Text by Shay Pratt; photos by Colleen O’Hara
How can you tell if a stream is healthy? One of the best ways is to look at the tiny organisms that live in it. The flies, larvae, and worms found there can tell the story of a stream’s overall health, if you know how to read them.
Many types of spineless underwater organisms, known as benthic macroinvertebrates, live in the silt and pebbles of a Virginia stream bottom. Arlington County has an illustrated guide to about 20 of them found locally (adapted from the Izaak Walton League’s Virginia Save Our Streams site.)
Some are hardier than others when it comes to tolerating pollution, heavy stormwater, heat and other pressures. For example, flatworms and black flies are tougher than mayflies and caddisflies. So, you could expect to find flatworms just about anywhere you look, even in streams that aren’t very healthy, because they can tolerate tough conditions. In contrast, when you find a casemaker caddisfly, you know the stream quality must be pretty high.
By tracking the stream bug populations across time, Arlington County staff can assess the effectiveness of pollution prevention practices and identify new problems. That’s why the County’s Department of Environmental Services coordinates regular stream monitoring at 10 stream sites, including Donaldson Run, Gulf Branch, and multiple sections of Four Mile Run, with the assistance of ARMN volunteers who are trained to methodically collect and analyze macroinvertebrates.
Macroinvertebrates make for ideal test subjects because they don’t move much and they’re readily found under rocks and stones. They can be scooped up with nets, identified with magnification devices like loupes, and returned to the water.
I learned all this one day in May, when I met other ARMN members at a picnic pavilion at Lubber Run park to collect and analyze samples. We split into teams of two to collect samples at designated spots along the stream, targeting shallow, rocky areas with fast-moving, oxygen-rich water—great habitat for macroinvertebrates.
My partner, Hutch Brown, a long-time volunteer who helps lead this monitoring group, dipped a long-handled net into the stream. I stood just upstream, picked up a few stones and pebbles from below the water, and gently rubbed them on all sides, loosening debris and any bugs that were living there. They floated down the water and into Hutch’s waiting net. I then scraped the silty floor with a tool shaped like a garden fork to release any macroinvertebrates living there. We carefully rinsed the contents of the net into a white plastic tub, then set off to repeat the process until our group sampled 10 total locations.
When all ten samples were collected, it was time to identify our findings. We carried our tub over to a picnic table strewn with sampling tools, ice cube trays, petri dishes, and microscopes. Using a shallow, white tray, I collected a sample of water from the collection tub and scanned its contents.
Within seconds, I saw movement. A faint yellow, hair-like organism only a few millimeters long twitched and spasmed. Elsewhere, a dark shape contracted and elongated on top of a water-logged maple leaf. I scooped up each critter with a pipette and plastic spoon, and transferred it to its own well in an ice cube tray. Using a loupe and a macroinvertebrate guide, I worked with the trained identifiers in our group to identify each one. Meanwhile, Stephanie Martin, another team leader, documented everything for the County.
Macroinvertebrates are easy to identify with basic magnification and practice. Most stream bugs have telltale physical characteristics. Mayflies exhibit 2-3 hair-like tails. Flatworms have triangular heads and a pronounced, cross-eyed look. Black flies look like mini-bowling pins, with a sucker on one end that they use to attach to surfaces. Our group made quick work identifying over 100 organisms.
In our survey of Lubber Run, we found mostly organisms that can tolerate poor to fair conditions—flatworms, black flies, a few aquatic worms, and lots and lots of midges. We also found some small minnow mayflies, which are much less tolerant of pollution, and indicate good riffle habitat found in the monitoring reach. These results confirmed that Lubber Run, like many of Arlington’s streams, is overall in fair condition.
Of all of Arlington’s stream monitoring sites, Lubber Run’s watershed has the most hard surfaces, with about half of its land covered with roads, roofs, parking lots, and other impervious surfaces. When a stream’s drainage area is more than 10% impervious cover, it is considered to be impacted as an urban stream. Arlington’s streams reached that development milestone decades ago, as land that once absorbed rainwater was developed. Over 40 percent of Arlington land is impervious to water, so the streams channel a lot of water after storms. That stormwater scours out the homes of many invertebrates and often carries pollution and sediment that impact habitat.
There are no easy fixes for these problems, but Arlington County is paying attention to the health of its streams—working to install Green Street rain gardens to capture rain runoff, reduce stream erosion, and educate the community about how we can all help prevent stream pollution. Data collected by volunteer stream monitors can make the case for intervention and track changes over time.
Lily Whitesell, who coordinates stream monitoring as Arlington County’s Stormwater Outreach Specialist, said the ongoing work has documented seasonal patterns (life cycles of the macro invertebrates) and patterns of disturbance and recovery. Longer term trends have been stable over time.
“We want to help our streams be as healthy as they can be, given our urban watersheds,” Whitesell said. “It’s good for our benthic macroinvertebrates, for the fish that rely on them, for the overall diversity of our local aquatic and terrestrial ecological communities, for the people that enjoy, appreciate, and rely on our streams, the Potomac River, and Chesapeake Bay.”
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