High school student at Gainesville Wilderness Institute with a crawfish collected from local waters.
Macroinvertebrate: The word itself is enough to keep most of us from reading any further. However, if we "break it down," it's really not as scary as it may seem. The first part of the word—macro—refers to organisms that are large enough to be seen without a microscope. The second part—invertebrate—refers to an animal that lacks an internal backbone. Put the two words together and you are describing a spineless animal that is big enough to be seen with the naked eye. Note: Technically speaking, macroinvertebrates include organisms that are larger than 500 micrometers (μm), which is equivalent to half a millimeter (i.e., half the thickness of a dime).
Aquatic macroinvertebrates include animals that live in wet environments such as lakes, marshes, ponds, rivers, streams, and wetlands. They can even be found in buckets of standing water or in puddles.
This unique segment of the animal kingdom includes a wide variety of organisms including beetles, crayfish, leeches, mayflies, mosquitoes, mussels, shrimp, snails, stoneflies, water bugs and a host of worms that live in bottom sediments. Many of these alien-looking creatures spend their entire lives in water, while others, like the dragonfly or mayfly, spend only the first two stages of life in water (i.e., during the egg and larval stages).
Macroinvertebrates serve several important functions within the aquatic environment:
They provide a valuable "cleaning" service by scavenging dead or decaying bacteria, plants, and animals, which helps recycle nutrients back into the system.
They are an important food for fish, birds, amphibians and reptiles.
In scientific circles, they are considered to be valuable biological indicators; that is, the presence and abundance (or absence) of certain types (species) provide clues as to the health of a waterbody. For example, in a healthy system one will usually find a wide variety of species, as well as an abundance of most of those species. In contrast, degraded environments tend to have far less variety, fewer numbers of most species, and an abundance of one or two species. For many aquatic systems, there are often one or two key indicator species, the presence and abundance of which typically indicate that the system is at or near optimum health, and the absence of which indicate that waterbody health has been impacted.
A "Macroinvertebrate Species List," may be available if macroinvertebrates are being studied for a waterbody. This list provides the scientific and common names of species living in that waterbody, as well as their abundance. If there is more than one report, be sure to compare them to see if species diversity or abundance has changed over the years.
Macroinvertebrates are relatively easy and inexpensive to collect. In some instances, sampling can be done with homemade equipment. Collection methods depend on the waterbody being sampled. For example, a lake with a mud bottom is usually sampled with a D-frame dip net (see Figure 1, taken from EPA Monitoring and Assessing Water Quality, Figure 4.7).
Other methods include sediment "grabs" which are accomplished through the use of devices such as Petite Ponar and Standard Ekman dredges (Figure 2). These take small "bites" of sediment when dropped to the bottom. Hester-Dendy artificial substrates are also used. These provide easy to mount and monitor multiple platforms on which macroinvertebrates can anchor, feed and lay eggs.
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Macroinvertebrate monitoring requires thorough training. Additionally, adequate lighting and magnification must be available when sorting and identifying organisms. A recent study found that individuals who have not been properly trained have a tendency to select and/or identify larger and slower moving organisms from samples, while ignoring smaller, quickly moving specimens (Nerbonne and Vondrack, 2003). For this reason, it has been recommended that non-professionals work in teams so they can verify each other's techniques.
Composition (i.e., number of species and their abundances) of the macroinvertebrate community do not always suffice as the only indicators of waterbody health. Other long-term water quality characteristics (e.g., nutrients, algal abundance) should also be considered. Lastly, reports may vary based on the methods used for collecting organisms. That's why it is a good idea to refer to multiple data sources, if available, before drawing any conclusions.