Activity 4: 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.
- 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?
- 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.
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)