Lake Water Quality Readings
The measurements a state or federal agency published for a lake, shown on its LakeQuality page: Secchi clarity, total phosphorus, total nitrogen, chlorophyll-a and turbidity, with the EPA National Lakes Assessment condition class for the lake’s ecoregion beside each nutrient and algae reading.
Water Quality
Lake Water Quality Readings is a term from limnology — the scientific study of inland waters. The detailed explanation below covers the definition, why the concept matters to lake water-quality interpretation, and how it relates to the readings shown on every lake page. Reading Lake Water Quality Readings alongside a lake's readings makes those numbers easier to follow: each measured parameter maps onto an established limnological concept.
The per-lake pages on LakeQuality always show the specific Lake Water Quality Readings value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
A US lake page shows the median summer readings its monitoring agency published for total phosphorus, total nitrogen, chlorophyll-a, and turbidity, and beside each one the condition class EPA assigns to that value in the National Lakes Assessment 2022 for the lake’s ecoregion: Good, Fair or Poor. The classes are set from the 75th and 95th percentiles of EPA’s least-disturbed reference lakes in each region, so there is no single national phosphorus or chlorophyll-a number that counts as Good; reference phosphorus varies roughly 30× across U.S. ecoregions, and Florida lakes are read against Florida LAKEWATCH breakpoints so subtropical, tannic waters are not marked down by a northern key. Water clarity (Secchi depth) is measured and shown on every lake page with no class, because EPA does not assign a Secchi condition class and dark water can come from algae, clay, or natural colour. The Carlson Trophic State Index is reported as a measurement; the trophic class is stated as fact only when a nutrient or chlorophyll-a parameter has at least three summer samples within ten years. A lake with no nutrient or algae reading on record shows what the record holds and says so. The readings are not a swimming, drinking, or fish-consumption clearance: they do not measure E. coli, cyanotoxins, mercury or PFAS, and whether a lake is safe to swim in or its fish safe to eat is a call for the state water agency and the county health department.
Frequently Asked Questions
What is lake water quality readings?
The measurements a state or federal agency published for a lake, shown on its LakeQuality page: Secchi clarity, total phosphorus, total nitrogen, chlorophyll-a and turbidity, with the EPA National Lakes Assessment condition class for the lake’s ecoregion beside each nutrient and algae reading.
Why does lake water quality readings matter for lake health?
A US lake page shows the median summer readings its monitoring agency published for total phosphorus, total nitrogen, chlorophyll-a, and turbidity, and beside each one the condition class EPA assigns to that value in the National Lakes Assessment 2022 for the lake’s ecoregion: Good, Fair or Poor. Th...
Related Terms
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
The systematic collection and analysis of water samples and measurements to track lake health over time and detect changes in water quality.
Source: EPA National Aquatic Resource Surveys, 2026.