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.
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.
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.