We have a new website go to gov.scot

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


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

7 Action

General requirements

7.1 Actions intended to reduce the probability of acute or delayed effects of algal toxins on people or animals are directed to reducing the probability of:

i. skin contact with or ingestion of algae in, or on the shore of, inland waters;
ii. ingestion of drinking water containing algae or algal toxins;
iii. exposure to toxins by eating fish or shellfish from algae-rich waters;
iv. delivery of contaminated water to patients undergoing haemodialysis.

7.2 Of these, the 7.1.i is the most likely. Acute effects from ingestion of publicly supplied drinking water containing algal toxins are considered unlikely in Scotland due to the effects of volume dilution and also the degradation of toxins during normal water treatment processes. Scottish Water will take appropriate action to ensure the safety of supplies where algal blooms are identified. The risk of longer-term exposure to toxins from contaminated private supplies can be reduced by practical measures discussed later in this section or, if necessary, by substitution of an alternative supply or bottled water.

7.3 Particular attention should also be paid to the health risks for patients undergoing haemodialysis. In normal circumstances, algal toxins are effectively excluded by the reverse osmosis units that are used to treat the water supply to dialysis units in Scotland. However, the possible consequences of exposure to algal toxins (and indeed to other pollutants) due to system failure should be addressed. Local NHS Boards should ensure appropriate resilience for this threat.

7.4 It is prudent to consider whether muscle tissue of fish from heavily affected waters should be eaten. Should toxins be detected by analysis, expert advice will be necessary on whether concentrations are sufficient to justify restrictions on the consumption of fish. The absence of taint does not indicate the absence of toxins since there is no correlation between the production of compounds affecting taste and odour and the production of toxins by blue-green algae.

7.5 Any proposed restrictions on the use of water because of the presence of an algal bloom should be based on a careful assessment of the resulting benefits and detriments. This assessment should, among other matters, take account of the circumstances of use and of the relevant WHO guidance documents for drinking water and for recreational waters referred to in Paragraphs 7.6 and 7.7.

Triggers for action

7.6 The WHO (1998) Guidelines for Drinking Water Quality (Second Edition, Addendum to Volume 2, Health Criteria and other supporting evidence. WHO, Geneva, Switzerland) have defined a provisional value of 1 g / l of microcystin-LR, (one of the commonly found hepatotoxins, in drinking water) for drinking water that is intended for lifelong consumption.

7.7 The equivalent WHO guidance document for recreational water is the 1998 Guidelines for safe recreational-water environments (Vol. 1: Coastal and fresh-waters. WHO Geneva, Switzerland. Unpublished document EOS/Draft/98.14) which is available online at http://www.who.int/water_sanitation_health/Recreational_water/Recreawat-II.pdf)

7.8 The 1998 WHO guidelines for recreational-waters state that "Health impairments from cyanobacteria in recreational waters must be differentiated between chiefly irritative symptoms caused by unknown cyanobacterial substances and the potentially more serious hazard of exposure to high concentrations on known cyanotoxins, particularly microcystins. A single guideline value therefore is not appropriate". For recreational waters, therefore, the document recommends "a series of guideline values associated with incremental severity and probability of health effects" and these values are then defined for low, moderate or high probabilities of adverse health effects.

7.9 A copy of the relevant section from this WHO document [section 7.5 Guideline Derivation] is appended here as Annex G. The guidance levels recommended by the WHO are summarised in Column 1 of Table 7.1. However, the advice given in Column 2 of Table 7.1 differs from that in the WHO Guidance document ( Annex G) by recommending that, as an additional precaution, all four of the "typical actions" defined by the WHO for 100,000 cells cyanobacteria/ml be adopted at the lower level of 20,000 cells cyanobacteria / ml. (It should be noted here that the general equivalence implied in the final row of Table 7.1 between cell numbers and chlorophyll-a concentration (1 g chlorophyll-a per 2000 algal cells) actually depends on cell type. Also, for some types, such as filamentous algae, individual cells are not easily identified or counted. These issues are considered in more detail in Annex E.)

Table 7.1;
Guidance levels and related "typical actions" derived from current WHO guidance.

Guidance level or situation

Typical Actions

Cyanobacterial scum formation in bathing areas

  • Immediate action to control contact with scums; possible prohibition of swimming and other water-contact activities
  • Public health follow-up investigation
  • Inform public and relevant authorities

20,000 cells cyanobacteria/ml or10 g chlorophyll-a/ l with dominance of cyanobacteria

  • Watch for scums or conditions conducive to scums
  • Discourage bathing and further investigate hazard
  • Post on-site risk advisory signs
  • Inform relevant authorities

Actions in response to an algal bloom

7.10 Column 2 of Table 7.1 summarises the "typical actions" that should be taken to protect people who might come into contact with recreational waters affected at the extent indicated in Column 1. The WHO guidance also notes that "actual action taken should be determined in light of extent of use and public health assessment of hazard". Section 4 of this document refers to the need for LAPs to make provisions for such public health (risk) assessments and Section 6 gives general requirements.

7.11 Actions defined in Table 7.1 relate mainly to provision of information and advice and discouraging or prohibiting water-contact activities. Responsibility for these actions will vary according to ownership and use of the waters in question and these responsibilities should be defined in LAPs and in pro-active risk assessments ( Annex F).

