Potomac Highlands Watershed School

Why Benthic Macroinvertebrates?


The Potomac Highlands Watershed School has a suite of benthic macroinvertebrate lessons that are accessed via the BMI Portal, and include: What is a Benthic Macroinvertebrate, Cast of Characters, Sedimentation Blues, Closer Look, Introduction to Stream Sampling, a high school and middle school version of the Animated Dichotomous Key and, in final development, a virtual stream sampling activity.


     Benthic macroinvertebrates are bottom living animals without backbones you can see without magnification.  Students, watershed groups, and professional biologists at universities and government agencies often use these animals to help them gage the health of our streams. 

     The idea that aquatic organisms can be used as indicators of water pollution has a long history. An indicator species is one that, by its presence, absence, or abundance relative to other organisms, indicates environmental conditions. For example, the presence of numerous midge (chironomid) larvae in a stream may indicate severe organic pollution.

     The advantage of using aquatic organisms over chemical indicators of water quality - such as the amount of a certain chemical in a water sample - is that animals are constantly "sampling" their environment and the communities found in benthic samples are indicative of water quality conditions over time. Chemical measures, in contrast, provide a momentary snapshot of conditions in a constantly changing environment.

     The main difficulty in using aquatic organisms, however, comes in understanding what the animals are telling us. For instance, during the baseline study of the Cacapon River and its major tributary (the North River), Institute researchers noted that they found no freshwater mussels at any Lost River sampling site, but often found them in other river sections. The absence of mussels alone, however, doesn't tell us very much. We don't know, for instance, if they were once present and then disappeared, or why they might have disappeared.

     Over the past four decades, researchers generally moved away from the use of individual indicator species and toward "indices" that look at groups of species. A typical index, for instance, might look at the total number of different species or the relative abundance of different species. For instance, if a researcher finds that species tolerant of degraded water quality outnumber kinds that are intolerant of pollution, it is more likely that degraded conditions exist  But the mere presence of pollution-tolerant organisms does not necessarily equate to water quality problems, because these organisms are often widely distributed.

     Water quality scientists once mostly used benthic macroinvertebrate studies to assess point sources of pollution, such as sewage flowing from a pipe. By sampling up- and downstream of the pipe, researchers could reach conclusions about the impact of the pollution.

     The use of benthic macroinvertebrate methods for assessing nonpoint sources of pollution, such as silt or fecal bacteria washing off the landscape, is more recent.  State environmental agencies throughout America now use benthic macroinvertebrates as a primary tool for assessing the health of the waterways.

     In recent years, the science of using animals to assess the vitality of a river ecosystem has gone public.  Volunteer monitoring programs, pioneered by the Izaak Walton League’s Save Our Streams (SOS) Program, have sprouted up around the country.  The SOS and similar volunteer methods use stream benthic macroinvertebrate diversity to rate stream quality and are similar in general design to the methods used by professionals, but tailored to the capabilities of non-professionals.  Such educational programs make the link between causes and effects of pollution using hands-on, in-stream activities, and make the societal benefits of clean water immediate and real.  One of the most active and inventive Citizen's Stream Monitoring Coordinators in the nation is West Virginia's own Tim Craddock, who runs WV Save Our Streams and has been an invaluable source of information and review of the Potomac Highlands Watershed School's BMI module:

The West Virginia Save Our Streams (WVSOS) program is a volunteer monitoring program that trains West Virginia citizens of all ages, how to monitor, and become watchdogs over their local wadeable streams and rivers. WVSOS uses a streamside biological approach to monitoring, which involves collecting a series of benthic macroinvertebrate samples, placing the organisms in trays of water, sorting them into look-alike groups, and calculating a stream condition index, which, assigns a rating to the stream site. This information, along with the habitat assessment survey, provides the volunteer enough information to make a general assessment of the stream site. By monitoring additional sites along the stream, the volunteers can make an assessment of the stream's overall health.

Cacapon Institute has studied benthic macroinvertebrates:

CI staff also teaches student groups how to study BMIs at schools, camps, and during its annual Stream Scholars Summer Camp. 


The Potomac Highlands Watershed School 

Why Benthic Macroinvertebrates

The Why Benthic Macroinvertebrates page is just one component of the Potomac Highlands Watershed School's watershed science and society curriculum, and is best used when accessed from within an eSchool classroom.  If you reached this page via a direct web-link, you might consider going to the Potomac Highlands Watershed School and visiting an eSchool classroom to use this and other activities, review the literature in the bookshelf or, if you are a teacher, visit the "Teacher's" room and check out the lesson plans.  This page is accessed by clicking on the center of the Benthic Macroinvertebrate Portal.


Cacapon Institute, PO Box 68, High View, WV 26808 

304-856-1385  E-Mail CI