What is polluting our lakes?

The short answer to that question is:  We are.

All types of human activities – turning forests into farms and turning farms into suburban developments, burning coal to power our homes and factories, fertilizing our lawns and fields, using antibiotics and other drugs, bringing plants and animals here from other places and allowing them to escape into an environment where they may have no natural enemies – pollute our lakes.

A few major types of pollution are:

Phosphorus, a non-metallic element found almost everywhere on Earth, may be our biggest pollution problem. It is in soil, plants of all kinds, animal tissue and wastes excreted by humans and animals. It is widely used as a farm fertilizer. Since 1972, state law has strictly regulated how much phosphorus can be contained in laundry detergents sold in Minnesota, and since 2005 how much phosphorus can be in lawn fertilizers.

Phosphorus is washed into lakes with eroded soil, leaves and grass clippings, and it falls on lakes as part of wind-blown dust particles. It damages lakes in several ways, mostly by feeding the growth of excessive algae. Large algae blooms are unsightly and nuisances for swimmers and boaters. Algae reduce lake clarity, make it hard for other water plants to survive and – when they decompose – deplete oxygen in the lakes’ deepest reaches . The result is a lake environment that is especially hostile for deep-water fish, like lake trout, and not so hostile for carp and bullheads.

According to an update of a 2007 report to the Minnesota Pollution Control Agency, the major sources of phosphorus in lakes and rivers across the state are: agricultural runoff from cropland and pasture, feedlots and farm tiles – 33 percent; atmospheric deposition — 23 percent; sewage treatment plants and industry — 15 percent; stream bank erosion – 13 percent; non-agricultural runoff — 6 percent; urban runoff — 5 percent; septic systems — 4 percent; and road deicers — 1 percent.

But those percentages vary dramatically between rivers and lakes, between wet and dry years, and among regions of the state and among individual lakes. Some lakes get almost all their phosphorus from the atmosphere; some get almost all theirs from stormwater runoff carrying fertilizer and soil.

Mercury is one of the most widespread pollutants in Minnesota. It is a naturally occurring element that is released around the world by the burning of coal. It falls to the Earth with rain and snow. In lakes, some mercury is converted by bacteria into methylmercury, which accumulates in fish and – in elevated concentrations – harms the nervous systems of people who eat significant quantities of fish, especially large, predatory fish. Methylmercury is especially harmful for fetuses, infants and children. Of the Minnesota lakes that have been designated as “impaired” by the Minnesota Pollution Control Agency, about two-thirds exceed standards for mercury, either in the water or in fish tissue.

Sediment is soil that is washed into lakes. It is a particular problem in stream-fed reservoirs, where current moves the soil along until the water slows down, for example, when it is stopped by an impoundment. The soil causes two problems: It covers gravel bottoms that walleyes and some other fish need for spawning, and it is a major source of phosphorus and mercury entering many lakes. We cause sedimentation when we allow soil from construction sites, farm fields, or yards to wash into lakes, and we cause it when wakes from boats and personal water craft erode fragile shorelines.

Bacteria and pathogens are organisms that sicken and sometimes kill people. Malfunctioning septic systems in lakeside cabins are a potential source. Wild geese often are a source of bacteria near swimming beaches. Lawns – rather than native vegetation — in beach areas give geese a sense of protection from predators. If you want to keep geese and bacteria out of the area where you swim, let the lakeside plants grow tall and natural and let the geese worry that something that could eat them might be lurking there.


Invasive species are a different type of pollutant. Animals and plants from around the world that we carelessly introduce into Minnesota lakes do great damage to native plants and fish, out-competing them for space and nutrition. Global climate change has the potential to dramatically aggravate the problem of invasive species by moderating the severe winters that now keep some non-native organisms from establishing themselves.Clickhere for more information on the most common and most threatening invasive species in Minnesota.

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