Usually, we measure turbidity to provide a cheap estimate of the total suspended solids or sediments (TSS) concentration (in milligrams dry weight/L). TSS measurement requires you to filter a known volume of water through a pre-weighed filter disc to collect all the suspended material (greater than about 1 micron in size) and then re-weigh it after drying it overnight at ~103ฐC to remove all water in the residue and filter. This is tedious and difficult to do accurately for low turbidity water - the reason why a turbidimeter is often used. Another even cheaper method is to use an inexpensive devise called a Turbidity Tube. This is a simple adaptation for streams of the Sec chi disk technique for lakes. It involves looking down a tube at a black and white disk and recording how much stream water is needed to make the disk disappear.
Turbidity is a standard measurement in stream sampling programs where suspended sediment is an extremely important parameter to monitor. It may also be useful for estimating TSS in lakes, particularly reservoirs, since their useful lifetime depends upon how fast the main basin behind the dam fills with inflowing sediments from mainstem and tributary streams and from shoreline erosion. In the WOW lakes, direct inputs of sediments from tributaries are probably too low to significantly affect the turbidity of the water column out in the main lake. However, algal densities, particularly in the more eutrophic lakes in the Minneapolis Metro area represent enough particulate material to be easily measureable by the RUSS turbidity sensors. Although chlorophyll sensors (fluorometers) would be the best way for us to estimate algal abundance (we lack the funding at present), in these lakes the turbidity sensors provide an alternate estimate of algae.
The figures below were developed to show how both organic (algae) and inorganic (silt and sediment) particulates affect turbidity values. The first set of images show filter discs prepared by filtering identical volumes of water from Lake Independence, with their corresponding values of turbidity and chlorophyll. The second set shows another set of filters generated using a nearshore water sample from an erodible area.
Turbidity is a standard measurement in stream sampling programs where suspended sediment is an extremely important parameter to monitor. It may also be useful for estimating TSS in lakes, particularly reservoirs, since their useful lifetime depends upon how fast the main basin behind the dam fills with inflowing sediments from mainstem and tributary streams and from shoreline erosion.