Macroinvertebrates are larger-than-microscopic invertebrate animals. Freshwater macroinvertebrates include aquatic insects, worms, clams, snails, and crustaceans. Because of their abundance and their sensitivity to environmental impacts, they are widely used in biomonitoring programs to assess water quality.
Understanding Aquatic Insects
Many insects lay their eggs in water. The young or immature insects grow and develop in water. When they are mature and ready to become adults, they go through a change called metamorphosis. After this change some may stay near water to eat and mate, but they no longer live in water. Insects that spend the first part of their lives in water include caddisflies, mayflies, and stoneflies. Other insects spend their entire lives in or on water, such as water scorpions, water striders, and many beetles.
An insect in an invertebrate. Invertebrates don’t have a backbone or an internal skeleton to support their bodies. Instead of a skeleton, invertebrates may have a hardened body or a protective shell.
Macroinvertebrates and Water Quality
Macroinvertebrates are key biological indicators of stream health and water quality. Some macroinvertebrates are very sensitive to pollution while others are very tolerant. Still others are somewhere in the middle. By collecting macroinvertebrates in a stream, one can very quickly get an overall idea of the health of a stream.
Organisms in this category are indicators of healthy or good quality water.
Pollution-sensitive organisms typically found in healthy streams include: mayflies, stoneflies, caddisflies, water pennies, dobsonflies or hellgrammites, and gilled snails. Image credits: University of Wisconsin Extension.
Organisms in this category indicate healthy or fair quality water.
Moderately pollution-sensitive organisms found in healthy or fair-quality streams include net-spinning caddisflies, alderflies, crane flies, damselflies, dragonflies, crayfish, scuds, riffle beetles, and clams and mussels. Image credits: University of Wisconsin Extension.
Organisms in this category are found in healthy, fair, or poor quality waters. They include black flies, leeches, aquatic worms, midge flies, pouch snails, and ramshorn snails.
The River Continuum Concept
The diets of aquatic insects vary. Some insects eat plants, some eat small animals, and some eat decaying things. Others eat some of everything – plants, animals, and decaying matter. Insects also provide food for larger creatures such as fish. The gradual change from the headwaters to the mouth of a stream affects the habitat structure and food base of the stream.
Shredders have chewing mouthparts which allow them to feed on large pieces of decaying organic matter, such as leaves and twigs which fall from trees and other plants in the riparian zone. They tend to inhabit headwater streams and other areas with a high percentage of canopy cover. Scuds, stoneflies, and some caddisflies are considered shredders.
Scrapers or Grazers
Scrapers or grazers remove attached algae from rock and wood surfaces in the current. By eating algae, they play an important role in keeping it from growing out of control. They are found in areas where sunlight is able to reach the stream bottom, because without sunlight, algae cannot grow. Because these conditions often occur in larger, wider streams, many scrapers have developed adaptations for hanging on in relatively swift currents, such as flat, streamlined bodies or suction disks. Water pennies, mayflies, and some caddisflies are considered scrapers.
Collectors eat fine particles of organic matter. Collectors tend to be found in all reaches of a stream because fine particles are present in all stream types to some degree. However, collectors make up a greater proportion of the bug population in the lower reaches of a system where fine sediments tend to accumulate and the habitat is not suitable for shredders and scrapers. Black fly larva, riffle beetles, midge larva, many mayflies, and some caddisflies are considered collectors.
Predators consume other macroinvertebrates; they have behavioral and anatomical adaptations for capturing prey. Many have extensible mouthparts or raptorial forelegs adapted for grasping prey, and strong opposable mouthparts for biting and chewing. Some predators pierce their prey and suck body fluids with tubelike mouthparts. They are found in all habitat types.
Dragonflies, damselflies, dobsonflies, alderflies, fishflies, aquatic beetles, and stoneflies are considered predators.
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