These little creatures usually go unnoticed in streams and ponds because they live in the mud and on the edges of the aquatic world, not to mention, they are very small and camouflaged.
As the water temperatures start to rise again in spring, their little ectothermic (cold-blooded) bodies start to warm-up too. There are many different types of aquatic macro-invertebrates and some of them act as biotic indicators for water quality. Biotic indicators are living things that have special habitat requirements or are very sensitive to pollution or changes in their environments.
Their presence in an ecosystem tells us something about the habitat's overall health. Take stoneflies for example, they are senstive to oxygen levels. They need high oxygen levels to survive, so if you find
them in a river or stream, you can conclude that the dissolved oxygen is at a healthy level.
Biologists consider 3 main orders of insects to be indicators of good water quality: Mayflies, Stoneflies, and Caddisflies. All three of these organisms have their gills on the outside of their bodies as larvae, so are more sensitive to their environments. Some of them can live up to 2-3 years as larva, so their continued presence can provide a long term view of water quality.
Monitoring these creatures at least once a year can alert stewards to changes in water quality. It's a fun and simple process that attracts lots of volunteers.