Attached algae are a natural component of many river ecosystems, playing a role in primary production providing food and habitat for invertebrates and fish. However, excessive growth of filamentous algae can occur in response to nutrient enrichment, altered flow regimes, or other environmental stressors. These growths can have detrimental effects on water quality, other aquatic life, and recreational uses of rivers. This means that monitoring and assessing filamentous algae can help us to understand the ecological health of rivers.
Advantages of using filamentous algae as biomonitors
Filamentous algae possess several characteristics that make them well-suited for biomonitoring:
- Widespread distribution: Filamentous algae are ubiquitous in rivers and streams across the UK,
- Rapid response to environmental changes: Algal growth and community composition are highly sensitive to changes in environmental parameters such as nutrient concentrations, shade, pH, and flow regime. This allows them to provide an early warning of environmental degradation.
- Relatively simple identification: Many common filamentous algae species can either be identified or placed into broad ecologically-relevant groups in the field using a hand lens and with basic microscopic techniques, reducing the need for specialised taxonomic expertise.
- Cost-effectiveness: Algal surveying and analysis can be less expensive and time consuming than traditional chemical and macroinvertebrate monitoring methods.
- Integration of environmental conditions: Algal communities reflect the integrated effects of environmental conditions at a location over time and, together with macroinvertebrates, can provide a more holistic assessment of river health than snapshot chemical measurements.
- Key part of the food web: Together with bacteria and other microscopic organisms they form the base of the food web in rivers and provide energy for higher trophic levels such as invertebrates and fish.
How are filamentous algae used?
Changes in the diversity and abundance of the filamentous algae community reflect responses to changes in water quality, especially nutrient concentrations, flow variability, water temperature, ambient light and habitat structure. These changes may be driven by changes in the catchment or river restoration, water abstraction and climate change.
What methods are available to use freshwater algae for river monitoring in the UK?
Algae offer a valuable tool for river monitoring with multiple uses:
- Assessment of biological and river health: Algal communities, together with other biological and environmental information contribute to an integrated assessment of the health of a river.
- Assessments of catchments with samples taken throughout a catchment: This can identify and highlight hotspots of nutrient enrichment, habitat degradation or flow issues. Management and restoration actions (such as nutrient removal from sewage effluents or improvements to agricultural practices) can then be identified and long-term monitoring can then provide a measure of the success of these actions.
- Above and below river restorations schemes or water abstraction schemes: algal communities respond relatively quickly to changes in their environments and are therefore good indicators of changes as a result of river restoration projects or alterations to abstraction.
Where and why to use algal monitoring?
Several metrics and indicators have been developed to quantify responses of attached algae including assessment of cover, community composition and diversity.
Several different methods are used around the world. Many of these focus on diatoms, a group of microscopic algae; however, assessments of algae visible to the naked eye are often included in surveys of macrophytes (higher plants) [including the UK macrophyte method, LEAFPACS – https://www.wfduk.org/resources/rivers-macrophytes. A few countries – notably Germany and Norway – have developed methods that focus solely on algae other than diatoms.
Most of these metrics, indicators and scoring systems have been developed by academics and regulators for professional use and there are no methods developed specifically for use by citizen scientists or volunteers.
To effectively utilise algae for river monitoring in the UK, several practical considerations must be addressed:
- Survey design: A well-designed sampling strategy is essential to ensure that the data collected are representative of the river reach being assessed. Factors to consider include the number of sampling sites, the frequency of sampling, and the method of sample collection. We also need to understand the inherent spatial and temporal variability of algae in a river.
- Species identification: Reliable identification of algae is critical for reliable biomonitoring. At a basic level this requires some training in algal identification and use of a hand lens to complement what can be seen with the naked eye.
- Collection of other environmental data: The contemporaneous collection of environmental variables such as flow, extent of shading, temperature, substrate composition and water quality parameters helps us to understand what may be shaping the algal community at a location.
- Data analysis: The abundance of different algal types or taxa should be estimated or quantified.
- Integration with other monitoring data: Algal data should be integrated with other monitoring data, such as chemical analysis, flow estimates, habitat and macroinvertebrate surveys, to provide a more comprehensive assessment of river health.
- Reference conditions: Knowing what to expect for algal communities in different types of rivers is essential for interpreting monitoring data and assessing the impact of anthropogenic stressors.
- Training and standardisation: Standardised protocols for surveying, the level of identification required, and data analysis are needed to ensure the comparability of data collected by different researchers, agencies or volunteers across different catchments. Training programs should be developed to build capacity in algal assessment.
