What are freshwater macroinvertebrates?
Invertebrates are animals which do not have a backbone. Macroinvertebrates are invertebrates which are visible (microinvertebrates are those which are invisible) to the naked eye but smaller than 50mm (megainvertebrates are larger than 50mm). Macroinvertebrates form an important link in stream food-chains providing a major source of food for freshwater fish including whitebait and trout.
- insect larvae
Insect Larvae, the most diverse range are juvenile insects which will eventually emerge as winged adults, e.g. mayflies, spend most of their life in the larval stage, feeding and growing in the stream for up to a year when they finally emerge as adults simply to reproduce, disperse and live for only a few days.
Why does Bay of Plenty Regional Council survey the macroinvertebrate communities?
Benthic (or bottom dwelling) macroinvertebrate communities provide a basis for the ecological monitoring of stream using the Macroinvertebrate Community Index (MCI).
Macroinvertebrates are suited to environmental surveying because they:
- are numerous
- are easily sampled
- are not very mobile
- have life cycles of a year or more.
The MCI can indicate the health of a waterway depending on the numbers and types of macroinvertebrates found during a survey. Each type of macroinvertebrate is given a value from 1 to 10 where 1 means it is capable of living in a less desirable environment with low oxygen, high nutrient levels, low clarity, etc while organisms with a value of 10 will only live in pristine stream environments.
Unmodified riverine systems tend to be dominated by insect larvae belong to a few common genera including:
- Ephemeroptera - mayflies
- Trichoptera - caddisflies
- Plecoptera - stoneflies.
Modified riverine systems tend to have higher proportions of:
- Diptera - two-winged flies; and
- non-insect groups such as snails and worms.
Macroinvertebrates have advantages over spot chemical sampling because they integrate the effects of a wide range of potential pollutants over a long period of time.
How does Bay of Plenty Regional Council survey macroinvertebrate communities?
Two methods are used to survey macroinvertebrate communities:
1. The river ecology programme which focuses on invertebrates
inhabiting fast flowing cobble riffles, primarily in the larger
streams and rivers.
sampling is targeted at riffle areas that are moderately shallow
- five replicate samples are collected from each site
- the sampler is positioned at random in the riffle
- water depth, and flow are recorded
- substrate type is described
- the underlying substrate is stirred vigorously for one minute to a depth of 10cm
2. The stream ecology programme which focuses on smaller streams and invertebrates living in a wider range of habitats. This programme is still undergoing development.
- a dip-net is used to optimise estimates of diversity rather than abundance for each sample
- sites concentrate on current or potential high resource use areas as well as reference sites to enable comparisons
- the physical features of the habitat are assessed including woody debris and cobbles
- impact sites are compared to forested streams within the same ecological district.