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Turbidity and total suspended solids (TSS) are a measure of the suspended material in the water column, which can interfere with respiration and light penetration.

Frequency of testing: Turbidity may be tested routinely where indicated (e.g. to assess effectiveness of mechanical filtration) or in response to an upcoming treatment (e.g. copper sulfate immersion).

Sampling: Testing is usually done on site in the affected system.

Testing: In ponds and other slow‐moving surface water, a black and white Secchi disk can be lowered into the water to record the depth at which the pattern cannot be seen (typically based on an average of two readings). Nephelometric tests measure the scatter from a focused light beam. Handheld probes are available.

Units: Secchi depth is reported in centimeters or inches; a low value is equivalent to high turbidity. Nephelometric tests are reported in nephelometric turbidity units (NTU), Formazin turbidity units, or a variety of other units depending on the meter.

Target values: Target for ponds may be Secchi depth of >50 cm (18 in) or turbidity < 1‐10 NTU.

Practical considerations:

 High turbidity can be an issue for fish because it can affect gill function, dissolved oxygen, feeding, fecundity, light penetration, and aesthetics. The particulates can also cause clogging of fine mechanical filters.

 High turbidity may be due to phytoplankton or bacterial blooms, insufficient mechanical filtration, disturbance of the substrate, or contamination (e.g. runoff).

 Low turbidity may be a concern for species that have evolved in brown or black pond or river systems.