Aquatic Invertebrates

Viewing wildlife is one of the best things about spending time along a river. Even aquatic invertebrates - the "bugs" living at the bottom of a river - are interesting if you take a close look at them!

The kinds of invertebrates found in a river can tell you a lot about the water quality.

Invertebrate Background Information

Viewing wildlife is one of the best things about spending time along a river. Even aquatic invertebrates - the "bugs" living at the bottom of a river - are interesting if you take a close look at them!

The kinds of invertebrates found in a river can tell you a lot about the water quality.

What Are Invertebrates?

Invertebrates are animals without backbones. They may have hard outer shells to protect and support their soft bodies. They are the most numerous and diverse kind of animal on Earth and they play a critical role in the functioning of ecosystems (ie: pollination, nutrient cycling, food webs).

There are several general categories of invertebrates:

Terrestrial invertebrates live on the land.

Marine invertebrates are common along ocean beaches.

Caddisfly Larva
Aquatic invertebrates live at least part of their life in freshwater ponds, lakes, streams or rivers. Some of the adult insects that we see flying around us have spent their early lives feeding underwater.

Macroinvertebrates are species large enough to be seen without using a microscope but they are generally less than 2 cm long

Benthic invertebrates are species that live underwater hiding in between bottom rocks and plants or in the mud.

Insect Life Cycles

Approximately 5% of insects spend all or part of their life cycle in water. The immature stages of these aquatic insects often have streamlined bodies and they breathe with gills.

Functional Feeding Groups

A healthy river has many different invertebrates feeding in many different ways. Diversity is a characteristic of stable, healthy ecosystems. Aquatic invertebrates can be grouped into four functional feeding groups according to how and what they eat:

Aquatic Food Chains

Energy flows from the sun through plants and then through each trophic level of a food chain. Food chains start with solar energy captured by green plants that use photosynthesis to produce simple sugars. Oxygen produced as a by-product is an added bonus for all animals!




plant chlorophyll








Collecting Invertebrates

Collecting aquatic invertebrates is a fun and interesting activity that can reveal useful information about the health of a river. The presence or absence of certain types of invertebrates can indicate the effects of pollution.

Collection Sites
Various invertebrates are adapted for life in areas with different river bottoms and water that may be shallow or deep, fast or slow. Since most observers don't have time to thoroughly sample all these habitats in one trip, a good idea is to focus areas that feature:

  • gravel or rocks
  • shallow water
  • moderate flow velocity

No one likes wading into mud and becoming stuck, and most people avoid wading into fast water that is deeper than their boots. It's obvious from these limitations that the most common choice for sampling sites will not accurately represent all of the habitats found along the length of a river. But, since different habitats have different invertebrates, it is easier to compare results if the same type of habitat is sampled each time. So, good advice would be to sample each time in shallow, moderately flowing water with gravel or rock bottom in order to provide consistency to a monitoring program.

Aquatic invertebrates should be captured by observers wearing rubber boots, hip waders or chest waders, especially if there is a concern over cold water, broken glass or sewage contamination.

Safety should always be the first priority during river monitoring. Observers should wear personal floatation devices (P.F.D.'s) and be under qualified supervision.

Observers collect invertebrates using equipment such as fine-meshed sweep nets, kick nets, Neill cylinders or deepwater artificial samplers (wire cages filled with rocks).

  • Nets have the advantage of being easy to carry, easy to use and inexpensive to build, however, they don't capture all the invertebrates present at a sample site.
  • Neill cylinders are expensive to buy and they capture an overwhelming number of invertebrates that can take many hours to sort and identify with a microscope back in a lab.

Using a Kick Net
Useful invertebrate information can be obtained with a simple kick net, especially if proper procedures are followed.

Identifying Aquatic Invertebrates

Aquatic invertebrates all look the same to the untrained eye - strange and bizarre! Honestly, with some background information, even the most reluctant observer can learn to appreciate and identify these truly amazing life forms hiding at the bottom of a river.

