This summer’s heat brought warnings about harmful algal blooms across Britain and Ireland. In August, swimmers were warned not to enter a newly designated bathing site on the River Thames at Ham and Kingston after blue-green algae were detected. The swimming section of an Ironman event in Leeds was also cancelled because of a bloom in Waterloo Lake.

It is tempting to blame these events entirely on the heat. Cyanobacteria, one of the main culprits behind these blooms, certainly benefit from warm, sunny and relatively still conditions. But a heatwave does not create a harmful bloom from nothing.

The underlying problem is usually already present: excess nutrients, particularly phosphorus and nitrogen, entering water from agricultural runoff, wastewater, septic tanks, urban drainage and other sources.

Warm weather is accelerating these blooms, but nutrient pollution is providing the fuel.

Blue-green algae are not really algae

The organisms commonly called “blue-green algae” are actually cyanobacteria. They occur naturally in lakes, rivers, reservoirs and ponds, and at low concentrations, they are a normal part of freshwater ecosystems.

Problems arise when conditions allow them to multiply rapidly and form dense blooms or surface scums. Some cyanobacteria produce toxins that can affect the liver, nervous system or skin.

Not every bloom is toxic, however, and toxin concentrations can vary greatly within the same water body and over short periods. This creates a public-health challenge as it is impossible to determine reliably whether a bloom is toxic simply by looking at it. A bloom........

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