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 may resemble spilled green paint, turquoise streaks, foam or clumps of particles. Harmless duckweed or filamentous algae (the stringy stuff) can sometimes look similar, while toxins may be present even without a dramatic surface scum.
Why hot summers make blooms worse
Cyanobacteria need light, suitable temperatures and nutrients. Many bloom-forming species thrive in warm water, while calm conditions allow them to accumulate near the surface, where sunlight is strongest.
Climate change is increasing the risk through warmer water, longer growing seasons, droughts and more intense downpours. Heavy rain after a dry period can wash fertiliser, manure, soil and wastewater-derived nutrients into rivers and lakes.
But climate change is not acting on pristine water. It is amplifying the consequences of decades of nutrient enrichment.
In a nutrient-poor lake, warm weather may increase biological activity without producing a major toxic bloom. In a nutrient-rich lake, the same weather can push an unstable ecosystem across a threshold.
Describing a bloom as simply “caused by hot weather” is therefore misleading. It presents the event as an unavoidable consequence of summer rather than a symptom of how surrounding land and wastewater systems are managed.
Lough Neagh in Northern Ireland, the largest lake in Britain and Ireland, provides a striking example. Since 2023, it has experienced extensive blooms dominated by cyanobacteria, including Microcystis aeruginosa, one of many cyanobacteria species that produce toxins.
Nutrients entering the lake from its agricultural and urban catchment provide favourable conditions for growth. Warmer water and calm weather can then increase the likelihood that blooms will develop and persist.
Invasive zebra mussels also contribute. These mussels are native to the Black and Caspian Seas and first made it to Ireland in the late 1990s. They were first recorded in Lough Neagh in 2005, attached to a moored boat’s hull.
By filtering the water, these mussels make it appear clearer, but clearer water is not necessarily healthier. Greater clarity allows sunlight to penetrate deeper, which can encourage the rapid growth of cyanobacteria. They may also alter nutrient cycling and the wider food web in ways that support harmful blooms.
Blooms rarely have a single cause. They emerge when nutrient enrichment, climate, water conditions and ecological disruption all interact.
More blooms are likely – but not inevitable
As the climate warms, Britain and Ireland will probably experience longer bloom seasons with more frequent periods that suit cyanobacterial growth. Blooms may appear earlier, persist later into autumn and even, in some waters, survive through winter.
That does not make increasingly severe blooms inevitable.
We cannot control heatwaves locally, but we can make rivers and lakes less responsive to heat by reducing nutrient inputs. This means improving farm nutrient management, preventing soil and fertiliser loss, upgrading wastewater infrastructure, managing septic systems and restoring wetlands that intercept runoff.
The benefits may not be immediate. Phosphorus can accumulate in lake sediments and be recycled for years. Nevertheless, reducing new inputs is essential. Otherwise, climate change will continue to expose and magnify the underlying pollution problem.
Monitoring also needs to change. A bloom can move, concentrate or disperse within hours as wind and water conditions change. A laboratory sample provides valuable information, but only about one place and time.
In our recent research, we detected the cyanobacterial toxin microcystin-LR at the waterside in Lough Neagh. The portable test we developed produced a result in about 35 minutes without filtration or bulky equipment. A mobile phone could interpret the colour change.
Rapid field testing could eventually create near-real-time maps of blooms and toxin detections, and show which samples need confirmatory laboratory analysis.
What the public should do
Follow warning signs and local advice, and avoid swimming or allowing children or pets to play in water where a bloom or scum is suspected. Dogs are particularly vulnerable, so keep them away from discoloured or scummy water, prevent them drinking from affected lakes or rivers, and rinse them with clean tap water after any accidental contact.
A heatwave can reveal a lake’s vulnerability, but it did not create that vulnerability. Harmful blooms are warning signs of water overloaded with nutrients as the climate warms. Treating the visible scum is only treating the symptom. Reducing nutrient pollution addresses the cause, and can make our rivers and lakes less vulnerable in a hotter future.
Suspected blooms in Northern Ireland and Britain can be reported through the Bloomin’ Algae citizen-science service.
Comments are closed.