Entering the World of Macroinvertebrates

For much of my life, the extent of my knowledge about organisms in streams consisted of the crayfish I would sometimes hunt for, the water spiders that skimmed across the water’s surface, and fish. That finally changed in college as I was searching for a summer internship. As I started my search, my mind went back to a presentation that Lamonte Garber gave in my high school environmental science class. He mentioned that his workplace, Stroud Water Research Center, hired entomology interns each summer to help analyze the aquatic macroinvertebrate data that was collected each year. Since it was my long-term goal to work in the environmental field, I applied for a summer entomology internship in 2017. I was accepted and I began the internship with no knowledge of aquatic macroinvertebrates. However the staff at Stroud fully trained me and my fellow interns, and I ended up returning the following two summers as well. During those three summers, I learned how to identify the aquatic macroinvertebrates of PA and the surrounding areas down to family-level and about their importance in indicating stream health.

This image, taken through the eyepiece lens of my microscope, shows what a majority of my summer internships looked like in the lab. This happens to be a mayfly, specifically from the Baetidae family (Photo credit: Rebecca Lauver).

Indicators of Stream Health

Aquatic benthic macroinvertebrates (affectionately called macros) are small, aquatic insects that are visible to the naked eye and live on the stream bed. Most prefer areas with rocky substrate and riffle habitat as it provides them with suitable habitat and higher oxygen levels. These insects serve as good measures of stream health as many are sensitive to the oxygen levels and pollution within the water. Due to their small size and relatively short life span, macros can not travel very far, making them especially susceptible to local pollution. On numerous occasions, I’ve heard them referred to as the “canaries in the coal mine” of stream water quality; when we start to lose sensitive macros, it’s likely that the stream they occupy is in trouble.

Some of the most sensitive macros are mayflies, stoneflies, and caddisflies or, as they are called in the entomology world, Ephemeroptera, Plecoptera, and Trichoptera (EPT). High levels of EPT abundance and diversity most likely means that the stream they were found in is healthy. However, not all macros are sensitive to pollution. A lack of EPT species and an overabundance of more pollution-tolerant organisms like worms, black flies, and midges can indicate poor stream health. Determining what macros are or aren’t present in the stream can help to give you a better understanding of what the water quality is in that area. Analyzing changes in macroinvertebrate density and diversity over time can also give you an idea of how the stream health is changing, for better or worse. And as stream health changes, and with it, macro levels, so can the density and diversity of fish populations.

Mayflies, stoneflies, and caddisflies (left to right) are important indicators of a healthy stream. More amazing photos like these and identification tips are available at Macroinvertebrates.org.

Angler’s Concern

With increased stream impairment across our landscape, many of the fishing spots that were once bountiful years ago no longer boast the fish, especially trout, abundances that they once did. Since trout and macroinvertebrates thrive in cool streams, forest buffer plantings are one of the best ways to restore these fishing spots. Trout Unlimited (TU) chapters are stepping up and partnering with stream restoration efforts in order to bring streams back to a high enough quality to see an abundance of fish return. Thanks to outreach efforts from local TU chapters within Pennsylvania including Donegal, Doc Fritchey, Muddy Creek, and Falling Springs, the Alliance was connected to 6 new sites and has gotten an additional 22 acres planted since 2019. 18 more acres are already lined up for the upcoming spring and fall planting seasons. The Alliance is also collaborating with the Maryland Chapter of Trout Unlimited to plant 9 acres of buffers along two native Brook trout streams. A recent planting done in partnership with the Muddy Creek TU in PA brought together over 40 volunteers to plant a buffer on a tributary to Muddy Creek. As Jimmy O’Connor, Muddy Creek TU president says in the featured video, “riparian buffer zones are a really important ecological tool in maintaining good water quality to make sure we’ve got cold clean water and good habitat for fish.”

Volunteers from the Muddy Creek TU chapter finish up the tree planting by protecting the trees with shelters (Photo Credit: Dan Dellinger, Muddy Creek TU #575 Member).

The Forests’ Input

Macroinvertebrates make up an important part of fish diet, so if you want fish diversity and abundance in streams, you need the conditions that support the macros that feed them. Many macros consume leaf litter and since these organisms can not travel far, they require nearby trees as a leaf source. Riparian forests provide this leaf input while also helping to cool the water, filter harmful chemicals or excess nutrients, and prevent sediment from covering the stream bottom. Without trees along streams, many of the macroinvertebrates in those streams would not have food or suitable habitat.

