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When we look out over a stream, it can sometimes seem peaceful and still, but even these serene streams have so much life on and just underneath the surface. We just need to know where to look! Lean in a little closer (but not too close!) and you’ll see that the top of a river is covered in all sorts of cool critters. Some of the most important of these are benthic macroinvertebrates, or “macros,” as they are lovingly called.

So what exactly are macros? First, let’s breakdown their full name. Macro means large enough to be seen without a microscope and invertebrate means without a backbone. These little creatures play a big part in the ecosystem of a stream as a food source for fish and other vertebrates. They also breakdown organic matter and therefore recycle nutrients.

Scientists use the presence of macros to help gauge the help of streams. Different species respond differently to the presence of pollution and changes in habitat (such as oxygen change sand temperature increases). By learning more about the macros in a particular habitat, scientists can better understand how healthy a stream is and if there is pollution present that could harm animals and people.

Pollution Tolerance of Macroinvertebrates

A species that has a low tolerance is sensitive to pollution, or negatively affected by it. A species that has a high tolerance is not sensitive to pollution and experiences very few negative effects.

Dobsonflies & Alderflies (Order Megaloptera) // Very low tolerance

Stoneflies (Order Plecoptera) // Low tolerance

Mayflies (Order Ephemeroptera) // Low-to-medium tolerance

Caddisflies (Order Trichoptera) // Low-to-medium tolerance

Snails (Class Gastropoda) // Low-to-medium tolerance

Sow Bugs & Scuds (Class Crustacea) // Low-to-medium tolerance

Water Beetles (Order Coleoptera) // Low-to-medium tolerance

Flatworms (Class Turbellaria) // Medium tolerance

Aquatic Earthworm (Class Oligochaeta) // Medium tolerance

Craneflies (Order Diptera) // Medium tolerance

Leech (Class Hirudinea) // Medium tolerance

Clams & Mussels (Class Bivalvia) // Medium tolerance

Mater Mites (Class Arachnids) // Medium tolerance

Crayfish (Order Decapoda) // Medium tolerance

Dragonflies (Order Odonata) // Medium tolerance

Water Boatman & Backswimmer (Order Hemiptera) // Medium tolerance

Midges (Order Diptera) // High tolerance

Blackflies (Order Diptera) // High tolerance

Macroinvertebrates Field Guide

Image for Halesochila taylori

Caddisfly

Halesochila taylori

Caddisflies have wings shaped like a tent and antennae that give a moth-like appearance. However unlike moths, caddisflies spend most of their lives living in the water as larvae. Their larvae build a protective case around themselves from pebbles, sand and plant material. See the insect guide for the adult form.
Image for Aquarius remigis

Common Water Strider

Aquarius remigis


photo credit: Gavin Slater

Common water striders use surface tension to “walk on water.”. Their legs have tiny hairs that repel water and capture air. By repelling water, the tiny water striders stand on the surface of the water and the airs allows them to glide easily. They eat insects such as mosquitos
Image for Rhynchobdellida

Leech

Rhynchobdellida

Leeches are commonly found in lakes and ponds and are eaten by fish, birds, reptiles and amphibians. Like most annelids they are hermaphrodites meaning they have both male and female sexual organs. Some leeches are used is reconstructive surgery.
Image for Potamopyrgus antipodarum

New Zealand Mud Snail

Potamopyrgus antipodarum

New Zealand mud snails are an invasive species and considered a threat to freshwater environments. They can become so numerous, up to 100,000 per square meter, that they out-compete our native aquatic snails and other macroinvertebrates for food.
Image for Chironomidae

Non-biting Midge

Chironomidae

Non-biting midge flies are commonly known as “blind mosquitoes” because they look like mosquitos but do not bite. Their larvae are tolerant of a wide range of environmental conditions and can be found in streams, deep slow moving rivers, stagnant ditches, lakes and ponds.
Image for Ophiogomphus severus

Pale Snaketail Dragonfly

Ophiogomphus severus

The Pale Snaketail habitat is rivers and streams with moderate current in forested and open landscapes, as well as large sandy lakes. The larvae feed on aquatic insects, small fish and tadpoles while the adults eat soft-bodied flying insects such as mosquitoes, flies, moths and mayflies. See the insect guide for the adult form.
Image for Turbellaria

Planaria

Turbellaria

Planarias are flatworms that can be found in ponds, streams and rivers where they eat small prey that they suck up with their muscular mouths. They have the ability to regenerate lost body parts. If you split one lengthwise or crosswise it will grow into two separate complete individuals.
Image for Pacifastacus leniusculus

Signal Crayfish

Pacifastacus leniusculus

Also called a crawfish or crawdad, this is the only native species in Washington. The signal crayfish inhabit both coastal and upland streams, lakes, and rivers where they seek shelter in rock crevices and under woody debris. It is an omnivore with most of its diet consisting of detritus.
Image for Sigara sp.

Water Boatman

Sigara sp.

Water boatmen are insects that occupy a wide range of aquatic habitats, including pools, ponds, backwaters of streams, and sometimes slow-moving streams. Most species are non-predatory, feeding on aquatic plants algae and detritus instead of insects and other small animals.
Image for Hydrachnidia

Water Mite

Hydrachnidia

Water mites are among the most abundant and diverse groups of benthic arthropods. Nearly all freshwater environments are inhabited by water mite species. The have a very complicated life cycle and all are parasitic at one stage.
Image for Margaritifera falcate

Western Pearlshell Mussel

Margaritifera falcate

Salmon play an integral part in the Western pearlshell mussel’s life cycles. The tiny mussel larvae attach themselves to salmon or steelhead that are headed to the ocean and live off of them until the fish return to the stream. There the mussels exit their hosts and bury themselves in the streambed.

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