Using a Benthic Dredge to Identify Insects (macroinvertebrates) in the Sediments

Maine Lakes Society and Lakes Alive!!

Using a Benthic Dredge to Identify Insects (macroinvertebrates)  in the Sediments

Few of us are familiar with what exists on the bottom of a lake or what kind of aquatic life inhabits the depths.  Most of us are familiar with the shallow, near-shore parts of a lake, where sunlight penetrates to the bottom and familiar plants and animals are common.  We don’t know the deep-water lake bottom as well.  These areas are highly variable and different from the shallows.

In this activity, we use a benthic dredge to scoop up a sample of sediment from the lake bottom and haul the sample to the surface for a closer examination. Our benthic dredge is like a metal clam shell; it has two jaws that are set in the open position until a metal weight (called a messenger) trips the instrument, causing the jaws to snap shut, trapping sediment in the process. We dump the whole sample i into a bucket, dilute it with water, and pour all this liquid through a strainer. Macroinvertebrates (large insects without spinal columns) remain in the strainer.  Then we identify the different types of critters using a key.


Essential Questions

  • What (if any) macroinvertebrates did you find in the sediment sample?
  • How does this deep-water sediment sample compare to the bottom of the lake in shallow areas where you can stand?
  • Where do many of the insects we see above and around the lake come from?

Learning Objectives

  • Floating plankton are not the only food sources in a lake; the lake bottom is the nursery for its insect life.
  • The healthier a lake is, the more aquatic insect life it supports. These aquatic insects have an important place in the food web of a lake.


Additional Info
Wonderful, Wacky Water Critters (pdf) guide to river and pond bottom dwellers
Macro invertebrate Life in the River (pdf)
Key to Life in the Pond (pdf)


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