PlainLakes

Guide · Impairment causes

Common Lake Impairment Causes Explained

Mercury, nutrients, pathogens, oxygen, pH, invasive species: what each listed cause means, where it comes from, and what it implies for using the lake.

The short answer

An impairment cause names the specific pollutant or condition that broke a water quality standard, and each cause carries its own source story, its own affected uses, and its own timescale for repair.

Mercury in Fish Tissue
the most common listing label on PlainLakes, on 2,961 lake waterbodies
6,851
lake waterbodies list mercury under one of its 7 filed names, across 42 states
Cause → use
every listing names a pollutant or condition and the uses it impairs
Cause → remedy
the cause determines whether a fix is fast or generational

The cause, not the word 'impaired', is what tells you whether a listing matters for your use of the lake.

According to the U.S. Environmental Protection Agency's ATTAINS program, state agencies and tribal nations report 60,590 lake-type waterbodies nationwide in the August 2026 extract, and PlainLakes renders those filed assessment decisions alongside linked U.S. Geological Survey daily gauge records where a conservative name match exists. See our methodology for reporting-unit limits, filer attribution, and refresh cadence.

The Most Common Causes in the Data

Counting how many lake waterbodies each cause appears on shows which problems dominate the national listing picture. The ranking below is computed from the PlainLakes database of state-reported causes.

Most common impairment causes on lake waterbodies

Number of lake waterbodies in the PlainLakes database listing each cause, top 10 by count, sorted descending

waterbodies

What this shows Mercury in Fish Tissue is the most frequently filed cause name, appearing on 2,961 lake waterbodies across 24 states. These bars rank names, not pollutants, and mercury holds 7 of them: read together, mercury is listed on 6,851 waterbodies in 42 states, more than twice what the leading bar shows.

Source EPA ATTAINS As of Reporting cycles 1987–2026

Mercury

Mercury reaches lakes mostly through atmospheric deposition from combustion and industrial sources, sometimes far away. In lake sediments, bacteria convert it to methylmercury, which bioaccumulates up the food chain and concentrates in long-lived predatory fish. Mercury listings therefore attach mainly to the fish-consumption use, and they can affect remote lakes with pristine shorelines.

For lake users: swimming and boating are unaffected; eating the fish is the question. Meal decisions belong to your state advisory, as explained in the fish consumption guide. Expect these listings to persist: the remedy depends on regional emission trends, not local action.

Nutrients and Eutrophication

Nutrient listings cover phosphorus and nitrogen arriving from fertilized fields, lawns, wastewater discharges, septic systems, and stormwater. Nutrients over-fertilize the lake: algae and aquatic plants grow beyond what the ecosystem can process, clarity drops, and the decay of all that biomass consumes oxygen. This enrichment process, eutrophication, is the engine behind several other listings and is measured on the trophic scale described in the trophic status guide.

For lake users: expect greener water, reduced clarity, more weeds, and in warm calm periods a higher chance of cyanobacteria blooms. Fisheries shift toward species tolerant of low oxygen. Nutrient problems respond to watershed action, but recovery is slow because phosphorus stored in sediments keeps recycling into the water for years.

Pathogens and E. coli

Pathogen listings rest on indicator bacteria, usually E. coli in freshwater, which signal fecal contamination from failing septic systems, sewer overflows, agricultural runoff, pet waste, or waterfowl. The affected use is almost always recreation: the standard protects people from swallowing contaminated water while swimming.

For lake users: this is the cause most directly relevant to swimmers, and also the most episodic. Bacteria counts spike after rain and fade in dry weather, so the listing marks a recurring vulnerability rather than a constant hazard. Current swim status comes from beach monitoring programs, as covered in the recreation safety guide.

Dissolved Oxygen

Aquatic life needs oxygen dissolved in the water, and states set minimum criteria to protect it. Low dissolved oxygen is usually downstream of nutrient enrichment: algal blooms die, decompose, and the decomposition consumes oxygen, especially in deeper water and during hot, stratified summers. Severe episodes kill fish outright; chronic low oxygen restructures the fish community toward tolerant species.

