Macroinvertebrates

waterbugsLots of tiny water animals live on and under the rocks, on water plants or on bits of wood lying in the stream. These animals include insects, crustaceans, molluscs, worms and leeches. Collectively these creatures are called macroinvertebrates – insects without backbones, which can be seen without the help of a magnifying glass or microscope.

 

What a name!
Lots of tiny water animals live on and under the rocks, on water plants or on bits of wood lying in the stream. These animals include insects, crustaceans, molluscs, worms and leeches. Collectively these creatures are called macroinvertebrates – insects without backbones, which can be seen without the help of a magnifying glass or microscope.

You wouldnt think it at first, but these animals can be very helpful, and tell us a lot about the surrounding environment, including water quality, or what kind of pollution is affecting our streams and lakes.

What do they look like?
They are all tiny – 3mm to 20mm long – and they are difficult to see at first glance.

Where do they live?
Turn over a rock in a shallow fast flowing stream and you should see small insect-like creatures scuttling for cover. Pieces of submerged wood or water plants are also good hunting grounds for macroinvertebrates, but they are often harder to see in these habitats. Sampling the stream with a net is the best way to find the variety of life in your stream. The National Waterways Project has instructions on collecting your own stream invertebrates.

Observations Analysis
High diversity, lots of stoneflies, mayflies and caddisflies No problem, good water quality
Moderate diversity, few if any stoneflies, fewer mayflies (present but not common) and an abundance of caddisflies. Mild enrichment present, moderate algal growth possibly the result of less shade and increased nutrients.
Low diversity, high density, lots of scrapers and collectors Organic pollution (nutrient enrichment) or sedimentation; lots of algal growth resulting from nutrient enrichment.
Only 1 or 2 types, high number of collectors Severe organic pollution or sedimentation .
Low diversity, low density, or no “bugs”, but the stream appears clean Toxic pollution

(e.g. chlorine, acids, heavy metals, oils, pesticides), or naturally unproductive due to limited light or nutrients

Did you know?
Biologists have determined the pollution tolerance of many common macroinvertebrates, and the type and number found living in a stream can tell us a lot about stream health.

As a general guide the following observations may help you to work out the health of your stream:

Chemical testing may be used to confirm the presence and particular type of pollutant/s, although this may be unsuccessful if pollution is intermittent. The information you gain from chemical testing may also be limited by the range of tests you do. For example, a phosphate and nitrate test will not reveal anything about the level of dissolved oxygen in your stream.

What do they eat?
There are many different food sources within a stream and a macroinvertebrate with just the right adaptations to take advantage of each these. Macroinvertebrates can be separated out into groups based on their feeding habits as follows:

Shredders
Mainly large insect larvae that chew up dead leaves. They are relatively uncommon in our streams;

Browsers
These consume fine particulate matter, algae and associated bacteria, fungi and slime, which are the main components of biological films on the surfaces of stones and plants. They are by far the largest and most diverse feeding group;

Collectors
These depend on fine particles of organic matter. Tend to be more abundant in the lower catchment;
Filtering collectors
Cleverly adapted for capturing particles from flowing water, using a range of devices, including snares, nets, brushes and filtering hairs.
Gathering
Collectors that gather small sediment deposits from the stream

bottom or others substrates.
Predators
These feed on other macroinvertebrates.

Did you know?
One species of macroinvertebrate is known as a toe-biter (dobsonfly) but you will be relieved to know that toes are not the main thing on the menu. At 30mm even the largest is unlikely to threaten life or limb.