Understanding Lake Water Quality: What the Numbers Mean

Every LakeQuality report card distills complex water chemistry into a single A-through-F grade. But what do the underlying numbers actually mean? This guide walks through each metric, Secchi depth, phosphorus, chlorophyll-a, and trophic state index, and explains how to interpret them for swimming, fishing, property values, and overall lake health across the 3,770 lakes we track in Minnesota and Wisconsin.

The Three Core Metrics

LakeQuality evaluates every lake using three independently measured water quality parameters. Each one tells a different part of the story, together, they provide a comprehensive picture of how healthy your lake is.

Secchi Depth: How Clear Is the Water?

Secchi depth measures water transparency, literally, how deep you can see. A trained observer lowers a black-and-white disk into the water on a measured line and records the depth at which it disappears. Across the 3,565 lakes with Secchi data in our dataset, the average reading is 15.9 feet, but values range from under 1 foot in the murkiest prairie lakes to over 25 feet in the Boundary Waters and northern Wisconsin.

Secchi depth is weighted most heavily in the LakeQuality formula (35%) because it directly reflects the swimming and boating experience. A Secchi depth of 15 feet or more earns an A, you can see the sandy bottom in most parts of the lake. Below 3 feet earns an F, the water is opaque green or brown, underwater hazards are invisible, and algae blooms are likely. Property value studies consistently show that every additional meter of Secchi depth correlates with a 2% to 6% increase in lakeshore property values.

Total Phosphorus: The Algae Fuel

Phosphorus is the nutrient that controls algae growth in nearly every freshwater lake. Measured in micrograms per liter (ug/L), total phosphorus tells you how much fuel is available for algae blooms. Across our dataset, the average total phosphorus is 43 ug/L. Lakes below 15 ug/L (grade A) are typically clear with minimal algae, while lakes above 60 ug/L (grade F) almost always have persistent green water and frequent surface scums.

Phosphorus enters lakes from three main sources: agricultural runoff carrying fertilizer and manure, stormwater from developed areas carrying lawn fertilizer and pet waste, and failing lakeside septic systems. Once phosphorus enters a lake, it can be recycled from bottom sediments for decades, a process called internal loading that makes eutrophication notoriously difficult to reverse. LakeQuality weights phosphorus at 30% of the overall grade.

Chlorophyll-a: How Much Algae Is Growing?

While phosphorus measures the potential for algae growth, chlorophyll-a measures the actual algae present in the water at the time of sampling. This green pigment is found in all photosynthetic organisms, and its concentration in a water sample directly correlates with the density of the algae population. Values below 5 ug/L (grade A) mean the water is essentially algae-free. Between 10 and 20 ug/L, the water has a noticeable green tint. Above 30 ug/L (grade F), algae dominates the water and may include harmful cyanobacteria species capable of producing liver and nerve toxins. LakeQuality weights chlorophyll-a at 20% of the overall grade.

Trophic State: The Big Picture Classification

The Carlson Trophic State Index (TSI) combines all three metrics into a single number from 0 to 100 that classifies lakes into four categories. This classification has been the standard framework for lake assessment in North America since Robert Carlson published it in 1977, and it accounts for 15% of the LakeQuality formula.

Reading the Grade: What Each Letter Means

Of the 3,770 lakes currently graded by LakeQuality, 1,697 earn an A, 776 earn a B, 581 earn a C, 406 earn a D, and 310 earn an F. The grade reflects the weighted combination of all available metrics using median summer-season values from the most recent five years of monitoring.

An A-graded lake is not guaranteed to be perfect on any given visit, weather, recent rainfall, and seasonal conditions all affect day-to-day water quality. But an A grade means that the lake is consistently in excellent condition based on years of monitoring data. Conversely, an F grade does not mean the lake is permanently ruined - it means current conditions are poor and the lake would benefit from active watershed management and nutrient reduction.

Trends: Is Your Lake Getting Better or Worse?

A single year of data is a snapshot, but multi-year trends reveal whether a lake is improving, declining, or stable. LakeQuality analyzes trend data when at least five years of monitoring records are available. An improving trend means Secchi depth is increasing (water getting clearer) and/or phosphorus and chlorophyll-a are decreasing. A declining trend indicates the opposite. Stable lakes are maintaining their current condition.

Trends are among the most valuable pieces of information for lakefront property owners and community leaders. A B-graded lake with an improving trend is on a positive trajectory and may reach A status with continued management. A B-graded lake with a declining trend is losing ground and may deteriorate to C or worse without intervention. When evaluating a lake, always consider both the current grade and the long-term trend.

Frequently Asked Questions