What Is Turbidity?

Turbidity measures how strongly suspended material scatters light. It is measured with a turbidity meter and reported in nephelometric turbidity units, or NTU.

Suspended clay, algae, cyanobacteria, bacterial floc, eroded soil, organic debris, and disturbed bottom sediment can all increase turbidity. A turbidity reading tells us how much suspended material is interfering with light, but it does not tell us exactly what that material is. Two lakes can have the same visible clarity while having completely different water-quality and fishery conditions.

What Is Water Clarity?

Water clarity describes how deeply someone can see into the water. It is commonly measured with a Secchi disk.

Clarity is influenced by suspended particles, but it is also affected by dissolved color, tannins, algal pigments, water depth, bottom color, glare, cloud cover, wind, and sun angle. This is why Secchi depth does not always improve at the same rate as turbidity.

A useful comparison is the difference between looking through muddy water and looking through tinted water. You may only be able to see 13 inches through either one, but the amount and type of material in the water can be completely different.

Useful Fishery-Management Benchmarks

Table 1. Turbidity and Secchi-clarity benchmarks used to interpret general fishery conditions. These values are practical guidelines rather than universal pass-or-fail standards. Appropriate conditions vary with fish species, lake depth, water color, habitat, watershed characteristics, and management goals

Why High Turbidity Matters to Fish

At these levels, both limited clarity and elevated turbidity can reduce the ability of sight-feeding fish to locate prey, lowering feeding efficiency, growth, and recruitment. However, turbidity at or above approximately 25 NTU can create additional direct stress that poor clarity alone does not. Fine suspended particles can irritate and clog gill surfaces, increase respiratory effort, and trigger physiological stress responses that alter feeding, movement, and overall fish behavior.

High turbidity limits sunlight penetration and can suppress beneficial aquatic plants and benthic algae. That may reduce spawning habitat, refuge, insect production, and food availability.

Suspended particles can also absorb solar energy near the surface. This can increase the potential for near-surface heating, especially in shallow or weakly mixed water. Warmer water holds less dissolved oxygen, while fish metabolism and bacterial decomposition generally increase as temperature rises. Together, those processes can reduce the margin between the oxygen available and what the fish community requires.

Warm water can also increase ammonia risk. As temperature and pH rise, a greater percentage of total ammonia may occur as toxic unionized ammonia. Even sublethal ammonia exposure can impair gill function, reduce appetite, and cause fish to stop feeding.

Case Study: The Fish Started Feeding Again

Before treatment, one pond looked extremely poor and the fish had largely stopped feeding. The water had an unhealthy brown-green color, turbidity measured 77.5 NTU, pH was elevated at 9.35, total phosphorus was 0.047 mg/L, and total nitrogen was 3.2 mg/L. Microscopy found very high levels of Raphidiopsis cyanobacteria, bacterial floc, and other suspended plankton.

A MetaFloc Pro treatment significantly improved the pond’s water quality. Turbidity declined from 77.5 to 12.3 NTU, an 84.2% reduction. Total phosphorus decreased from 0.047 to 0.018 mg/L, total nitrogen declined from 3.2 to 2.4 mg/L, and pH returned to normal 7.81. Ammonia was not detected after treatment. The water visibly looked healthier, and the fish began feeding again.

Figure 2. Before-and-after appearance of the case-study pond following MetaFloc Pro treatment. The water visibly improved in color and overall condition as turbidity declined from 77.5 to 12.3 NTU; however, Secchi clarity remained near 13–14 inches, illustrating that a major reduction in suspended material does not always produce an equally large increase in measured clarity.

Despite those improvements, Secchi clarity remained near 13–14 inches. Someone judging the pond only from the shoreline or by Secchi depth might conclude that very little had changed. From a fishery perspective, however, the before-and-after conditions were dramatically different.

A Secchi depth of approximately 13 inches at 77.5 NTU represents water carrying an extreme suspended load. Fish must live and feed within that material, which can reduce their ability to locate prey, irritate their gills, increase respiratory effort, reduce light penetration, and contribute to unstable oxygen and water-quality conditions.

At 12.3 NTU, the same 13–14 inches of clarity still may not look impressive, but the fish are exposed to far less suspended material. The remaining visibility limitation was associated with persistent background clay, dissolved color, ultra-fine particles, glare, and recurring sediment resuspension.

The exact temperature difference between water at 77.5 NTU and 12.3 NTU cannot be calculated from turbidity alone. It depends on particle type and color, sunlight, depth, wind, mixing, shade, and residence time. However, the much heavier suspended load at 77.5 NTU created a greater potential for near-surface heat absorption and associated thermal stress.

The pond may have retained nearly the same measured clarity, but it was functioning much better biologically. The return of fish feeding, removal of detectable ammonia, lower phosphorus, normalized pH, reduced cyanobacterial

biomass, and dramatic reduction in turbidity were stronger measures of treatment success than Secchi depth alone.

Looking Beyond Appearance

A successful lake treatment should not be judged by Secchi depth alone. Turbidity, dissolved oxygen, temperature, ammonia, nutrients, algae, and fish behavior all help determine whether conditions actually improved.

When turbidity and clarity do not respond at the same rate, additional testing can help identify the cause. Settling experiments can show whether particles fall out naturally or remain suspended. Total and volatile suspended solids can help separate mineral sediment from organic material. Chlorophyll-a and microscopy can determine whether algae are driving the problem. Temperature and dissolved-oxygen profiles, ammonia testing, sediment sampling, and watershed inspections can reveal additional stressors.

The goal should not simply be to make a lake look clearer. The goal is to determine whether poor visibility is caused by algae, clay, dissolved color, organic material, sediment resuspension, or continued watershed loading—and then manage the actual source.