Taking the time to identifying aquatic invertebrates is worthwhile for several reasons:

  • Aquatic invertebrates are interesting and easy to see.
  • Invertebrates are an important part of the food chain.
  • The presence or absence of a particular invertebrate can be an indication of pollution.

Invertebrate Taxonomy

Scientists organize living things into a series of categories. This process is called taxonomy. Taxonomy categories are listed below from largest to smallest along with an example that shows how humans are classified as Homo sapiens.

For quick shoreline and field trip identification, all river organisms can be identified down to the level of phylum, maybe to class, some to orders and only a few to families. Accurate genus and species identification is very complicated and requires the viewing of microscopic features back in a lab.

There are many thousands of species of invertebrates and it helps to know how they are related to each other. Some of the more common river invertebrates are classified below. Special emphasis is placed on the insects that are commonly seen. Click on the highlighted names for more detailed information.

Invertebrate Identification

Shoreline identification and the release of live invertebrates is easily done by merely viewing illustrations.

  • Crustacean
  • Resemble small shrimp
  • Swims on its' side
  • Swims quickly before burrowing into clumps of vegetation
  • Omnivores and scavengers on plant or animal material
  • Requires well-oxygenated water
  • Moderately tolerant of pollution
  • May indicate fair water quality
Blackfly Larvae
  • Insect
  • Complete metamorphosis
  • Blackfly larvae resemble small grubs
  • Head dark coloured
  • Bottom-end swollen and fatter than the head-end
  • Attach to the upper smooth surface of rocks using suckers on the bottom end
  • Heavily populated rocks appear to have a stubble beard
  • Larvae often attach to the bottom of a sorting tray
  • Found in flowing water
  • Omnivores
  • Filtering collectors
  • Pollution tolerant
  • May or may not indicate poor water quality
  • Segmented worm
  • Bristleworms esemble thin, reddish earthworms
  • Bristles on each segment are not visible to the unaided eye
  • Can tolerate low oxygen levels
  • Pollution tolerant
  • Large numbers may indicate poor water quality
  • May indicate organic pollution
Caddisfly Larva
  • Insect
  • Complete metamorphosis
  • Some larvae build tube-like cases to hide in
  • Larvae resemble caterpillars with skinny legs
  • Mostly herbivorous on algae and plants
  • Some are predators that eat nymphs
  • Some are collectors that build nests
  • Larvae and adults are a favourite trout food
  • Larvae are moderately tolerant of pollution and warm water
  • Large numbers may indicate fair water quality
Clams and Mussels
  • Mollusc
  • Found in slow moving, warm rivers
  • Clams are small, round and symmetrical
  • Mussels are larger, oblong and lopsided
  • Filter feeders on plankton and organic debris adrift in the current
  • Can tolerate degraded or polluted environments
  • Moderately pollution tolerant
  • Large numbers may indicate fair water quality
Cranefly Larva
  • Insect
  • Complete metamorphosis
  • Cranefly larvae resemble plump caterpillars with a knobby butt
  • Larvae are found more often in the fall
  • Herbivorous larvae shred leaf material (shredders)
  • Adults do not feed
  • Adults look like "giant mosquitoes" - what a scary thought, but they don't bite
  • Moderately pollution tolerant
  • Large numbers may indicate fair water quality
Damselfly Nymph
  • Insect
  • Incomplete metamorphosis
  • Nymphs have three paddle-shaped tails
  • Extendable lower lip is used to grab prey
  • Predatory on mayfly nymphs and mosquito larvae, worms and anything else small enough to grab
  • Moderately pollution tolerant
  • Large numbers may indicate fair water quality
Diving Beetle
  • Insect
  • Complete metamorphosis
  • Large diving beetles are fun to discover
  • Breaths air from a "scuba tank" air bubble trapped under the wing covers
  • Found in water both as adults and larvae
  • Strong swimmers
  • Carnivorous on larvae and small fish
  • Adults are not useful as an indicator of water quality because they breathe from surface air bubbles
