What a stream can tell you

A water test tells you about one moment. The creatures living on the stream bed tell you about the last few weeks and months, because they have been living in that water the whole time. That makes them one of the best clues to a stream's health.

Macro what?

Scientists call them macroinvertebrates. Macro means big enough to see without a microscope. Invertebrate means an animal without a backbone. Most are young insects (larvae or nymphs) that will one day fly away as adults, plus snails, worms and freshwater crayfish (kōura).

Sensitive and tolerant

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): each kind of creature has a number, and a high total points to a healthy stream. Community groups and schools use a simpler version in NIWA's Stream Health Monitoring and Assessment Kit (SHMAK).

Why a stream changes

Streams warm up when the trees along the banks are cleared, and warm water holds less oxygen. Soil washes in after rain and clouds the water, smothering the gaps between stones where larvae hide. Fertiliser and animal waste feed slimy algae, and when the algae die and rot, oxygen drops even further. Each change pushes sensitive creatures out, and the fish and birds that eat them go too.

Being a fair observer

Sample the same spot, the same way, for the same time. Repeat it at least three times. Write down the weather, because a flood can wash creatures downstream for days and make a healthy stream look empty. And always return every creature where you found it.