The ‘other’ metrics

One of the greatest benefits of invertebrate monitoring is the ability to use the results to calculate values for river health. When we receive your SmartRivers invertebrate data, we generate a water quality ‘scorecard’ for your sites, made up of these values. 

Typically, policing of our inland rivers mostly relies on family-level invertebrate analysis of watercourse sites and/or family level metrics with no species biodiversity targets. SmartRivers goes beyond this, analysing invertebrates at a higher resolution, allowing the generation of species-level metrics that can indicate specific water quality pressures. Examples of these metrics are PSI for sediment pressure and SPEAR for chemical pressure (more detail on these here).

There are a huge number of different metrics available. As well as the species-level pressure metrics, we calculate other, more general water quality metrics from SmartRivers data. If you’ve viewed your data on the database, you’ve probably noticed these additional abbreviations. To help you understand a bit more about these metrics and what they indicate, we’ve put together some information below.

Examples of other metrics on the database indicated by green arrows
Examples of other metrics on the database indicated by green arrows

BWMP (Biological Monitoring Working Party) Score

BMWP is an older metric which has been used for a long time as a general index of biological quality.

Invertebrates, mostly at family-level, all have scores related to organic pollution sensitivity (wastewater discharges from cities and intensive livestock farms constitute the main organic pollutant loads into rivers). The invertebrate scores range from 1 to 10, with 10 being the least tolerant. More ‘low tolerance’ scoring taxa in a sample, means a higher cumulative score and indicates better water quality.

High scores represent both tolerance and taxonomic richness (good diversity of invertebrates, which is a key component of a healthy river). However, abundance or resolution to species level is not taken into account in the metric. Plus, each taxa only scores once.

 

ASPT (Average Score Per Taxon)

ASPT is calculated from BMWP.

It is obtained by dividing the BMWP score by the number of invertebrates that were assigned a tolerance score. An index from 0 - 10 is generated.

This score is based on the concept that a river under stress will be less diverse and support a lower number of scoring invertebrates. 

BMWP/ASPT gradings

WHPT (Walley Hawkes Paisley Trigg) Index

WHPT index values are on the same scale as BMWP and respond to the same pressures (organic pollution) but are thought to be more accurate as they are derived from analysis of a very large set of field results, rather than just expert judgement.

It should be remembered that this metric still essentially remains only at family-level versus other species and abundance level metrics for assessing organic enrichment like the Saprobic Index. 

There are no direct observed water quality bandings for WHPT, unlike BMWP and the score relies on Observed:Expected ratio values generated from family-level faunal data via predictive modelling software called RIVPACS/RICT (River Invertebrate Classification Tool).

 

CCI (Community Conservation Index)

CCI scores take into account both the richness and rarity of invertebrate species present.

Conservation scores are assigned to species based on definitions (outlined in Chad & Extence 2004). The scores are added together and divided by the number of scoring invertebrates. The figure is then multiplied by a community score which is based on the highest conservation value species, or the BMWP score, depending on which gives a higher value.

Although CCI is a helpful tool to assist in value judgement, it must never be allowed to preclude expert opinion.

CCI grading

EPT (sp)

A simplified index that adds together the numbers of species from the following groups: Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies).

As these groups are more sensitive to pollution, a greater number of these species indicates higher water quality.

An expert water quality interpretation of EPT scores
An expert water quality interpretation of EPT scores