Dragonfly Nymph
  • Insect
  • Incomplete metamorphosis
  • Nymphs are large, ferocious creatures
  • Jet-powered butts squirt water for propulsion
  • Lip is hinged and extendable to capture prey
  • Nymphs express huge attitude with a large lower lip
  • Predatory on larvae, nymphs, tadpoles and small fish
  • Carnivorous - there's something scary about an insect that can eat a fish!
  • Adults don't fold their wings; the wings lay flat and outspread
  • Found in slow moving and still water
  • Moderately pollution tolerant
  • Large numbers may indicate fair water quality
  • Small, pale blobs found in the vegetation or under rocks.
  • Omnivorous on living or dead plants and animals.
  • Old science textbooks are full of drawings that show flatworms growing two heads. What mad scientist would split those heads in half in the first place? Doesn't that hurt, or at least result in a "splitting" headache?
  • Pollution tolerant
  • Large numbers may indicate poor water quality
  • Segmented worm
  • Leeches are fun to watch swimming or inching along the glass of an aquarium.
  • Parasitic on the blood of fish and birds.
  • Pollution tolerant.
  • Large numbers may indicate poor water quality.
Mayfly Nymph
  • Insect
  • Incomplete metamorphosis
  • Three long tails
  • Swims like a dolphin with up and down undulations
  • Feathery gills are located along sides of the abdomen
  • Diverse body types - flat, armoured, short or long and skinny and are adapted for different flow conditions
  • Mainly herbivorous on algae and detritus.
  • Nymphs require clean, oxygenated water.
  • Pollution intolerant.
  • Large numbers likely indicate good water quality and high oxygen levels.
Midge Larva
  • Insect
  • Complete metamorphosis
  • Larvae occur in astronomical numbers and dominate many aquatic samples
  • Some larvae have red blood
  • Larvae resemble a short worm
  • "C-shaped" and swim by flexing rapidly
  • Wiggles back and forth vigorously
  • Appear to have no legs
  • Omnivores feeding on small organisms, decaying matter and algae
  • Pollution tolerant
  • Large numbers may or may not indicate poor water quality and organic enrichment
Damselfly Nymph
  • Mollusc
  • Herbivores that feed on algae scraped from stones and leaves
  • Detritivores that feed on decaying matter
  • Browse by means of a radula - a ribbon-like tongue embedded with thousands of "teeth" - scraped along rocks or leaves
  • Lung-breathing snails have shells coiled like a tuba or spiral shells opening on the left side without a door. (Lung = Left) They obtain air from above the water's surface and therefore are not as sensitive to pollution and are not really good indicators of water quality
  • Gill-breathing snails have spiral shells opening on the right side with a door (operculum). They rely on oxygen dissolved in the water and may be more susceptible to pollution
  • Pollution tolerant
  • Large numbers of lunged snails may indicate poor water quality and organic enrichment
  • Large numbers of gilled snails may indicate good water quality
Stonefly Nymph
  • Insect
  • Incomplete metamorphosis
  • Can be very large
  • Great to find but kind of scary looking with armour and big legs
  • Most are herbivores feeding on decomposing leaves coated in bacteria and fungus
  • Two long tails and antennae
  • Swim like sharks with side-to-side undulations
  • They do push-ups to move water past the "arm-pit" gills
  • Leave their dry, shed skins attached to dry rocks
  • Found in deeper, faster water
  • Very pollution intolerant
  • Indicate good water quality with high oxygen levels
Water Boatman
  • Insect
  • Complete metamorphosis
  • Are water boatman the cutest little bugs, or what?
  • Can fly or swim
  • Adults fly in search of deeper water for breeding and overwintering.
  • Swarms of these swimming insects blacken shallow water in the North Saskatchewan River in Edmonton each fall.
  • Omnivorous and feed on algae, detritus, micro-animals, small midge and mosquito larvae.
  • Found in all types of water, moving or still.
  • Boatman "scuba dive" with an air bubble trapped on their body.
  • Not necessarily useful as indicators of water quality because the adults breathe surface air.

