The Mighty Macros: What They Do and What They Can Tell Us About Our Waters

Collecting benthic macroinvertebrates in the Jemez River at the Valles Caldera. (Photo courtesy of Rossana Sallenave/NMSU)

By Rossana Sallenave, Extension Aquatic Ecologist, New Mexico State University

Spring is upon us, and with it comes warmer waters and one of my favorite pastimes: wading around in streams and turning over rocks to look for benthic macroinvertebrates!

Benthic (bottom-dwelling) macro (large enough to see with the naked eye) invertebrates (no backbone) are aquatic organisms that are generally small enough to catch with a fine mesh net or cloth, but large enough to be easily collected. These creatures inhabit the bottom layers of streams and lakes, and include insects, worms, crayfish, snails, and freshwater clams. Some of these aquatic macroinvertebrates, such as insects, spend at least part of their life cycle in water, while others, such as clams and snails, are entirely aquatic.

Blackfly larvae (Family Simuliidae) are semi-tolerant of pollutants and can survive with moderate oxygen levels. (Photo courtesy of the California Dept. of Fish & Game Bioassessment Laboratory)

Benthic invertebrate sampling requires little more than rubber boots, a net, and a bucket, and is every bit as rewarding to us stream ecologists as birdwatching is to birders. Granted, beauty is in the eye of the beholder, but to me, these tiny critters are much more than just creepy crawlers. They are magnificent examples of adaptation and resilience, and play a critical role in maintaining ecosystem function of our aquatic systems.

Aquatic macroinvertebrates are an integral part of the aquatic food web, and play a key role in nutrient cycling in aquatic ecosystems because they are the primary processors of organic materials. They do this by eating leaves, algae, bacteria, and other invertebrates, and in turn, are eaten by larger invertebrates, fish, amphibians, birds, and other vertebrates. Macroinvertebrates can be divided into different groups based on what physical characteristics of streams they exploit to obtain their foods. They include grazers, shredders, gatherers, filterers, and predators. The many roles and ecosystem services provided by aquatic macroinvertebrates highlights the importance of their conservation.

Stonefly nymphs (Family Perlidae) require clean, oxygenated water. (Photo courtesy of the Society of Freshwater Science).

Benthic macroinvertebrates are also reliable indicators of water quality and are used in biological monitoring programs. Macros spend all or most of their lives in water and are therefore affected by the physical, chemical, and biological conditions of the aquatic system. Unlike taking chemical measurements, which is like taking a single snapshot of the aquatic ecosystem, biological measurements provide a more long-term view of the water quality, as reflected by the community of organisms able to live in that environment.

Some aquatic macroinvertebrates are more sensitive to pollution than others. Therefore, if a stream is inhabited by pollution-tolerant organisms and the more sensitive organisms are absent, pollution is likely. Biomonitoring programs use the presence or absence of indicator species or indicator communities to reflect environmental conditions. To give an example, the presence of high numbers of adult riffle beetles and gilled snails can serve as indicators of good water quality. These organisms require high levels of dissolved oxygen and are highly sensitive to organic pollution. Many species of stonefly nymphs are also very sensitive to pollution and require high levels of dissolved oxygen to survive. On the other side of the spectrum, high numbers of bloodworm midges or tubificid worms of the genus Tubifex, both of which can be found in severely polluted waters, indicate poor water quality.

Do you want to learn more about how macroinvertebrates are used to assess water quality and how to identify them? Check out this publication from the NMSU Extension Office.

Clam shrimp under the microscope

An excited family came to the nature center on Friday with an interesting discovery. While hiking in Pueblo Canyon, they spotted a pool of water full of something orange and moving! They put some in a bottle and brought it to look under our HD video-microscope. We were all shocked to see little animals zipping around at lightning speed!

Clam shrimp under the microscope

The teeny creatures they found are clam shrimp. If you look closely at the video, you can see their clam-like shape. Clam shrimp are a type of crustacean, closely related to fairy shrimp, and a bit more distantly related to the shrimp we eat at the table. Like many shrimp, they appear pink or orange due to the presence of carotenoids, which are a group of orange pigments that also give carrots their color.

A huge thank you to the family for sharing their unique discovery with us at the Los Alamos Nature Center! We encourage everyone to bring these natural finds to us and have a closer look under our microscope!

Clam shrimp under the microscope