannot absorb more phosphorus than they can use. Spreading manure and other fertilizers on lake watershed soils already saturated with phosphorus contributes to chronic nutrient loading, as nutrients can run directly into the lake and/or leach into our lakes from groundwater for years. Manure runoff from detention ponds and/or saturated land can also have acute effects; sudden influxes of nutrients can deplete oxygen in the water and trigger fish kills.

Excess algae and aquatic plant growth caused by an over-abundance of nutrients can impact boating and other water recreation and increase lake management costs for lake groups.

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Sediments in polluted runoff: effects on lakes

Sediment—soil particles that end up in the water—reduces water clarity causing the water to become cloudy or “turbid.” Sunlight cannot penetrate as deeply into turbid waters, which restricts aquatic plant growth to smaller and shallower areas. Nutrients are plant food. If aquatic plants are unable to grow in turbid water conditions, algae may use the available resources and become dominant.

Turbid water conditions can also affect fish by damaging gills and impacting their ability to find food. Some species of fish are unable to tolerate persistent turbid water conditions, and may no longer be able to survive in the lake. Some undesirable fish—like the invasive carp—do well in cloudy waters and contribute to continued turbidity by stirring up and re-suspending lake bed sediments into the water. Fine sediments also bury fish spawning areas, and effect egg incubation and fry-rearing.

Aquatic insects are an important part of the food chain. They are eaten by many fish, frogs, birds, and other shoreland animals. Many insect species find shelter in the spaces among rocks, cobbles, and boulders. When fine sediments fill these spaces, it reduces the quality and quantity of habitat available to aquatic insects.

Sediments can be eroded from construction sites, developed areas, and cropland. In addition to the impact the sediment particles can have themselves, sediment runoff can pick up and transport additional pollutants such as metal flakes, debris, toxics, and even more phosphorus into our lakes.

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