Home / Blogs / Our Hidden Urban Neighbors: Macroinvertebrates
December 4, 2023
Sligo Creek in October (Photo credit: Ariane Gottlieb)
Sligo Creek glides through heavily urbanized Takoma Park, Maryland on its way to the Anacostia River, then into the Potomac River, and ends in the Bay. Surrounding the creek are single family homes, a footpath, public parks, and businesses. Just beneath the creek’s waters lives an entirely different ecosystem of critters that would not look out of place from the movie franchise, Alien, clinging to rocks and crawling on the submerged substrate. Despite their less than loveable features, a stream’s aquatic benthic macroinvertebrates are great indicators of stream health.
Aquatic benthic macroinvertebrates (also called macros in the field) are small aquatic insects that can be seen by the naked eye. They have no vertebrate (or spinal column), and spend a lot of their early lives on the bottom of streams. These macros can’t travel very far, and many species are sensitive to changes in their local environment. By collecting, identifying, and analyzing the macros’ diversity and abundance of different species, along with basic habitat assessments, we can determine stream health at a single point in time. Additionally, with repeated monitoring, we examine the health trends over time.
In spring of 2023, I joined Nature Forward’s volunteer aquatic benthic macroinvertebrate monitoring program as part of my professional development funding from the Alliance. I have been coordinating the Alliance’s DC Citizen Science Water Quality Monitoring project, which has collected E.coli bacteria samples in the summer, since 2022. I wanted the opportunity to dip my feet back into field work, and saw there was an opportunity to continue expanding my knowledge on water quality monitoring techniques. In April, July, and October, Nature Forward staff, volunteers, and I headed down to Site 49 in lower Sligo Creek, one of the many sites in the Maryland they monitor.
Sligo Creek is very accessible to the public, with close proximity to housing and the Sligo Creek Parkway, a paved walkway popular among walkers and bikers. At a glance, the stream is a pretty, urban oasis for community members to take a stroll with the sound of water running over the rocks and the sight of sunlight streaming through the tree leaves overhead.
This urban stream, like many others in the area, is facing significant challenges in water and habitat health. Common problems with rising urbanization around our waterways include increased impervious surfaces, leading to increased stormwater runoff, which carries a variety of human pollutants like pet waste, motor oil, heavy metals from construction, litter, and fertilizers into the streams.
Along with deforestation and historical channelization, the process of artificially straightening a stream, the creek’s bank shows heavy signs of erosion. The trees on the west side of the bank struggle to hold onto the crumbling bank, as their roots are heavily exposed to the elements. The east side of the stream fairs a little better, with boulders strategically placed into the side of the bank to hold the soil in place. In addition, the lingering hot summer months and the long, regional drought drank down the creek’s water level. This left a large portion of the streambed visibly dried out, limiting aquatic habitat to what was left of the narrow and shallow creek.
Sligo Creek’s bottom is comprised of sand, pebbles, and boulders of different sizes that promised good homes for the macros. The best spots are where the water moves fast and shallow over rocks in what is called a riffle. Riffles are important because all the splashing adds oxygen to the water. A net is placed inside the riffles and then hands are plunged into the chilly water to grab rocks or leaves to scrub off the critters into the nets and buckets. Volunteer monitors wade through the shallow creek with these nets and buckets to collect samples of the stream’s inhabitants.
A petri dish with macros inside water droplets in order to identify them. (Photo credit: Maya Sterett)
Identifying which type of macros clinging to the rocks and leaves under the stream is as important as how many there are. Many of these macros are too small to identify by the naked eye, so we isolated around 100 of them to identify under the field microscope. The order and family, think phylogenetic tree, of the macros identified in the petri dish will indicate stream health. Some are sensitive to pollution and changes in stream health, while others are much more tolerant. The higher the diversity of macros and the greater number of pollution intolerant orders and families, the better the stream health.
Left, a Uenoid Case-maker Caddisfly in its case and right, and caddisfly outside of its case. (Photo credit: macroinvertebrates.org)
As I visited Sligo Creek, I came to know the regulars caught in the net. In the summer, the stream had higher numbers of pollution-intolerant macros, like caddisfly larvae and mayfly nymphs. The first macros I learned to recognize were caddisfly larvae. The adults are rather unimpressive, but the larval stage can produce stunning visual adaptations depending on the family. Caddisfly larvae play an important engineering role, helping stabilize the streambed through their ability to create web-like silk which they connect to rocks and sticks in order to build their shelter in the forms of cases. They can also use this webbing to hunt. Sprinkled throughout the net, there would be a flash of bright green from the common netspinner caddisfly larvae clinging to the leaves. The case-making caddisflies are the most exciting, as these cases can vary in size and material depending on the family. Some build with shredded leaves, small pebbles, or bits of wood. Occasionally, we excitedly found Uenoid case-maker caddisfly larvae hiding in their small pebbled cases, that resembled bejeweled sleeping bags.
A hellgrammite captured during monitoring (Photo credit: Maya Sterett).
Hellgrammites are the exciting, heavyweights of Sligo Creek monitoring. These are the larval stages of the dobsonfly, and they are more sensitive to pollution. Despite the threat of their large mandibles, they are gentle when picked up from the leaf packs.
Difficult to identify without a microscope, mayfly nymphs are excellent indicators of improving water quality. Mayflies spend a majority of their lives in the water, then emerge from the water into the skies in swarms to mate. After this dramatic display, mayflies, depending on their species, will fly around for a few days to a few weeks, until dying.
The sampling tray holding all the benthic macroinvertebrates captured, waiting to be identified. (Photo credit: Maya Sterett).
As the leaves fell and daylight grew shorter, the number of more pollutant-tolerant macros increased and the caddisfly larvae and mayfly nymphs dwindled from our nets. In fall, a majority of the macros plucked out of the streams were red worms and midge fly larvae.
One of the funniest and cutest macros, most numerously found in the fall, are the soft bodied planaria. Planaria are arrow shaped flat worms with eye spots on the top of their heads that make them look cross-eyed. This macro is extraordinary, with the ability to regenerate almost any part of their body, including their head, and classified as tolerant of pollution.
Sligo Creek is one example of many creeks hiding a whole ecosystem of fascinating creatures tucked within the stream bed! Aquatic benthic macroinvertebrates in our waterways are our neighbors, and most of them live out their entire lives near the patch of riffles they were born in. They are the locals, and by observing them, they can tell us the backyard gossip about how healthy our water is for playing, fishing, and living by. Next time, if it’s safe to do so, explore a nearby stream and find a rock within a riffle. Pick it up and take a close look, there might be a fingernet caddisfly larva waving its legs at you!
Water Quality Monitoring Projects Coordinator