Some slow-moving, wide streams with riparian buffers can experience pile-ups of leaves at certain times of the year. Piles of leaves in streams are called “leaf packs” (Photo Credit: Rebecca Lauver).

When the leaf litter enters a stream, it undergoes “microbial conditioning” during which the leaves are broken down slowly by enzymes secreted by microbes in the water. Some macroinvertebrates, categorized as “shredders,” will take these slowly decomposing leaves and shred them into smaller bits to eat. Certain types of caddisflies will even use leaves to build the protective casings that they surround their bodies with. Others, called “filter-feeders” and “collector/gathers,” either filter small leaf particulates from the water or scavenge along the stream bottom for small leaf pieces, as their names imply.

Caddisflies use a mixture of materials to make their cases including pebbles, plant fibers, and leaves. Different species have different material preferences and this particular caddisfly opted for a mixture of materials, including a few leaf bits (Photo Credit: Jim Rathert, MDC).

Faux Flies

Since macroinvertebrates make up a portion of fish diet, the bait used to catch them often attempts to mimic macros. While I am not very familiar with the fly fishing world, Adam Miller, Communications Director for the Alliance, is. He has years of experience making his own fishing flies and work-related Zoom calls frequently feature his fly-making station in the background. “I never imagined I would one day be a passionate bug nerd,” says Miller. “However, as a dedicated fly fisherman and fly tyer, I quickly learned that understanding the aquatic macroinvertebrates in a given stretch of water is one of the most efficient ways to gauge what’s on the menu for trout.” The ties used by fly fishers take careful precision and time to make and without the understanding of what is attractive to fish, it would be wasted effort. However, those that commit the time and effort are often rewarded and, as Miller says, “there’s no greater feeling than educating yourself about local hatches, spending the time to tie an imitation, and using it to fool a feeding trout.”

Handmade flies mimic different parts of the macroinvertebrate body like its tail, segmented body, and external gills. This fly was made by Adam Miller and is next to its counterpart, a preserved stonefly. (Photo Credit: Adam Miller).

Swarms of Insects

Sometimes fishing flies mimic the aquatic life stage of a macroinvertebrate, while others mimic their terrestrial flying stage. Mayflies, stoneflies, caddisflies, dragonflies, and damselflies, among others, all complete portions of their life cycle in and out of the water. They start out as eggs in streams and then hatch into their nymph stage where they may remain for several months to a few years. After growing and going through a varying amount of instars, they will emerge from the water, ready to metamorphose into their winged form. Similar to how butterflies leave behind a chrysalis or a cicada its exoskeleton, these transitioning organisms will leave behind the shell of their aquatic forms. Once their wings are ready, they take to the sky in search of a mate.

For some, this period of time is extremely short. Mayflies and stoneflies are often of particular interest because for some species, this out-of-water period may only last a day. This day-long frenzy can result in large swarms of insects flying over the water’s surface. On a few occasions, swarms can become extremely dense, especially if the flying insects are attracted to a light source which happened on the Columbia-Wrightsville Bridge that crosses the Susquehanna River. These swarms are signs of improving water quality. Efforts to reduce unnecessary insect death, like turning off insect-luring lights, can help to ensure more macroinvertebrates will return in the future.

A large swarm of mayflies or stoneflies can easily blanket the sky. (Photo Credit: Adam Miller).

A Path Forward

During my senior year of college, my experience with studying macroinvertebrates and my passion for environmental restoration came together. The research project I completed focused on the water quality of an urban watershed through macroinvertebrate analysis and water sampling. This coldwater, limestone stream was an ideal fishing location but in recent years, it hadn’t produced the catches that many anglers were used to. Through my research and some previously collected data, I determined that the macroinvertebrate diversity and abundance in the watershed was lacking.

At the same time, I was working to buffer the waterways that flowed through my college’s campus. With the help of a friend, I was connected to the Alliance’s buffer program which provided the much-needed, invaluable support and resources to make the planting possible. When I graduated the buffer expansion process was handed off to Leah Stern, who took over my position. The plantings have continued (and now include lawn conversions as well) and I am excited to see how they will continue to expand.

As I have transitioned to working at the Alliance myself, it is even more clear how tree plantings can change the narrative of impaired streams and entire watersheds. From research centers, to TU chapters, to restoration organizations, the momentum is here, and growing, to change what our current landscape looks like. Whether they are on agricultural, urban, or residential land, streams have the potential to support a wide range of life. Exploring the world of macroinvertebrates is just a small, but important, part of that range.