A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 2 - Lakes

Total Suspended Solids and Turbidity in Lakes

Why Is It Important?

Total suspended solids (TSS) concentrations and turbidity both indicate the amount of solids suspended in the water, whether mineral (e.g., soil particles) or organic (e.g., algae). However, the TSS test measures an actual weight of material per volume of water, while turbidity measures the amount of light scattered from a water sample (more suspended particles cause greater scattering). This difference becomes important when trying to calculate total quantities of material within or entering a lake. Such calculations are possible with TSS values, but not with turbidity readings.

High concentrations of particulate matter affect light penetration and productivity, recreational values, and habitat quality, and cause lakes to fill in faster. Particles also provide attachment places for other pollutants, notably metals and bacteria.

Reasons for Natural Variation

TSS and turbidity values vary for two main reasons – one physical, the other biological. Heavy rains and fast-moving water are erosive. They can pick up and carry enough dirt and debris to make even an unpolluted inflowing stream look muddy. So, heavy rainfall may cause higher TSS concentrations or turbidity, especially where the stream flows into the lake. In lakes, the most important reason for variation in these parameters is caused by seasonal changes in algae growth. Warm temperatures, prolonged daylight, and release of nutrients from decomposition may cause algae blooms that increase turbidity or TSS concentrations.

Expected Impact of Pollution

Pollution or general human activities usually result in higher TSS concentrations or turbidity. For example, loss of vegetation due to development exposes more soil to erosion, allows more runoff to form, and simultaneously reduces the watershed’s ability to filter the nutrients and organic matter from runoff before it reaches the inflowing streams. Although much of the particulate matter may settle to the lake bottom, the addition of nutrients will eventually cause increased algae growth.

TSS concentrations are reported in units of milligrams of suspended solids per liter of water – mg/L. Turbidity is reported as nephelometric (NTU) or Jackson turbidity units (JTU), depending on the instrument used to perform the measurement. The Washington State water quality standard is based on turbidity as measured by a nephelometer. The standard states, "turbidity shall not exceed 5 NTU over background conditions." Turbidity measurements for three Western Washington lakes are shown here for comparison purposes. TSS measurements are not available.

The next section discusses chlorophyll a levels in lakes.

Chapter Four provides information about how to measure TSS and turbidity in lakes.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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Last updated on April 01, 2008