7.12 Information and advice might be provided by leaflets, warning notices, letters to stakeholders or public announcements (for example on local radio). Leaflets can provide more information to water-users than is possible in a warning notice and might be particularly appropriate in circumstances where there is extensive recreational use of a water. More detailed consideration of public information provisions is given in Section 8.

7.13 In addition to advice aimed at minimising public health risks, advice should also be given to dog owners to protect dogs from ingestion of blue-green algal material in the water or on the shoreline. Parallel advice should be given to farmers to protect stock.

7.14 A suggested text for a warning notice is given in Annex H.

7.15 LAPs and proactive risk assessments should also consider the need for advice to avoid eating freshwater shellfish.

7.16 Fish should not be consumed if fish mortalities, or behavioural abnormalities, are observed at waterbodies containing mass populations of blue-green algae. In the event of blue-green algal scum being present, or blue-green algal cell numbers exceeding 20,000 per ml ( Annex G), toxin analysis of fish intended for consumption should be carried out. Expert advice will be necessary on concentrations of toxins, which if present, may be sufficient to justify restrictions on fish consumption. The liver and gut from fish caught in waters affected by blue-green algae should not be fed to pets.

7.17 LAPs and proactive risk assessments should consider the use of standing waters for irrigation of crops. Advice should be given on precautions that are appropriate to local circumstances.

7.18 Table 7.1 also raises the possibility of "Public Health follow-up investigation". This would be a matter for the local Director of Public Health but Paragraph 7.19 gives some general indications.

7.19 The long-term adverse effects of blue-green algal toxins are not fully understood. In cases of exposure to skin, a need for long-term follow-up is not indicated. If toxic bloom or scum have been ingested, medical or veterinary monitoring might be needed for adverse health effects and further advice should be sought ( Annex C).

7.20 Where water is used for potable supply, toxin analysis should be planned and carried out as appropriate as an aid to hazard management.

7.21 Guidance levels for recreational waters are defined in terms of concentrations of the algae themselves rather than algal toxins. However, toxin analysis for recreational waters should be considered, depending on individual circumstances, in conjunction with advice from relevant sources ( Annex C). Where a bloom is highly localised or confined to one area, the risks of exposure to significant quantities of toxin are likely to be low except where there is immersion in or ingestion of water in close proximity to the bloom itself.

7.22 Pre-emptive action should also be considered if, from knowledge of the water and recent weather, the probability of bloom formation is judged to be high.

Action to prevent algal blooms

7.23 In water bodies where persistently high algal concentrations occur or regular blooms take place (Categories 1 and 2 in Table 5.1), and where the attendant risk is categorised as "High" (Table 6.1) the source(s) of the problem should be established, and where possible, appropriate action taken. These measures should be enacted in consultation with SEPA and Scottish Water.

7.24 Algal blooms normally result from a combination of natural factors, including availability of nutrients and light, water temperature and wind conditions. Availability of nutrients is a principal concern, as they are essential for plant growth. Typical nutrient sources from human activity in Scotland include discharges from sewage works, industry and agriculture.

7.25. Information on reducing nutrient enrichment of surface waters from agricultural sources can be found in 'Prevention of Environmental Pollution From Agricultural Activity Code of Good Practice (1997)' and in 'Prevention of Environmental Pollution from Agricultural Activity: Nitrogen and Phosphorus Supplement' both of which are available from the Scottish Executive ( http://www.scotland.gov.uk/library3/environment/pepfa.pdf). Methods for preventing nutrient losses from urban areas are included in the Sustainable Urban Drainage Systems - Design Manual for Scotland and Northern Ireland (2000)'. Further information and guidance can also be found in the 1998 WHO Guidance document for recreational waters. It is likely that SEPA will take account of such matters under the European Union Water Framework Directive.

7.26 Reducing nutrient inputs from the catchment is part of the long-term solution to the cause of algal blooms, but other measures may be effective in reducing these symptoms in the short-term. These measures include the use of barley straw, biomanipulation, forced circulation and chemical control.

Barley straw This method involves the use of (small) bales or nets of barley straw submerged at the inlet to the water body and at other suitable locations. It is only effective in small water bodies. Further details are available at: http://www.iacr.bbsrc.ac.uk/pie/JonathanGrp/Straw_Information_sheet.pdf

Biomanipulation The aim here is to make the aquatic ecosystem less conducive to algal blooms. Systems include interventions aimed at increasing the populations of zooplankton (which feed on algae) and of aquatic macrophytes (plants that are large enough to be distinguishable from algae) which compete with algae for light and nutrients. The first of these might involve manipulation of the fish community to reduce the rate of zooplankton predation by fish.

Forced circulation Water circulation can be forced by a wind- or electrically-driven turbine within the water body, or by sparging with compressed air. This ensures that the water body is evenly mixed and reduces internal release of phosphorus from the bottom sediments of the water body under anaerobic conditions. It also forces the algal cells to spend an increasing proportion of the day away from sunlight.

Chemical control methods Precipitating agents have been used to encourage binding of phosphorus to sediments (reduce internal release); their application is not, however, recommended without expert guidance. Certain algicides and herbicides have indicative approval for use on or near waters in the UK. However, proposed use of any control chemicals on or near waters in Scotland must be notified to SEPA for approval where appropriate.

Further information on catchment management, biomanipulation and other control methods can be obtained from the Centre for Ecology and Hydrology ( Annex C).