What Water Clarity Reveals
Water clarity is determined by the concentration and character of particles suspended in the water column. In most lakes, the dominant sources of reduced clarity are phytoplankton (algae), resuspended bottom sediment, and dissolved organic matter that colors the water. Each source responds to different drivers and requires different management responses, so understanding what is causing reduced clarity is as important as measuring the clarity itself.
In general, declining water clarity over time indicates increasing eutrophication, typically driven by excess phosphorus promoting algal growth. Sudden decreases in clarity after storms often reflect sediment loading from the watershed. Persistent tea-colored water suggests high concentrations of dissolved organic carbon from wetland or forest runoff, a natural condition that is not necessarily a management concern.
Secchi Depth: The Simplest Clarity Measurement
The Secchi disk, a 20-centimeter (8-inch) circular plate painted in alternating black and white quadrants, is the oldest and most widely used tool for measuring water clarity. Developed by Angelo Secchi in 1865 for the Pope's navy to measure Mediterranean Sea clarity, it remains in routine use today because it is inexpensive, requires no calibration, and produces results that are directly comparable across decades of historical records.
To take a Secchi depth reading, lower the disk on a calibrated line from the shaded side of a boat until it just disappears from view, then slowly raise it until it reappears. The Secchi depth is the average of the disappearance and reappearance depths. Readings should be taken around midday when the sun angle is high, without polarized sunglasses (which reduce surface glare and artificially increase the apparent reading), and ideally by the same observer each time to minimize perceptual variability.
Secchi depth can contribute to trophic-state interpretation when algae dominate light attenuation and methods are comparable. Fixed oligotrophic, mesotrophic, and eutrophic bins are classification conventions, not universal lake-health or treatment thresholds. Dissolved color, mineral sediment, waves, sun, observer conditions, and macrophytes can decouple transparency from nutrients and chlorophyll-a.
- Oligotrophic (low productivity): Secchi depth typically greater than 4 meters
- Mesotrophic (moderate productivity): Secchi depth 2 to 4 meters
- Eutrophic (high productivity): Secchi depth less than 2 meters
- Hypereutrophic: Secchi depth less than 1 meter, often with visible algal scums
Turbidity Sensors and Meters
Turbidity is an optical measurement of how much light is scattered by particles in the water, reported in Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU). Unlike Secchi depth, which integrates the effect of all light-attenuating substances through the water column, turbidity meters measure scattering at a specific point, making them suitable for depth-specific profiles and continuous monitoring.
Modern turbidity sensors use an infrared LED light source and a detector positioned at 90 degrees to the beam. Particles in the water scatter light into the detector, and the intensity of scattered light is proportional to particle concentration. These sensors can be incorporated into multi-parameter sondes for continuous deployment or used as handheld meters for spot checks.
For long-term deployments, the primary maintenance concern is biofouling, the growth of algae and biofilm on the sensor optics. Many continuous turbidity sensors include mechanical wipers that periodically clean the optical window. Even with wipers, sensors should be retrieved and manually cleaned on a regular schedule, typically every two to four weeks during the growing season.
Total Suspended Solids: The Laboratory Approach
Total suspended solids (TSS) is a gravimetric measurement: a known volume of water is filtered through a pre-weighed glass fiber filter, the filter is dried and reweighed, and the mass of retained solids is reported in milligrams per liter. TSS provides a direct, quantitative measure of particle concentration that is not influenced by particle color, shape, or refractive index the way optical turbidity readings can be.
TSS analysis requires proper sample collection (a well-mixed, representative grab sample), transport on ice, and processing within a defined holding time, typically seven days. While less convenient than field turbidity readings, TSS values are essential for regulatory reporting, sediment loading calculations, and calibrating turbidity sensors to actual particle concentrations in a specific waterbody.
Building a Clarity Monitoring Record
Because water clarity responds to both long-term nutrient trends and short-term weather events, a useful monitoring record requires consistent methodology over multiple years. Secchi depth measured at the same station, at roughly the same time of day, at biweekly or monthly intervals through the growing season builds a dataset that reveals genuine trends when analyzed with appropriate statistical methods.
Plotting growing-season average Secchi depth year over year is one of the simplest and most effective ways to communicate lake condition to stakeholders. It answers the most common question lake users ask: is the water getting clearer or murkier? Combining Secchi depth with total phosphorus and chlorophyll-a data provides the three-parameter basis for trophic state index calculation, a standardized scoring system used by most state water quality agencies.