For lake users: anglers feel this cause first, through fish kills, shrinking coldwater habitat, and shifted catches. It is a symptom listing; fixing it means fixing the enrichment that drives it.

pH

States set acceptable pH ranges because water that is too acidic or too alkaline stresses aquatic organisms and changes how other pollutants behave; acidic water, for instance, makes metals more soluble and more toxic. Drivers include acid deposition, mine drainage, naturally acidic watershed geology, and, on the alkaline side, intense photosynthesis during algal blooms.

For lake users: pH excursions rarely affect swimmers directly. The significance is ecological, and a pH listing alongside a nutrient listing often points to bloom-driven swings rather than acidification.

Invasive Species

Some states list invasive plants and animals as causes when they prevent a lake from supporting its uses: milfoil mats that close swimming areas, mussels that reshape the food web, carp that uproot vegetation and stir sediment. Unlike chemical causes, there is no load to allocate, so management runs through prevention, containment, and control programs rather than a conventional pollution budget.

For lake users: the practical burden is often on boaters, through inspection and decontamination requirements meant to stop the spread between lakes, and on anglers as fisheries change.

Frequently Asked Questions

Can one lake have several impairment causes at once?

Yes, and many do. Causes are listed independently: a lake can carry mercury in fish tissue, excess nutrients, and low dissolved oxygen simultaneously, each attached to the designated uses it affects. Multiple causes often interact, since nutrient enrichment drives the algal growth whose decay depletes oxygen. PlainLakes ranks lakes by cause count on the rankings page, which is one rough proxy for how layered a lake's problems are.

Does a nutrient impairment mean the lake is dangerous?

Not by itself. Nutrients, mainly phosphorus and nitrogen, are not directly toxic to people at the levels found in lakes. The concern is what they feed: dense algal growth, reduced clarity, oxygen depletion that harms fish, and conditions favorable to cyanobacteria blooms, some of which can produce toxins. A nutrient listing describes an over-fertilized ecosystem, and the practical implications depend on how that enrichment expresses itself in a given year.

What does a pathogen impairment actually measure?

States rarely measure disease-causing organisms directly. They monitor indicator bacteria, most often E. coli for freshwater, whose presence signals fecal contamination from sources like failing septic systems, sewer overflows, livestock runoff, or waterfowl. When indicator counts exceed the recreation standard, the waterbody is listed as impaired for recreation. The indicator does not identify which pathogens are present, only that the contamination pathway exists.

Why is "dissolved oxygen" listed as a cause when it is not a pollutant?

Listings name the condition that violates the standard, and states set minimum dissolved oxygen criteria because fish and other aquatic life suffocate below them. Low oxygen is usually a symptom of something else, most often nutrient-driven algal decay, but it is the measurable exceedance, so it appears as the listed cause. The same logic applies to pH listings: the measured condition is listed even when the driver is acid deposition, mine drainage, or intense photosynthesis.

Are invasive species treated like pollutants in the data?

They are listed as causes when they prevent a waterbody from supporting its designated uses, for example when invasive plants choke out habitat or invasive mussels restructure the food web. Because an organism is not a pollutant with a daily load to allocate, these listings often sit in integrated-report categories where a conventional TMDL is not the tool, and management runs through containment and control programs instead.

Sources

What to do with this

Match the cause to your use of the lake, then check the data pages.

Cause names are reported by state programs; naming conventions vary slightly between states.

Keep reading

All guides

PlainLakes is rendered directly from state Clean Water Act assessment decisions in EPA ATTAINS and lake level records from USGS NWIS, no number is typed in by an editor. The cause ranking in this guide is computed at render time from the cause records in the PlainLakes database; no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of Reporting cycles 1987–2026. Primary sources: EPA ATTAINS and USGS National Water Information System.