Interpreting Invertebrate Data

Aquatic invertebrates are living indicators of pollution levels. The numbers and types of invertebrates in a river change if pollution is present. Invertebrate data can serve as a quick check of water quality.

A survey of invertebrate populations can reveal information about the health of a river. However, the data may have limited value if collected on only one day of one season in one year. Data is more useful if it can reveal trends spanning an entire season, an entire year, several years or along the length of an entire river.

Types of Pollution

Pollution is any substance that has a negative effect on living things. There are several categories of pollution including sediment, toxic chemicals, warm water and organic nutrients.

Sediment Pollution

Particles that wash into a river may originate from street runoff during storms or during spring snowmelt. Sediment can also originate from construction areas, trampled banks or flood events.

  • Sediment pollution does damage when suspended particles gradually settle over the river bottom. The effects of sediment pollution can include:Measuring the turbidity (clarity) of the water can serve as a test for sediment pollution.
  • reduced number of invertebrates and invertebrate types
  • smothering and killing fish eggs, algae and invertebrates
  • murky water that blocks sunlight for photosynthesis
  • rocks and plants covered in silt
Toxic Pollution

Chemicals that are harmful to life can originate from storm sewer outlets, water treatment plants, factories, rail yards, lawns, golf courses and mines. These chemicals can include paint, diesel fuel, chlorine, oil, acid, pesticides, herbicides and heavy metals.

  • The effects of toxic pollution can include:Analysis of invertebrate data can serve as a measure of toxic pollution. Testing for specific toxins is usually beyond the scope of school and public monitoring programs
  • reduction or absence of all types invertebrates
  • water appears clear and clean
Thermal Pollution

Human activities can return warm water to a river. Sources of thermal pollution can include power plants, wastewater treatment plants, fish hatcheries and oil refineries.

  • Recording changes in water temperature can document thermal pollution.

The effects of thermal pollution can include:

  • increased water temperatures
  • increased plant growth
  • slowing of river velocity because of planet growth
  • fewer kinds of invertebrates
  • large numbers of pollution tolerant invertebrates
  • lower dissolved oxygen levels
Organic Nutrient Pollution

Too much of a good thing can be harmful to life. While nutrients are necessary - like nitrogen and phosphorus - too much can result in massive algae and plant growth. Excessive plant growth can be followed by oxygen depletion as dead plant material decomposes and bacteria uses oxygen. Lower oxygen levels can result in fish kills.

Organic nutrients can originate with human and livestock wastes, feedlots, meat packing plants, sewage and fertilizer runoff from yards and farms.

  • Invertebrate data can be used along with testing nitrogen and phosphorus levels to measure of organic nutrient pollution.

The effects of organic pollution can include:

  • fewer kinds of invertebrates
  • large numbers of pollution tolerant invertebrates
  • an increase in collectors and scrapers such as caddisfly larvae or roundworms
  • unpleasant odours
  • rocks covered in algae
  • excessive weed growth
  • high concentrations of nitrogen and phosphorus
  • lower oxygen levels

Invertebrate Pollution Tolerance

Caged canaries were once taken deep inside coal mines to alert the miners of deadly, odourless gases. If a canary died in its cage, it was time for the miners to quickly evacuate to the surface.

In a similar way, benthic (bottom dwelling) invertebrates can indicate the presence of pollution in a river. Some invertebrates are very sensitive to pollution and quickly die off.

Invertebrates are good "bio-indicators" of pollution for several reasons:

  • Invertebrates are basically stationary even though the river is constantly moving past them. The impact of any pollution can be seen in the surviving organisms long after all traces of a chemical have been washed away.
  • Invertebrates have a relatively long life cycle of one to three years. They are available to measure pollution over long periods and at low concentrations.
  • If scrapers or collectors become more common, they may be an indicators of organic nutrient pollution and increased algae growth.
  • If the water quality has been impacted by pollution, it will be home only to those invertebrate species that are tolerant of pollution and these may be present in very great numbers.

