Algal blooms occur when conditions are optimum for growth (high concentrations of nutrients in the water and favourable weather conditions: warm, light, calm). Algal growth is normally limited by availability of nutrients, but eutrophication (an excess of rich nutrients from a runoff from land) can remove this limitation, allowing algal populations to grow until they become self-limiting as the cells shade each other and light becomes a limiting factor. In extreme cases the high density of cells causes the water to take on an obvious green colour and to be very turbid. As algal cells die, they can then create a floating scum on the surface of the water.

Algal blooms are becoming an increasing problem year on year and one which is only going to get worse. An additional challenge is having to address the effects of climate change on watershed hydrology, temperature, mixing and nutrient loading/cycling dynamics. An increasingly common scenario that promotes algal bloom development is a period of excessive rainfall and nutrient runoff followed by prolonged drought and heat wave conditions. Additionally, earlier spring warming and later autumn cooling results in a longer growing season, enabling blooms to develop early and persist for longer.

Some of the problems associated with algal blooms include: depletion of oxygen levels; blocking of sunlight from reaching other plants in the water; increased pH; blocking/clogging of filters during water treatment; visual issues; and from cyanobacteria the production of toxins affecting fish and humans, production of taste and odour compounds affecting drinking water supplies and accumulation of substances such as iron, manganese and hydrogen sulphide.

Algal bloom in a river

Identifying the source of and diagnosing causes of algal blooms

Key to managing algal blooms is identifying the source of and cause of the blooms. Only once you know what species are blooming and what is driving this can you manage the situation effectively. Some species react very differently to different management techniques, for example some species favour mixing when artificial aeration is used as a management option. Only once the causes of blooms are fully understood and quantified, can effective management of these be undertaken.

Management and mitigation options

There are three levels of approach to the assessment of management and restoration options:

  • Short term – where blooms are occurring or anticipated, options to control algal levels in the short term
  • Medium term – options for controlling numbers of algae so that they do not form blooms
  • Long term – options for reducing nutrient loads so that low concentrations ensure no bloom development

Want to know more about algal blooms and how APEM can help?

APEM is the leading UK provider of algal bloom assessment and management, providing professional advice on the source and impacts of algal blooms, and on monitoring and management options, to government bodies, water companies, recreational centres, event organisers and angling associations.

To find out more about how APEM have advised on and managed algal blooms why not read one of our case studies?

Salford Quays: a water quality success story

Glasgow Commonwealth Games: triathlon swim at Strathclyde Loch