The bounty of the benthos

Leaving an airport, you see signs that say Ground Transportation. After flying at 36,000 feet and a few hundred miles an hour, that travel mode seems quite unglamorous.

So it is with life on at the bottom of a lake, which the limnologists (freshwater biologists) call the benthos. Up above in the water column the fish are like the aircraft and birds of our dry-land world. Creatures less appreciated live on (an in) the “ground” below.

It’s appropriate at this season to think about the benthos, because that’s where a lot of lake life is heading as the water gets cold and winter comes on. The term “benthos” comes from a Greek word, “bathys,” which means “deep.” It’s a zone much richer in life than most of us appreciate.

Of course, crayfish live on the bottom, as do clams, mussels and snails. Aquatic insects like mayflies and damselflies also live on the bottom, or buried in sediment, at stages of their metamorphosis from egg, to nymph, to winged adult.

These creatures are important links in the lake food chain. They eat algae or sunken plant matter and in turn provide food for fish (as anyone who has ever caught bluegills with nymphs or perch with wigglers can attest). An assortment of worms can also be found in upper layers of bottom sand and muck.

Leopard frogs and bullfrogs become benthos dwellers in winter. They do not (as many believe) dig into the bottom – the sediment contains too little oxygen to get them through until spring. Instead, they lie on the bottom, or only partly bury themselves. Some may even swim around slowly from time to time.

Painted and snapping turtles, on the other hand, do burrow into soft lake bottom mud and hibernate. In that state, they need very little oxygen and can absorb it through exposed mucous membranes in the mouth and throat.

An important function of the small benthic creatures (the worms and inserts) is that they allow scientists to assess water quality in a lake (or stream). A researcher can take “grab samples” of the bottom sediment, sort out and identify the organisms it contains, and get a good idea how healthy the lake is.

One measure they use is species diversity. In general, the more different creatures they find, the better the water quality. Another criterion is pollution tolerance. If a bottom sample is rich in immature forms of mayflies and stoneflies, which are sensitive to pollutants, that indicates good water quality. But if only midges and worms are present, that signals polluted water.

So while we get ready to “hibernate” for the winter, it’s good to think about the importance of all those creatures spending the cold season on and under the benthic blanket.