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

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.


2.1 Cyanobacteria occur in fresh, brackish and sea-waters throughout the world. In Scotland, they can occur in quantity in lochs, ponds, canals, rivers, reservoirs and coastal waters. While usually green or blue-green in colour (hence the term blue-green algae), they may be khaki, blue, black, dark brown or red (hence the preferred terminology now used is cyanobacteria).

2.2 When present in high concentrations, colonies of cyanobacteria can often be seen with the naked eye: they may resemble fine grass cuttings or take the form of small irregular clumps or pinhead-sized spheres. Cyanobacteria in high concentrations in the water column can form 'blooms' and, when blown on to a downwind shore, form scums which can be up to a few centimetres thick. Scums may also be seen in slow-flowing rivers and streams downstream from lochs. Decaying scums, due to other naturally-occurring microbes or bright sunlight for example, can appear bleached as sky-blue, grey, or white masses.

2.3 Cyanobacteria may also grow on the bottom of shallow water bodies and on shoreline rocks and sediments. They occasionally form thick gelatinous mats, which may be exposed as the water level falls or may detach from the bottom and reach the shoreline. These mats are usually very dark in colour (black, dark brown or green), are cohesive and are sometimes mistaken for sewage.

2.4 Growths of some varieties of water plants (particularly duckweed) that float on the water surfaces can be mistaken for surface scums of cyanobacteria.


Email: Janet Sneddon

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