Publication - Advice and guidance

Cyanobacteria (Blue-Green Algae) in Inland and Inshore Waters: Assessment and Minimisation of Risks to Public Health

Published: 12 Apr 2012
Part of:
Health and social care

Guidance to Directors of Public

Health, to Heads of Environmental Health in Local Authorities (LAs), and to others in

Scotland, on possible risks to public health of cyanobacteria (blue-green algae) in inland

and inshore waters. It updates previous guidance under the same title that was published

by the Scottish Government Health Directorate previously in 2002, and revised in 2007.

Cyanobacteria (Blue-Green Algae) in Inland and Inshore Waters: Assessment and Minimisation of Risks to Public Health


General provisions

5.1 Wherever possible, assessment of cyanobacterial populations should be co-ordinated by those with relevant interests. The principal aim should be to identify any need for further action. Provisions for assessments should be defined in the LAP and should include, as appropriate, procedures for visual inspection of the site, monitoring (sampling and analysis), and reporting and assessment of results.

Defining an assessment programme

5.2 The factors; physical, chemical, biological, and climatic, that lead to the development of cyanobacterial blooms in inland waters (or to the growth and detachment of algal mats) are complex. However, for each individual waterbody, the frequency, duration and magnitude of such blooms often (though by no means always) follow a predictable annual cycle.

5.3 In Scotland most blooms occur between April and October, though in some waters, blooms can occur outside this period. Therefore, where activities with a high risk of human exposure take place throughout the year, the possibility of continuing inspection and/or monitoring activities for example from November to March, should be considered. The Bathing Water Directive requires that a profile of each designated inland and coastal site bathing water shall be undertaken to include identification of those bathing waters deemed to be susceptible to cyanobacterial proliferation. The Directive also includes a particular obligation to ensure that any risks identified are properly managed during the official bathing season, which in Scotland runs annually from 1 June to 15 September. This task will be undertaken by SEPA.

5.4 The frequency, duration, and magnitude of blooms will affect the needs for inspection and monitoring. For present guidance purposes, it is therefore useful to categorise waters in accordance with these "occurrence" related factors, as shown in Table 5.1.

5.5 Column 3 of Table 5.1 gives general indications for the likely efficacy and information yield of monitoring for waters in each of the four categories. As a general rule, planned regular monitoring of cyanobacterial cell populations for waters in Categories 1 and 4 will add little to existing knowledge. Planned monitoring might be of use for waters in Categories 2 or 3 but in circumstances where monitoring at fixed time intervals is likely to miss blooms, only frequent visual inspection and monitoring is useful.

Table 5.1:Categorisation of waters in terms of the frequency and intensity of algal blooms.

Column 3 indicates the likely benefits of monitoring and inspections.

Category Description of algal blooms Implications for planned regular monitoring
1 Waters that consistently contain large populations of cyanobacteria for many months in every year Not indicated. Will add little to what is already known.
2 Waters that have algal blooms for short periods in most years Of value, depending upon use made of waterbody. However, the ability to detect short-lived blooms will depend on frequency of monitoring and some might be missed.
3 Waters that have only intermittent algal blooms in occasional year. Of value depending upon use made of waterbody. Frequent sampling will yield many negatives and less frequent sampling might fail to detect short blooms.
4 Waters that never have algal blooms Not indicated. All samples likely to be negative.

5.6 Decisions on the frequency of inspections and/or monitoring will also depend on other factors relating to the nature and frequency of use of the water in question. Circumstances will vary widely but the following general indications might apply:

i. Where exposure via recreational waters (including designated bathing waters) might be predictably persistent or recurring (Categories 1 or 2), it might be appropriate to provide permanent or semi-permanent warning notices, and to carry out inspections or monitoring to determine the beginning and end of the period of the hazard.

ii. For Category 3 recreational waters and for designated bathing waters visual inspection at intervals determined by the use of the water, with or without sampling, should be carried out.

iii. For waters in Categories 3 or 4 where blooms have been transient and infrequent and where there is limited scope for exposure of people or animals, frequent visual inspection and sampling are unlikely to be cost-effective.

5.7 In general, therefore, monitoring and inspection requirements should be determined by local circumstances and should be defined in the LAP.

Visual inspections

5.8 Visual inspections for cyanobacteria should record the appearance and apparent concentration of cyanobacteria in the water or on shorelines. This will normally involve collection of samples of water from the furthest extent of reach from the shore using a suitable container. The presence of cyanobacteria in the samples should be assessed, as distinct from general turbidity, by their characteristic colour and possible particulate appearance. Where possible, samples from a number of points including the lee and windward shores should be assessed and recorded in this way. Annex E gives further guidance on recognition and identification of cyanobacterial blooms.

5.9 Visual inspections should particularly note the presence and quantity of cyanobacterial scums on the surface of the water and of stranded scums and mats on the shorelines.

Sampling and analyses

5.10 Methods of sampling and analysis should, wherever possible, follow guidelines provided by SEPA (Annex E).

5.11 In some circumstances, depending on the uses made of the body of water, proactive or reactive monitoring for the toxins themselves should be considered. This is also considered in Annex E.

5.12 In incidents involving human or animal exposure and reports of associated illness where cyanobacterial poisoning is suspected as a cause or contributing factor, sample collection and analysis for toxins should be carried out to assist identification or elimination.

5.13 SEPA can provide an analytical service to identify and quantify algal blooms (including cyanobacterial blooms) in samples taken by others from recreational waters by others, SEPA itself has specific duties regarding sampling of bathing waters. Scottish Water can also provide a similar service, including sampling of potable supplies that are thought to be at risk.

5.14 The LAP should define procedures for local assessment, recording, reporting, and storage of monitoring data. For public drinking water, Scottish Water should provide customers or users with appropriate information to minimise any health risk from exposure to cyanotoxins. For private water supplies, the owners and users would rely on the Environmental Health Department of the local council and SEPA for advice. For bathing waters, SEPA or the beach owner/operator should provide bathers with appropriate information to minimise the health risk from exposure to the toxins.

5.15 Results from investigations should also ideally be reported, along with details of the incident, to HPS via the electronic surveillance system, SEISS. Given predictions regarding the potential increase in cyanobacterial incidents associated with climate change, an improved capacity to monitor trends in incidents will become increasingly important. The completeness of incident-reporting via SEISS could be improved however. As SEISS is the only multi-agency reporting system covering Scotland, ideally incidents associated with water bodies should be reported and should use the risk categorisation criteria set out in Table 6.1 (high, medium, low) to improve the quality and usefulness of the data. Improved reporting would enable improved tracking of trends and would help in assessing if predictions on climate change impacts are accurate.


Email: Janet Sneddon