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A User Guide for the Macroinvertebrate Community Index

Acknowledgements

Part 1: Background to Biotic Indices

1 Introduction

2 What are biotic indices?

3 New Zealand's MCI-type biotic indices

Part 2: Guidelines for using the MCI, QMCI and SQMCI

References

Appendix 1: Calculating the MCI: Excel Macros and User-defined Functions

Appendix 2: Use of MCI, QMCI and SQMCI Throughout New Zealand

2 What are biotic indices?

Traditionally, stream quality or "health" assessments were based on analysing water quality and focused on chemical data. The problem was that these measures reflect only the conditions at the moment the sample is taken, and only a defined set of parameters. In contrast, most macroinvertebrates (e.g. mayflies, caddisflies, true flies, snails) possess a life cycle of at least a year or more, do not move great distances, and are more or less confined to the area of stream being sampled. The macroinvertebrate community of a stream lives with the stresses and changes that occur in the aquatic environment, whatever their cause, including those that are due to human activities (such as nutrient enrichment from diffuse and point-source discharges) as well as natural events such as floods and droughts. They are ideal candidates for "biotic" (rather than chemical) measures of stream health.

Biological data can be complex and difficult to understand for laypeople, so various "biotic indices" have been developed to make them easier to understand. Biotic indices rely on the fact that biological communities are a product of their environment, in that different kinds of organisms have different habitat preferences and pollution tolerances. So when an organic effluent is discharged into a stream, intolerant organisms reduce in numbers or disappear, while those that can tolerate such stresses increase in number.

This principle is well illustrated in Figure 1, which shows three sites on the Waiongana River in Taranaki that were subjected to enrichment and pollution from both diffuse and point sources (e.g. dairy farmland, dairy factory, and a piggery) in 1981. The response of the stream macroinvertebrate community is shown visually by photographs of the macroinvertebrates collected in a hand-net sample from each site. The MCI quantifies the stream condition with a single number.

Figure 1: Three sites on the Waiongana River in Taranaki, sampled in October 1981, showing the macroinvertebrate community and MCI response to increasing enrichment

A single number that characterises the stream community is useful to the specialist biologist as well as to those non-specialists charged with managing stream health. Raw macroinvertebrate data are lists of scientific names and counts or relative abundances, which are meaningless to most people. A biotic index provides a single number that summarises this complexity (albeit with some loss of information), provides a measure of stream health, and can be related statistically to a wide range of physical, chemical, and biological measures. These relationships are fundamental to understanding how ecosystems work and respond to stressors. Although methods that use a number of variables provide one way to manage this complexity, a single index value has been shown to work well both in New Zealand and overseas. This simplicity has allowed the MCI to double as a tool for scientists to characterise complexity, and as a measure of stream health that is easily understood by non-scientists.

To give a formal definition, biotic indices are numerical expressions coded according to the presence of bioindicators differing in their sensitivity to environmental conditions (Graca and Coimbra 1998). They generally are specific to a type of pollution (usually organic enrichment). They involve assigning tolerance values to various types of organisms (or taxa), based on either generally accepted organism sensitivities to pollution and habitat disturbance (BMWP 1978), or on calculations based on the distribution of taxa at a range of stream sites, grouped (or ranked) according to the degree of human impact (Stark 1985; Chessman et al 1997; Chessman 2003; Stark and Maxted 2004, 2007). Because biotic indices incorporate the pollution tolerances of indigenous taxa, they are regionally specific.

Most of New Zealand's freshwater macroinvertebrates are not found in other countries, so we cannot apply any of the biotic indices developed overseas in this country without first deriving tolerance values for local taxa. It is worth noting that tolerance values (Hilsenhoff 1977, 1987, 1988) have been variously referred to as "taxon scores" (Armitage et al 1983; Stark 1985, 1993b, 1998), "quality values" (Chutter 1972), or "sensitivity grade numbers" (Chessman et al 1997; Chessman 2003).

Biotic indices such as New Zealand's MCI (and its variants, see Stark 1985, 1993b, 1998; Stark and Maxted 2004, 2007) can be thought of as indicator species applied at the community level. An indicator species is one that is taken to be a measure of stream health. To a large extent, biotic indices were developed to overcome particular shortcomings of the indicator species approach. We know, for example, that good populations of the spiral-cased caddisfly Helicopsyche indicate that a stony stream is in excellent health, and that the mayfly Zephlebia is an indicator of a healthy soft-bottomed stream. However, there are healthy stony- and soft-bottomed streams that do not support populations of these taxa. Conversely, red bloodworm midge larvae (Chironomus) and tubificid oligochaetes are indicators of grossly enriched conditions, but they are not found in all highly polluted places and are also found occasionally (generally in low numbers) in high-quality environments. Another problem arises when the particular indicator organism is not found. This is not to say it was not present - just that it was not collected in samples − so in this case the indicator organism approach tells us nothing about stream condition.

Macroinvertebrates are found in almost all aquatic habitats, so by assessing entire communities, rather than one or two indicator species, whatever species are present can be used to convey information about the health of their habitats. There is no doubt that well-performing biotic indices could be produced based on a subset of the entire community, but the philosophy and value behind the Macroinvertebrate Community Index, as its name implies, is that the assessment is based on the entire macroinvertebrate community.