Methods include semi-quantitative methods (surveying an area of the river and estimating abundance or cover and composition of the algal community) and quantitative methods (measuring chlorophyll a or biomass in a known area or quadrat).
Professional methods use standardised sampling procedures with laboratory-based identification using microscopes and detailed identification keys, or metabarcoding.
Semi-quantitative methods are suited to citizen science initiatives and methods allow for rapid assessment of algal communities. One example of a citizen science method is RAPPER-CS:
The Rapid Assessment of PeriPhyton in Rivers (RAPPER) method was developed for use by The Scottish Environment Protection Agency (SEPA) and other UK statutory agencies for assessing attached algae in rivers (Kelly et. al., 2016). This method was simplified further for the Local Authorities Water Programme in Ireland and this simplified method was tested and refined as part of an undergraduate project at Lancaster University (Woodfield et al, 2023) in North-West England.
Trials of this Citizen Science method across six catchments in England and Wales with volunteers from local Rivers and Wildlife Trusts were carried out during the summer of 2025 and the method was further revised following feedback from volunteers (RAPPER report, 2025). The trials, although limited, were considered to be a success as 1) volunteers were able to carry out the monitoring and 2) it provided results across 6 differing catchments and 3) the limited results mostly agreed with assessments made by professional biologists working in the UK’s statutory agencies.
Challenges and Limitations
While filamentous algae are a valuable tool for river monitoring, there are also some challenges and limitations to consider:
- Seasonal and inter year variability: Algal communities can exhibit significant seasonal and inter year variability, which can complicate data interpretation.
- Influence of natural factors: Algal growth and community composition can be influenced by natural factors such as light availability, temperature, and flow regime, which can make it difficult to isolate the effects of anthropogenic stressors.
- Lack of standardised protocols: While some protocols exist, there is a need for more standardised protocols for sampling, identification, and data analysis to ensure the comparability of data across different studies.
- Public perception: The presence of excessive algal growth can be perceived negatively by the public, even when it is not necessarily indicative of poor water quality. Citizen Science methods such as RAPPER-CS help people to better understand their local rivers and can be useful in driving local advocacy and evidence-based actions.
- Taxonomic expertise: For professional monitoring, accurate identification of algal species often requires specialised taxonomic expertise and the use of a microscope, which may not be readily available to citizen scientists.
Summary
Algae are already a valuable tool for assessing the ecological health of rivers in the UK and now their use can be extended to the growing band of “citizen science” volunteers. Their widespread distribution, rapid response to environmental changes, and cost-effectiveness make them an attractive alternative or complement to traditional monitoring methods. However, successful implementation of algae-based monitoring programs requires careful consideration of survey design, identification level, data analysis, and integration with other evidence. Addressing the challenges and limitations associated with this approach will be crucial for realising its full potential and contributing to more effective river management in the UK.
There is great potential to further roll out and establish nationwide rapid citizen science method such as RAPPER-CS to complement the monitoring carried out by professional biologists in regulatory agencies and other bodies. RAPPER-CS permits assessments at a greater temporal and spatial intensity than is possible for statutory monitoring and this is extremely important as conditions can change so much over the course of a summer and between years.
Further monitoring and data analysis is still needed to improve our understanding of the factors influencing algal communities in UK rivers.
References
Kelly, M. G., Krokowski, J., & Harding, J. P. C. (2016). RAPPER: A new method for rapid assessment of macroalgae as a complement to diatom-based assessments of ecological status. Science of the Total Environment, 568, 536-545. https://doi.org/10.1016/j.scitotenv.2015.12.068
Woodfield, K., Kelly, M.G., Brierley, S.J. & White, B. (2023). Feeling our way towards macroalgal assessments for citizen scientists. https://www.fba.org.uk/articles/feeling-our-way-towards-macroalgal-assessment-for-citizen-scientists.
References
RAPPER (Rapid Assessment of PeriPhyton in Rivers): trials with citizen scientists in six UK catchments (September 2025).
RAPPER – Rapid Assessment of Periphyton Ecology in Rivers: a manual for Citizen Scientists. (September 2025). Martyn Kelly and Bill Brierley.
Algal Survey sheet – RAPPER v3 September 2025.
Three blog posts (microscopesandmonsters) by Martyn Kelly and Bill Brierley:
The greening of our rivers (1) …… (November 2025)