‘Canary’ of the river
- Good indicators of water quality with a wide range of sensitivities, from highly sensitive to stressors, to highly tolerant.
- Highly diverse taxonomically. Different groups are sensitive to different stressors giving a nuanced response. Using monitoring metrics can interpret changes in invertebrate diversity in terms of responses to specific stressors, including metals, acidification, organic pollution, pesticides, siltation, low flows, and road run-off.
- Macroinvertebrates are highly abundant, making them amenable to data analysis.
- Can be used to interpret a response to changes in hydromorphology.
- Easy to sample using quantitative and semi-quantitative methods.
- Relatively long-lived and present throughout the year. This means that the response of the invertebrates to a stressor is apparent in the sample for a fair amount of time after the stressor may no longer be detectable because it has passed downstream. This provides a major advantage over monitoring water quality variables unless sondes are used at high spatial and temporal frequency. Recovery time of the macroinvertebrates will indicate the severity of the stressor.
- Key part of the food web. Process algae, bacteria, detritus and make this energy available to other organisms at higher trophic levels. Therefore, declines in abundance and diversity of macroinvertebrates will have impact on entire ecosystem.
- A long history of use as indicators of river quality (tried, tested and trusted), so the pros and cons of sampling methods are well-understood and robust.
How are freshwater macroinvertebrates used?
Changes in diversity and abundance reflect responses to changes in water quality and habitat structure. These changes may be driven by pollutants in the water column to which the inverts are exposed; changes in hydromorphology (including river restoration, bankside management (either vegetation clearance leading to habitat reduction and exposure to sunlight, or from shading), water abstraction, channelisation; climate change.
Several metrics developed to quantify response, e.g. PSI (silt), TRPI (phosphate), SPEAR (pesticides), Saprobic (organics), LIFE (flow), CCI (biodiversity /conservation value), BMWP, WHPT, ASPT (used by EA), ARMI, WQS (water quality score Extended Riverfly), SFS (silt flow score Extended Riverfly). When the stressor has been identified this enables informed remedial actions.
Trigger Level is applied to ARMI scores based on the known typical abundance and diversity of the invertebrate assemblages of the sampling site. If scores fall below this trigger level the Agency is informed of a potential pollution incident.
What methods are available to use freshwater macroinvertebrates for river monitoring?
Long- and short-lists specifically for macroinvertebrate monitoring methods are under development.
- Sampling methods include timed kick-sampling (semi-quantitative, using standard-sized net bag and mesh); surber sampling (quantitative, using standard-sized frame and net bag and mesh).
- Samples sorted to specific taxonomic level depending on which monitoring metrics are to be used. The higher the taxonomic level employed by the sorting method, the higher the potential for interpreting which stressor may have driven the response.
- Citizen science methods allow bankside sorting of sample. ARMI identifies 8-taxonomic groups (mixed level from order to genus); Urban Riverfly identifies 14-taxonmic groups (mixed level order to genus); Extended Riverfly identifies 33-taxonomic groups (mixed level order to genus).
- Professional methods use lab-based sorting of preserved samples. BMWP uses family-level identification; SmartRivers and WHPT use species-level identification.
- Log abundances used for samples collected by kick-sample, except Smart Rivers which uses absolute abundance in analysis of kick-samples.
- The explanatory power of the macroinvertebrate method is improved if the method is used in conjunction with other methods that provide information on the status of variables that may be causing a response by the macroinvertebrates (e.g. methods that measure flow, silt, substrate composition, water quality variables, temperature, distribution of macrophytes and algae).
Where to use freshwater macroinvertebrate monitoring
- Above and below outfalls. A comparison of the two samples will show any downstream impacts from the outfall, and also how far the effects of the outfall can be detected downstream and the recovery time (e.g. Thompson et al. 2015). ARMI take monthly samples for early warnings of pollution. SmartRiver, BMWP, WHPT use seasonal samples.
- Above, within and below river restorations. A comparison of the results will show the effectiveness of RR in improving water quality and habitat. Seasonal or annual samples. Best to take quantitative samples as abundance changes are the most significant effect of RR. Fine mesh net bags (e.g. 200 micron) are best used as best indicators of biodiversity response tends to be chironomids, which are not captured efficiently in 1 mm mesh bags (e.g. Thompson et al. 2018)
- Samples taken throughoutthe river catchment will show any changes from upstream to downstream. It will highlight pollution hotspots and show downstream recovery. Long-term samples will show positive and/or negative trends through time and highlight the impacts of chronic pollution. Expect samples naturally to increase in abundance and diversity from upstream to downstream.
References
Brooks et al 2019. ARMI …. Freshwater Science
Murray Bligh et al 2022. Practitioners guide ….. FBA
Thompson et al 2015 FW Biol.
Thompson et al 2018 J Appl Ecol