What is turbidity, and what is NTU?
Turbidity is a measure of how cloudy water is, caused by particles suspended in it: sediment, plankton, organic matter, or anything else scattering light as it passes through. It is measured optically, not by weighing or filtering the particles out.
How it's measured
A turbidity sensor shines light into the water and measures how much of it scatters back to a detector at a fixed angle (typically 90°). More suspended material scatters more light, which reads as a higher turbidity value. This optical, scatter-based method is what "nephelometric" refers to.
NTU (Nephelometric Turbidity Unit) is the standard unit this reading is reported in. It is calibrated against a known reference suspension, typically formazin, at defined concentrations, so that a reading of, say, 40 NTU means the same thing across different instruments that have each been calibrated the same way.
Why turbidity matters at a marine farm or restoration site
- Light for photosynthesis. Suspended sediment blocks light, which slows growth in seaweed and other photosynthetic species.
- Smothering and clogging. High turbidity events can settle out as sediment that smothers seabed habitat or clogs the feeding structures of filter-feeding shellfish.
- A signal of disturbance. A turbidity spike is often the first visible sign of a storm, dredging, construction, or other physical disturbance nearby.
- A baseline and compliance metric. Continuous turbidity logging is a standard input for environmental baseline surveys and BACI (Before-After-Control-Impact) monitoring designs.
Calibrating a turbidity sensor
Turbidity sensors are typically calibrated with a two-point procedure: a zero (or near-zero) reference and a known-concentration formazin standard, to correct for drift in the specific unit's optics. PEBL's GrowProbe is calibrated this way over a 0 to 40 NTU range using a supplied formazin standard, see the turbidity calibration guide for the full bench procedure.
Need help?
Get in touch at hello@pebl-cic.co.uk.