Lake Water Quality Glossary
Plain-language definitions for 33 lake water quality terms used in LakeQuality report cards. From Secchi depth to trophic state index, understand what the numbers mean for your lake.
Water Quality
Phosphorus
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
Dissolved Oxygen
The amount of oxygen gas dissolved in lake water, measured in milligrams per liter — essential for fish survival and a key indicator of lake health.
Water Clarity
The transparency of lake water, typically measured by Secchi depth — a primary indicator of water quality and recreational suitability.
Lake Grade
An A-through-F letter grade assigned by LakeQuality to summarize overall lake water quality based on clarity, phosphorus, and chlorophyll-a measurements.
Internal Phosphorus Loading
The release of phosphorus stored in lake-bottom sediments back into the water column, often triggered by low oxygen conditions in deep water.
Measurement
Secchi Depth
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
Chlorophyll-a
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
Trophic State Index
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
Total Suspended Solids
The total amount of particulate material suspended in lake water, including sediment, algae, and organic matter, measured in milligrams per liter.
Ecology
Oligotrophic
A lake classification indicating low nutrient levels, high water clarity, and minimal algae growth — the cleanest category of lake.
Mesotrophic
A lake classification indicating moderate nutrient levels, reasonable clarity, and balanced algae growth — the middle category between oligotrophic and eutrophic.
Eutrophic
A lake classification indicating high nutrient levels, reduced clarity, and frequent algae blooms — a nutrient-rich and highly productive lake.
Hypereutrophic
A lake classification indicating excessive nutrient levels, very poor clarity, and persistent algae blooms — the most impaired category.
Algal Bloom
A rapid increase in algae population in a lake, often visible as green discoloration or surface scums, triggered by warm temperatures and excess nutrients.
Cyanobacteria
Photosynthetic bacteria (commonly called blue-green algae) that can form harmful blooms and produce toxins dangerous to humans and animals.
Blue-Green Algae
The common name for cyanobacteria — photosynthetic bacteria that form potentially toxic blooms in nutrient-rich lakes during warm weather.
Lake Turnover
The seasonal mixing event when surface and deep water layers in a stratified lake exchange positions, driven by temperature equalization.
Thermocline
The transition layer in a stratified lake where water temperature changes rapidly with depth, separating warm surface water from cold deep water.
Invasive Species
Non-native organisms that spread aggressively in lake ecosystems, outcompeting native species and often degrading water quality and habitat.
Zebra Mussels
Small, striped freshwater mussels native to Eastern Europe that attach to hard surfaces in lakes, filter large volumes of water, and alter lake ecosystems.
Eurasian Watermilfoil
An aggressive invasive aquatic plant that forms dense surface mats, displaces native vegetation, and impairs swimming, boating, and fishing.
Lake Stratification
The separation of a lake into distinct temperature layers during summer, with warm water on top and cold water on the bottom.
Aquatic Macrophytes
Visible aquatic plants growing in lakes, including emergent, floating-leaf, and submersed species that provide critical habitat and help maintain water quality.
Pollutants
Nonpoint Source Pollution
Water pollution from diffuse sources across the landscape — including agricultural fields, lawns, streets, and construction sites — rather than from a single identifiable discharge point.
Point Source Pollution
Water pollution from a single identifiable source, such as a factory discharge pipe or wastewater treatment plant outfall.
Stormwater Runoff
Rainwater and snowmelt that flows across impervious surfaces like roads, driveways, and rooftops, carrying pollutants into lakes and streams.
Septic System Impact
The contribution of failing or inadequate septic systems to lake pollution, particularly through phosphorus and bacterial contamination.
Agricultural Runoff
Water flowing off cropland, feedlots, and pastures that carries fertilizer, manure, pesticides, and eroded soil into lakes and streams.
Management
Watershed
The entire land area that drains water, sediment, and nutrients into a particular lake — the primary factor determining lake water quality.
Shoreline Buffer
A strip of natural vegetation maintained along a lake shoreline to filter runoff, stabilize soil, and provide wildlife habitat.
Lake Management Plan
A comprehensive strategy developed by lake associations, local governments, and agencies to protect or restore water quality in a specific lake.
Water Quality Monitoring
The systematic collection and analysis of water samples and measurements to track lake health over time and detect changes in water quality.
Recreation
Understanding Your Lake Report Card
LakeQuality assigns A-through-F grades based on water clarity (Secchi depth, 35%), phosphorus levels (30%), chlorophyll-a concentration (20%), and trophic state index (15%). Look up any term above to learn how it affects your lake's grade.