On a warm morning in early Spring, Mountain Watershed staff made their way north on the Indian Creek Valley Trail, carrying a net, a measuring tape, empty mason jars, metal sieves, and various water sampling equipment. A whole week of field work was dedicated to an unlikely but significant form of water quality testing: benthic macroinvertebrate sampling.
Benthic means “bottom-dwelling” and macroinvertebrates are animals without backbones that can be seen with the naked eye. In this case, aquatic insects are the ones we are sampling. The larval stage of common streamside flying insects such as dragonflies, damselflies, craneflies, caddisflies, mayflies, and stoneflies spend part of their life in streams. During the larval stage, these insects help break down leaf litter that falls to the bottom of our waterways during the Fall. As an integral part in the stream food chain, they are a key food source for many fish. People who fish for trout are familiar with benthic macroinvertebrates, creating fishing flies that mimic the body shape and movements of these tiny stream insects to attract the trout to their line.
Once they reached their destination, they made their way down to the banks of Indian Creek, preparing their specific roles within the sampling day. Morgan, our Americorps member, made her way into the water with a large net which she would use to catch the sediment, leaf litter, and rocks she gently shuffled on the streambed in various locations within the stream site. Jace, our Field Technician, grabbed the measuring tape and flow meter, heading to a spot in the stream where the deepest part wouldn’t flow over his chest-high waders. He then stretched the measuring tape across the length of the creek. Carla, MWA’s Director of Conservation, did quick math to determine spots along the measuring tape where Jace needed to measure the flow of the water.
“Typically, we do 6 samples at each site, but we will take 10 samples at this site because it is a glide pool, not a ripple,” says Carla. Ripples in the creek are areas where the water is moving swiftly over rocks, usually at shallower depths. Glide pools tend to have slower-moving water with deeper depths. “Since there are no ripples, we are less likely to find stream insects. So, we will take additional samples.”
Let’s circle back to why we are spending a week sampling for stream insects in the first place. Benthic macroinvertebrates are considered a “bio-indicator”. Because some species of “macros” are sensitive to pollution and impaired streams, the presence of sensitive species is a sign that water quality is good.
When specifically talking about streams impaired by abandoned mine drainage (AMD), some macros can’t live with a low pH while others are highly impacted by aluminum, a byproduct of AMD in some cases. High aluminum content in streams can impact fish and other aquatic life with gills because the aluminum becomes caked in their gills and ultimately suffocates the aquatic life. Because these metals settle on the streambed and create a film along the top of the rocks, this creates an issue for macros who primarily make their homes among the small pebbles and underneath large rocks on the streambed.
Although we have other ways of testing water quality to assess the specific amounts of iron, aluminum, manganese, and other pollutants, enlisting the help of bioindicators allows us to gauge the health of stream ecosystems downstream from our AMD treatment sites. As we continue to clean up the Indian Creek watershed and restore the mainstem and tributaries to a better water quality, we will see an increase in the presence of sensitive macros and fish species. As a testament to the positive impact of our treatment sites, we have discovered through fish shocking surveys that wild populations of trout have returned to areas within the Indian Creek watershed where they previously hadn’t been in quite a while.
Now that we understand WHY we’re sampling for stream insects, let’s get back to how we do it.
After Morgan finished shuffling the rocks, dirt, and leaves into the net, she returned to a large rock-turned-makeshift-work-table and dumped the contents into a metal sieve.
To ensure there weren’t little critters left behind in the D-net, Carla took on the task of searching the net with a pair of tweezers.
Because all of the sediment in the D-net would end up in mason jars until staff are able to sort through the material for macros, large rocks are rinsed off into the sieve then thrown back into the creek.
After the large rocks have been rinsed and the net has been picked through, all of the contents are then stuffed into mason jars and labeled with the date and site name. Back at the office, ethanol will be added to the jars to preserve the macros for when they will be sorted and identified at a later date. Our resident Macro expert, Carla, uses her microscope to identify the macros down to family and genus.
Want to learn more about all of the benthic macroinvertebrates found in streams near you? Check out www.macroinvertebrates.org and Stroud Water Research Center’s collection of resources about macros.