Macroinvertebrates   One Big Word for Many Small Creatures

 

The word “macroinvertebrate” is derived from the latin.  The word means creatures big enough to be seen by the naked eye (macro) and without a backbone (invertebrate). Some of these animals like clams and whirligig beetles spend all of their life in the water while others like mayflies and dragonflies spend just a part of their life cycle in the water.  Most but not all macroinvertebrates are larvae of insects. These animals are found attached to rocks or buried in the substrate of a stream or river for at least part of their life cycle.  For this reason they are often called benthic (meaning on the bottom) macroinvertebrates..  Food comes to them as the stream flows by and they usually take in dissolved oxygen through gills as fish do.  If adverse changes occur in the stream they are profoundly affected as they do not move with the ease of fish and can not escape.

 

 Each species has a certain range of physical and chemical conditions in which it can survive.  Some can survive in a wide range of conditions and can tolerate pollution while others are very sensitive to changes and can not tolerate pollution.  Biologists label those not very sensitive to water quality TOLERANT.  Tolerant species include orb snails, some midgefly larva and water striders.  Those that require water of good quality with a lot of dissolved oxygen are labeled SENSITIVE.  Pollution intolerant or sensitive species include the larvae of mayflies, stoneflies and most caddisflies.  There is a middle group of animals that are somewhat sensitive and require water of good quality.  They are labeled FACULTATIVE and include crayfish, dragonfly nymphs and all water beetles.  Pollution tolerant organisms can be found in both polluted and nonpolluted streams but only a few or no sensitive organisms would be found in polluted waters. These sensitive organisms are the first to decline in numbers when streams are degraded. Biologists study benthic macroinvertebrates because they act as continuous monitors of the water in which they live whereas chemical monitoring gives important information about water quality but only at the time of measurement.    

 

Macroinvertebrate sampling takes place in the spring when stream waters are flowing abundantly. Two people will take a “kick net” to a stream site. A kick net is just 3 square feet of screening material attached to 2 poles.  The 2 poles of the net are dug into the substrate so that the net is snug with the bottom.  While one person holds the net by the poles the other person goes upstream a few feet and kicks to disturb the bottom so animals will flow downstream and into the net.  The net is then placed on the bank.  The animals are carefully removed from the net and sorted often into ice cube trays filled with stream water where they can be easily identified and counted.  The variety as well as the numbers of each macroinvertebrate species found is recorded. The animals are then carefully returned to the stream.  In order to protect macroinvertebrates people doing the census must have a valid Pennsylvania fishing license.

 

 Two streams in our watershed have been regularly sampled for macroinvertebrates by the Pike County Conservation District.  One site is Walker Lake Creek just off Lee Road and the other site is Twin Lakes Creek just below the confluence with Walker Lake Creek.  Both streams are classified as clean water streams based on the variety and numbers of macroinvertebrates found.           



Freshly emerged dragonfly and its shed nymph skin (exuvia). Taken at Walker Lake by Scott Rando, May 2011


It's not the predator but the prey that testifies to the water quality of this stream. This female ebony jewelwing holds a freshly emerged mayfly in its mandibles. Mayflies are indicative of good conditions as they are intolerant of poor water quality. (Taken at the inlet of Walker Lake during Jun 2012, TGP construction monitoring)