The Art of “Elutriation”
By Will Bartsch
The benthic survey is progressing nicely. The Lake Explorer II has already sampled all of the deep spots that it can safely access and was docked yesterday.
The 90-foot vessel can operate safely in water as shallow as five meters. However, much of Chequamegon bay is shallower than that. So, we’ve enlisted other boats to join the effort. The Prairie Sounder, being able to operate in one meter of water, got an early start and has been out taking Ponars (a heavy metal sampling device as described in my first post; see below for more about this) in the shallow, coastal areas of the Bay near Ashland. In anticipation of needing another small vessel that can operate in shallow areas, we towed the Research Vessel Tullibee from Duluth on Monday. The Tullibee is 26 feet long and has a cathedral hull that offers extra stability, and was also taking Ponars.
More About Ponars
A Ponar is a metal contraption that is designed to be lowered to the bottom of the lake using an overhead cable and winch. It is lowered in an open position and, because it is so heavy, will sink into soft or sandy sediment. Upon hitting bottom, the locking mechanism releases. This allows the jaws to close and collect a sample when it is retrieved. Because it needs to sink into the sediment to work properly, it is not an effective sampling method for areas with hard or rocky bottoms. Even sporadic gravel can cause problems as it doesn’t allow the jaws of the Ponar to properly shut.
Once the Ponar is out of the water, its contents are emptied into a large bin. The next step is to separate the sample into two parts. The first part is all the benthic organisms. The second part is everything else that we don’t want: mud, clay, rocks, sand, sticks and other large organic detritus.
This process is called elutriation. To elutriate, we mix the sample with water in a hinged basin that has an outlet connected to a fine mesh net with a bottle at the end. After mixing we pour the top water, along with any benthic creatures that were suspended in the process, into the bottle. This is repeated multiple times until only the things we don’t want remain in the basin. The sample that is captured in the bottle is preserved with ethanol and taken back to the lab.
Next we will investigate sites to deploy the benthic sled. We’ll let you know how it goes.
About the Author: Will Bartsch is an ORISE fellow with EPA’s Midcontinent Ecology Division, part of the Agency’s Office of Research and Development. He primarily analyzes data collected as part of the National Coastal Condition Assessment survey, and works on early detection methods for invasive species in Great Lakes coastal embayments.