Background reading · Science

What a stream can tell you

Stream creatures are living measuring instruments. Here is how to read them.

A single water test tells you about one moment. The creatures living on the stream bed tell you about the last few weeks and months. That makes them one of the best clues to a stream's health.

Sensitive and tolerant

Scientists sort stream invertebrates (animals without backbones) by how much pollution they can cope with.

Sensitive: mayfly, stonefly and many caddisfly larvae need cool, clear water with plenty of oxygen.

In between: some snails, beetles and freshwater shrimp.

Tolerant: worms, midge larvae and some snails can live in warm, muddy, low-oxygen water.

Lots of sensitive creatures usually means healthy water. Only tolerant ones usually means a problem. NZ scientists turn this into a score called the Macroinvertebrate Community Index (MCI).

Why the water changes

Streams warm up when trees along the banks are removed. Soil washes in after rain and clouds the water. Fertiliser and animal waste feed algae, and when the algae die and rot, oxygen drops. Each of these changes pushes sensitive creatures out.

Being a fair observer

Sample at the same spot, the same way, for the same time. Repeat it. Note the weather, because heavy rain changes everything for a few days.

Sources and further reading

Written for Kōkiri Learn students in our own words. Check facts against the sources.

Used in: Ngā Tohu o te Taiao: Signs in Our Environment, Te Pūnaha Hauropi: Investigating Ecosystem Change