Every year hundreds of streams in the United States are contaminated by pollutants that degrade their overall health. What most people do not realize is that a large portion of this stream degradation is attributed to us. After a rainfall event, paved areas and rooftops create large volumes of surface runoff that pick up contaminants on our roads and lawns (such as oil and fertilizer) before flowing into storm drains which channel the polluted runoff directly to our streams. Fairfax County has 30 major stream watersheds, all of which feed into the Potomac River and Chesapeake Bay. Both the increased runoff volume as well as the contaminants can have very adverse effects on our aquatic ecosystems, and in particular, on our populations of benthic macroinvertebrates.
What are macroinvertebrates and why are they so important to the health of our streams? Benthic macroinvertebrates are small but visible aquatic animals without a backbone that live at the bottom of streams for at least some portion of their lives. They can be found under logs, sediment, rocks and aquatic vegetation.
Some common examples of these animals are: crayfish, clams, snails, aquatic worms, and a variety of aquatic insect larvae, such as stoneflies, caddisflies, mayflies, dragonflies and damselflies. In addition to being an integral part of the aquatic food chain, they are also used to draw conclusions about the overall health of our streams.
How are macroinvertebrates used to measure stream health? Macroinvertebrates, unlike fish and other aquatic vertebrates, are less mobile and therefore less able to escape the effects of habitat degradation. Their abundance and species diversity can be used to measure stream health. They live year-round in the stream which helps in continual water quality studies. Each species responds to contaminants and stress differently. This allows for better determination of problems that are plaguing our streams. In addition, certain macroinvertebrates such as stoneflies, caddisflies and mayflies can also act as early warning indicators of changes to the stream, in particular, through contamination from point and nonpoint sources. These specific macroinvertebrates are very sensitive to stream impairment and need good water quality, cool temperatures, and a high concentration of dissolved oxygen to survive.
Often, when we think about determining water quality, we imagine chemical tests such as pH, dissolved oxygen or nitrate testing. Although these give us a numerical value as to the quality of our water, the numbers are subject to high variability within the stream. A less expensive, more efficient method involves using biological monitoring to evaluate the abundance and diversity of benthic macroinvertebrates found in the stream to assess habitat quality.
Macroinvertebrates are collected using a net positioned on the stream bed as rocks and sediment are overturned. After each catch, the macroinvertebrates are identified, sorted by species and recorded on a data sheet. At the end of the stream monitoring session, some simple calculations are made and an ecological score is determined. A typical stream monitoring site should be sampled four times a year during each season to get an accurate assessment of the changing quality of the stream.
Currently only 36% of the rivers in the US are monitored by the government. We know very little about the condition of many of the streams and rivers, even in our own backyards, and it is up to volunteers to do additional monitoring to protect the health of our streams.
Stream monitoring is important for the Chesapeake Bay watershed, home to thousands of different species of plants and animals that all depend on clean water. The macroinvertebrates serve as the first indicators in determining human impairment of our streams.
Stream monitoring is a fun, easy way to determine if your local stream has been impacted by pollution sources. To get involved and find out about upcoming events, contact Dan at Daniel.Schwartz@FairfaxCounty.gov or 703-324-1422, TTY 711.