Biomonitoring Working Party index (BMWP)

Is a measure of water quality generated through the analysis of macroinvertebrate species within a river system as biological indicators for the overall quality of a fluvial system. 

Background theory

This is a method of biological assessment as different species have different tolerances to organic pollution (OP). As such different species are given a tolerance score depending on the environment that they are able to survive in such as mayflies and stoneflies that are given a score 10 as they indicate the cleanest waterway while species that have a higher tolerance to for OP such as worms are given a value of 1. The biodiversity within the water system is a highly influential factor as the more diverse the number of species sampled within the channel it can be assumed that the system contains fewer pollutants as the pollution sensitive species.

Methodology

Fieldwork

Systematic sampling is used along the course of a river taking samples from within the river course at specifically designed intervals. At each point a GPS position is taken and kick sampling is used where the bed of the river is agitated using your foot, for a period of 3 minutes, with a net placed downstream. Any invertebrates collected are transferred to sample bags where they are preserved in alcohol solution. If any invasive species are identified within the stream it must be noted and removed from the system and a failure can be noted for the biological quality of the system. 

*Always complete an adequate risk assessment before conducting field research to prevent any serious injuries/issues from occurring

Laboratory

The collected sample is taken to a laboratory where the species are identified using a microscope, invertebrate identification guide and the BMWP index to assign scores to the species within. The BMWP score equals the sum of the tolerance scores of all macroinvertebrate families in the sample. This data can then be combined with each of the GPS positions taken during fieldwork to show the changes in the ecological health of a freshwater system.

Statistical Analysis

Descriptive statistics can be generated through analysing all of the BMWP scores at each sample site as a single data set that can provide insight into the general information related to environmental health, such as the Mean, Standard deviation and range of the data. As well as comparing the BMWP score to the standards for ecological status to interpret the health of a fluvial system,

The BMWP can be further analysed to calculate the Ecological Quality Index (EQI), a value that represents a more comprehensive estimate of environmental quality. This is calculated by dividing the Observed BMWP score for each sample with a BMWP of a pristine water system, generally assumed to be a BMWP of 85. This allows for greater understanding of the environmental health of a river system.

Both the BMWP and EQI can then be further analysed through the production of graphical outputs, Bar charts, where the limits for ecological status can be used to provide a visual interpretation of the environmental quality of the river system. It can be assumed that the range for Good Ecological status (GES) in relation to the EQI ranges between 0.7 and 0.84.