The Macroinvertebrate Community Index (MCI) is a tool used in New Zealand to measure the health of freshwater streams. Scientists look at the types and numbers of macroinvertebrates, such as insects, worms, and snails, in a stream to understand how clean or polluted the water is. Each species is given a score based on how sensitive it is to pollution. The MCI calculates an average score by adding these numbers. A higher MCI score usually means the stream is healthier.

The MCI works by assigning scores to macroinvertebrates based on how well they can survive in polluted water. Species found in very clean water get higher scores than those in polluted areas. The MCI can be calculated using this formula:

MCI = (site score ÷ number of scoring species) × 20

In the past, water quality was measured using chemical tests and habitat studies. However, these methods are not always reliable because pollution can come from many sources, like farms or cities. Living things in water, such as macroinvertebrates, can show how healthy an ecosystem is. They respond to changes like too much dirt, city growth, farming, or cutting down trees. When pollution harms a stream, some species may disappear, and pollution-tolerant species, like certain worms and flies, may become more common. Fewer types of species and lower MCI scores can signal poor water quality. Streams in low areas with lots of farming or cities are most at risk of poor water quality.

Macroinvertebrates are often used to check water health because they are easy to find and identify. They also live short lives, so they react quickly to changes in their environment. The MCI method is widely used around the world because it is reliable and effective.

There are two other versions of the MCI: the Quantitative Macroinvertebrate Community Index (QMCI) and the Semi-Quantitative Macroinvertebrate Community Index (SQMCI). Both are used in New Zealand. The QMCI was created to measure how much organic waste is in water. The SQMCI was developed to make testing easier by requiring fewer samples. Both QMCI and SQMCI use a scale from 1 (very pollution-tolerant) to 10 (very pollution-sensitive). Unlike the MCI, which uses presence-absence data, the QMCI uses numbers or percentages. Scientists are still deciding when each version is best to use. A study found that the MCI is better than the QMCI for tracking changes in water quality over time.

The Ministry for the Environment in New Zealand has suggested using all three indexes—MCI, QMCI, and another called Average Score Per Metric (ASPM)—to measure water quality.

QMCI is calculated using this formula:

QMCI = (sum of (number of each species × its pollution score)) ÷ total number of species

SQMCI is calculated the same way, but instead of counting every individual, scientists use estimates of how many are present.

Several factors can affect MCI results. Streams with hard surfaces, like rocks, and soft surfaces, like mud, may need different versions of the MCI because the types of species and their pollution scores differ.

Changes in the stream’s shape, like areas with fast-moving water or deep pools, can also influence MCI results. However, studies show that MCI and QMCI results are not affected by water depth, how fast the water flows, or the type of streambed. This makes the indexes more reliable for testing.

Seasons also affect MCI results. Scientists recommend using data collected during the same season as the reference data for accurate comparisons. Warmer seasons, like summer, can lower water quality by increasing water temperature and reducing oxygen levels. This makes it harder for macroinvertebrates to survive, which changes the MCI results. Other factors, such as water temperature, the life cycles of invertebrates, and oxygen levels in the water, also influence seasonal changes.