TURBIDITY SUSPENI)ED SEDIMENT, AND WATER CLARITY: A REVIEW¹

TLDR

Suspended sediment damages ecosystems mainly through light attenuation that reduces visual range and photosynthesis, making optical measurements—such as beam attenuation—more relevant, cost‑effective, and real‑time than mass concentration or turbidity alone. Visual clarity, measured by Secchi depth or black disc visibility, is a precise, environmentally relevant optical metric that should replace nephelometric turbidity in water‑quality assessments.

Abstract

ABSTRACT: Suspended sediment causes a range of environmental damage, including benthic smothering, irritation of fish gills, and transport of sorbed contaminants. Much of the impact, while sediment remains suspended, is related to its light attenuation, which reduces visual range in water and light availability for photosynthesis. Thus measurement of the optical attributes of suspended matter in many instances is more relevant than measurement of its mass concentration. It is more relevant than measurement of its mass concentration to optical effects of suspended matter. Unlike many molecules, light scattering is typically dispersive and its effect on biological and ecological processes is well understood. It is less expensive to measure than mass concentration, and it can be used for real‐time monitoring of environmental contamination. However, turbidity is only a relative measure of scattering (versus arbitrary standards) that has no intrinsic environmental relevance until calibrated to a ‘proper’ scientific quantity. Visual clarity (measured as Secchi or black disc visibility) is a preferred optical quantity with immediate environmental relevance to aesthetics, contact recreation, and fish habitat. Contrary to common perception, visual clarity measurement is not particularly subjective and is more precise than turbidity measurement. Black disc visibility is inter‐convertible with beam attenuation, a fundamental optical quantity that can be monitored continuously by beam transmissometry. Visual clarity or beam attenuation should supplant nephelometric turbidity in many water quality applications, including environmental standards.