It should be noted that surface-breathing invertebrates such as water striders, lunged snails and adult beetles do not depend on dissolved oxygen and therefore have limited use as bio-indicators of pollution. They may be able to live in oxygen poor water by breathing with surface air.

"Chemical measurements are like taking snapshots of the ecosystem, whereas biological measurements are like making a videotape."

- Professor David M. Rosenberg
University of Manitoba

Pollution Tolerance Index

Invertebrates can be assigned to three groups depending on their tolerance to organic nutrient pollution. In this way, the presence or absence of a particular invertebrate is a bio-indicator of water quality.

After collecting, identifying and counting invertebrate samples, the results can be checked against the pollution tolerance index. A majority of invertebrates tending falling into any one category will indicate a certain level of water quality.

Organic Pollution Tolerance Index

For Aquatic Macroinvertebrates

Decreasing Pollution Tolerance --->

Pollution Tolerant


Pollution Intolerant

Increasing Water Quality --->

Presence in great numbers may indicate poor water quality but can be found in any type of water

Presence in great numbers may indicate fair water quality

Presence in great numbers may indicate good water quality

blackfly larvae
midge larvae
round worms
lunged snails

caddisfly larvae
cranefly larvae
dragonfly nymphs
damselfly nymphs
gilled snails

mayfly nymphs
stonefly nymphs

Seasonal Considerations

The numbers and types of invertebrates can change according to the seasons without any relationship to pollution. When analyzing invertebrate data, keep in mind these seasonal considerations:


  • Unusual scouring by high water flows, ice dams or truck loads of snow dumped into rivers may cause a decrease in invertebrate populations.
  • Spring invertebrates tend to be easier to see, easier to count and easier to identify because of their low numbers and large size.


  • Rivers are shallow in the autumn, making it easier for people to wade-in and collect invertebrates.
  • Reproduction over the summer gives rise to higher invertebrate populations.
  • Invertebrate samples collected in the autumn tend to contain higher numbers of organisms.
  • Autumn immature insects are smaller and more difficult to identify.

River Characteristics

The characteristics of a river can change naturally without the influence of human activity or pollution. River characteristics can affect invertebrate populations without necessarily indicating the occurrence of organic pollution. When analyzing invertebrate data, keep in mind the effects of these river characteristics:

  • Rivers tend to age or mature naturally as they flow toward the ocean from cold mountain headwaters and out across the warm prairies.
  • Generally, the downstream sections of a river become warmer, slower, deeper, more nutrient-rich, more turbid and muddier. Some invertebrates prefer warm water.
  • Downstream sections of a river are more likely to support organisms typical of fair or poor water quality.
  • The type of river bottom (substrate) can affect the numbers and types of invertebrates. Rivers with muddy bottoms are more likely to support organisms typical of fair or poor water quality.

So, how do you tell if "older" rivers are polluted? That's a good question! The answer probably lies in evaluating a number of factors such as bacteria and surrounding land-use.

Drawing Conclusions

After collecting, identifying and counting aquatic invertebrate samples, graphing the results will help illustrate differences or changes.

Differences in the graphed invertebrate data may be caused by the time of day, the season, the type of bottom substrate or by pollution. Were the differences natural or the result of human impact?

The following questions may help with the analysis of invertebrate data:

  • How much of the river was affected?
  • Were differences measured on both sides of the river?
  • Were all organisms affected or just specific types?
  • Were the differences related to the seasons?
  • Did each sample site have the same type of substrate and flow rate?
  • Were the invertebrates sampled with the same type of equipment and care?
  • Is there any chemical data corresponding to the invertebrate changes?
  • Were there any unusual smells detected in the area?
  • Did previous sampling, floods or snow dumps disturb the study site?
  • How is the surrounding land being used?