Barb Liukkonen, Water Resources Center, 612-625-9256

Those three terms are often used interchangeably, which is incorrect. They don’t mean the same thing, although they are closely related. Here’s a quick guide to understanding what these parameters are telling you about water clarity.

Transparency is a measure of how well light passes through the water column. Transparency is usually measured with a Secchi disk (for lakes) or transparency tube (for streams), although it can be measured in the field with a light meter. Secchi disk readings are probably the most commonly collected water quality data across the U.S. Transparency measurements are typically made in situ (on site) and can be affected by suspended sediment, algae, and water color (i.e., humic acids that stain the water red or brown).

Turbidity is a measure of the how much light is scattered by particles in the water. It is reported in NTUs (Nephelometric Turbidity Units) and is measured with a nephelometer, which may cost several hundred dollars. Turbidity measurements can be made in situ with a meter or back in the lab. Algae blooms or suspended sediment can increase turbidity because light is scattered by particles in the water, whether those particles are sediment or algae.

Total Suspended Solids (TSS) is a direct measurement of the particles suspended in the water - by weight. That means you must collect a sample and take it back to the lab where the water is filtered and dried in an oven, before being weighed. Sediment weighs more than algae, so TSS is a more accurate measurement of how much sediment is in the water, whereas turbidity is affected equally by sediment or algae.

If you collect samples for turbidity or TSS, be sure to shake the container thoroughly before taking a measurement, so whatever has settled out is re-suspended. Neither TSS or turbidity measurements are affected by colored water.

TSS and turbidity are inversely correlated with transparency. That means that as turbidity or TSS increases, transparency decreases (more stuff in the water translates to reduced light penetration).

A secchi disk is used to measure water clarity.

Thanks to committed volunteers we’re building an extensive database for transparency across Minnesota. Those data can be used to establish long-term trends or to identify significant changes in water clarity. Most volunteers will continue to gather important transparency data with their trusty Secchi disk, but if you want to know more about the cause of reduced transparency, you might want to sample for turbidity or TSS.

Posted in Lakes |