How Lake Water Quality Readings Are Classified
As of September 3, 2026, LakeQuality tracks 37,603 lakes, each shown with the measurements its agency published. Each nutrient and algae reading is shown beside the condition class EPA assigns to it for the lake's ecoregion, a repeatable classification of measured data, not a subjective judgment and not a swim clearance. This guide is the short version; the cutpoint table and the sampling floors are on the methodology page. Figures here are the published census for September 3, 2026 (37,603 tracked). Not a new data run.
Step 1 — Pull the measurements
The raw data comes from the EPA Water Quality Portal, which aggregates state, federal, and tribal monitoring records into one standardized feed, plus licensed international chemistry catalogs. The readings shown are total phosphorus, total nitrogen, chlorophyll-a, turbidity, and Secchi disk depth. For each parameter the page shows the median of recent summer surface samples, so a single anomalous reading cannot swing the picture.
Step 2 — Read each one against EPA ecoregion cutpoints
Each nutrient and algae median is classified Good / Fair / Poor against the EPA National Lakes Assessment 2022 condition classes for that lake's ecoregion. Reference phosphorus varies roughly 30× across U.S. ecoregions, so a concentration that is Good in the Coastal Plains can be Poor in the Western Mountains. There is no single national Secchi, phosphorus, or chlorophyll-a number that counts as Good. The transcribed cutpoint table is on our methodology page.
Florida is the deliberate exception: subtropical lakes are read against Florida LAKEWATCH breakpoints so naturally tannic, productive waters are not marked down by a northern key.
Step 3 — Show clarity as a measurement, with no class
EPA assigns no condition class to Secchi depth, so none is shown. Low clarity can be algae, which chlorophyll-a already captures, or suspended clay or natural dissolved colour, neither of which is pollution. Clarity is displayed in feet on every lake page and explained on the Secchi depth guide. In a clay-turbid lake, phosphorus can read Poor while the lake grows little algae, because phosphorus binds to clay; chlorophyll-a is the reading to trust there.
Step 4 — Say how much sampling stands behind each reading
Every lake page shows the sample counts and date range behind its readings. A trophic class is stated only when a nutrient or chlorophyll-a parameter has at least three summer samples within ten years, and a lake enters the cleanest and clearest rankings only when a parameter has eight or more samples across two or more years with the most recent within five. A lake with no nutrient or algae reading on record shows what it does have and says so. That is EPA Integrated Reporting Category 3, insufficient data, a statement about the record and never a finding about the lake.
Step 5 — Classify trophic state separately
Independently of the EPA classes, the same chemistry can feed the Carlson Trophic State Index, a logarithmic scale that places a lake on a productivity spectrum: oligotrophic (TSI under 40, clear and nutrient-poor), mesotrophic (40–50), eutrophic (50–70, nutrient-rich and algae-prone), or hypereutrophic (over 70). TSI is reported as a measurement. The trophic class is stated as fact only when the sampling floor above is met.
What the classes cover
Applied to US lakes with EPA and state agency measurements. Ontario and Québec lakes carry provincial measurements. Other country hubs publish the class their own agency issued, attributed to that agency, and never converted onto the EPA scale. Classes are comparable only inside the same ecoregion key (or the Florida profile). A phosphorus reading that is Fair in Minnesota's Upper Midwest key is not automatically Fair in Ohio's Temperate Plains key.
Frequently Asked Questions
Does LakeQuality weight one reading more than the others?
No reading is weighted into another. Each of total phosphorus, total nitrogen, chlorophyll-a, and turbidity is shown with its own EPA National Lakes Assessment 2022 condition class for that lake’s ecoregion. Water clarity (Secchi depth) is measured and shown with no class. In a clay-turbid lake, phosphorus can read Poor while the lake grows little algae, because phosphorus binds to clay; chlorophyll-a is the reading to trust there. The methodology page lists the full cutpoint table.
Is the Trophic State Index one of the EPA classes?
No. The Carlson Trophic State Index (TSI) is a separate measurement that places a lake on a productivity spectrum: oligotrophic, mesotrophic, eutrophic, or hypereutrophic. TSI is shown as a number. 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 clarity-only TSI is not published as a water-quality finding.
Why do some lakes show no nutrient or algae reading?
Most lakes have never been sampled for nutrients or algae, or not recently enough to count: fewer than three summer samples within ten years, clarity as the only reading, or a location outside every EPA ecoregion so the condition classes cannot be applied. Those pages show what the record holds and say so. As of September 3, 2026, LakeQuality tracks 37,603 lakes on the site, each shown with the measurements its agency published.
Why does Florida read differently?
Subtropical Florida lakes are naturally more productive, and many are tannic (tea-colored) rather than nutrient-clouded. Read against Upper Midwest cutpoints they would look degraded for being what they naturally are, so Florida lakes are read against breakpoints derived from the Florida LAKEWATCH program rather than the default ecoregion key.
Sources: EPA Water Quality Portal (measurements); EPA National Lakes Assessment 2022 Technical Support Document (ecoregion condition classes); R.E. Carlson, “A trophic state index for lakes,” Limnology & Oceanography (1977); Florida LAKEWATCH (subtropical profile). The classes are a classification of measured data, not a safety certification.
More Lake Guides
Learn how to read Secchi depth, phosphorus, chlorophyll-a, and trophic state measurements, and what they tell you about your lake.
Practical steps every lakefront property owner can take to reduce nutrient runoff, maintain shoreline buffers, and protect water quality.
Use LakeQuality data to evaluate lake properties before buying, what the water-quality readings, depth, fish species, and trends mean for your investment.
How to read a lake’s nutrient and algae readings before a swim, why a reading is a record of past measurements rather than a same-day clearance, and the visual signs of a harmful algal bloom.
How water clarity, temperature, and algae shift across the seasons, from ice-out through fall turnover, and what that means for the clearest swim, the best bite, and the sharpest Secchi reading.
What a Secchi disk measures, what a Secchi reading shows (clarity, not nutrients), what drives clarity up or down, and how the lakes in the LakeQuality dataset compare.
How to pick a lake for kids and first-timers: forgiving panfish, public shore access, and clean, low-algae water. What to look for in the readings, which species to target, and how to plan the trip.
How the fishing changes through the year as lakes warm, stratify, and turn over — where fish hold in each season, why water quality and temperature drive the bite, and how to time a trip.