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Benthic macroinvertebrates are multi-cellular animals that live in, on or under aquatic substrate. Benthic means they are bottom dwellers. They do not have backbones (invertebrates). They are large enough to be seen without the aid of a microscope (macro).
In streams, many macroinvertebrates are immature or larval forms of insects, but other examples include snails, clams, leeches and crayfish. See Appendix C for details on classification of organisms.
Benthic macroinvertebrates may also be divided into groups based on their food-gathering techniques. This method of classification can help identify water quality problems in a stream. For example, increased numbers of scrapers may indicate a nutrient runoff, but increased numbers of collectors may indicate organic enrichment. Described below are the six major groups listed in Merritt and Cummins (1984):
Scrapers graze on algae attached to sticks, stones and other surfaces. Many of these creatures have flattened bodies, which enable them to hold onto surfaces as they feed. Organisms of this type include water pennies, limpets and some mayfly larvae.
Collectors can be subdivided into gatherers and filterers. They feed on fine particulate organic matter (FPOM) and organic debris such as bacteria and waste products from other organisms. Many mayfly, caddisfly and true fly larvae fall into this group.
Predators feed directly on prey organisms. These include dobsonfly larvae, snipe fly, dragonfly and stonefly larvae.
Shredders feed on coarse particulate organic materials (CPOM) such as rooted plants and leaf material from trees in the surrounding watershed. They are important because they break down materials to a size that other organisms can use as food. Some caddisfly and stonefly larvae, scuds and sowbugs are included in this group.
Parasites feed either internally or externally on living animal tissue. Many types of roundworms are in this category.
Piercers feed on the fluids from living vascular plants and tissue fluids of filamentous algae. Spongillaflies and some types of caddisfly larvae are grouped as piercers.
Habitat Description
Macroinvertebrates have the same needs as other animals: oxygen, food, water and shelter. As aquatic organisms, many of them are particularly dependent on pollutant-free, oxygen-rich water. Monitors can look for macroinvertebrates in several types of types of habitats, including shallow riffles, deeper runs and quiet pools.
Riffles are areas where there is a significant increase of the water velocity. The surface of the water is broken while flowing over rocks or other objects found in the stream bed. This turbulence allows for a greater concentration of dissolved oxygen, or DO. Because of this, organisms that require higher DO are usually found in riffles. Riffles are usually about 12 to 18 inches deep.
Pools are characterized by deeper, slower water found above and below the riffle. Here, the environment differs from the riffle area particularly in the water velocity, depth and chemistry. Typically sites of decomposition and sedimentation, these areas act as catch basins for organic materials delivered from the riffle areas. The pool areas develop where the stream gradient or slope decreases. When the water velocity decreases, suspended materials drop out of the stream flow. As silt and sediment are deposited, fast-water organisms are replaced by organisms such as crustaceans and burrowing organisms (worms, midge larvae). With these changes in the biological community, pools often take on the appearance of a small lake or pond.
The substrate or bottom surface of rivers often determines the type and abundance of organisms found in an area. Cobble and gravel bottoms appear to offer the best habitat for certain macroinvertebrates. These medium-sized particles have enough surface area to provide protection for benthic organisms from predators and higher stream velocities, as well as providing a good substrate for algae growth.
Sandy bottoms tend to be less productive because there are few stable surfaces for attachment and there tends to be less algae for scrapers to feed on. Heavily silted bottoms may not allow for enough oxygen penetration to support life. Generally, these two substrates are not productive habitats for most types of macroinvertebrates. Solid bedrock can be treacherous depending on the water velocity. The organisms must have special adaptations for attachment or they may be washed off when flow rates increase.
Possibly, a stream may have excellent water quality and still have only a few benthic organisms. Monitors should not be disturbed. The numbers may simply reflect the amount of available habitat. For example, if the creek has a sandy bottom, there might be fewer macroinvertebrates due to a lack of plant life and appropriate shelter.
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