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Blue-Green Algae (Cyanobacteria) in Inland Waters: Assessment and Control of Risks to Public Health
5 Assessing the nature and intensity of algal blooms
General provisions
5.1 Wherever possible, assessment of blue-green algal populations should be co-ordinated between 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 and climatic, that lead to the development of blue-green algal blooms in inland waters (or to the growth and detachment of algal mats) are complex. However, for each individual water, 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 outwith this period. Therefore, where activities with a high risk of exposure take place throughout the year, the possibility of continuing inspection and/or monitoring from November to March should be considered.
5.4 The frequency, duration and magnitude of such blooms will affect the needs for inspection and monitoring. For present purposes, therefore it is useful to categorise waters in accordance with these "occurrence" factors, as shown in Table 5.1.
5.5 Column 3 of Table 5.1 gives general indications for the likely efficacy of monitoring for waters in each of the four categories. As a general rule, planned regular monitoring of blue-green algal 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 intervals of time is likely to miss blooms, only frequent visual inspection and reactive monitoring is useful.
Table 5.1:
Categorisation of waters in terms of the frequency and intensity of algal blooms. Column 3 indicates the likely efficacy of monitoring and inspections.
Category | Description of algal blooms | Implications for planned regular monitoring |
1 | Waters that consistently contain large populations of blue-green algae 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 some value. However, the ability to detect short blooms will depend on frequency of monitoring and some might be missed. |
3 | Waters that have only intermittent algal blooms for short periods. | Of less value than for Category 2. 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 for the water in question. Circumstances will vary widely but the following general indications might apply:
i. Where exposure at recreational waters might be predictably persistent or recurring, (Categories 1 or 2) it may 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, 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 the scope for exposure of people or animals is limited, 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 blue-green algae should record the appearance and apparent concentration of algae in water. This will normally involve collection of samples of water from the furthest extent of reach from the shore using a glass container. The presence of algae 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 algal blooms.
5.9 Visual inspections should particularly note the presence and quantity of algal scums on the surface of the water and 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 SEPA can provide an analytical service to identify and quantify algal blooms in samples taken by others from recreational waters. Scottish Water can provide a similar service for samples taken from potable supplies that are thought to be at risk.
Reporting and assessment of results
5.13 The LAP should define procedures for local assessment, recording, reporting and storage of monitoring data. For public or private drinking water, Scottish Water or the water-provider should provide customers or users with appropriate information to minimise any health risk from exposure to blue-green algal toxins.
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