The Wonderful World of Algae: Why Some Blooms Appear and What We Can Do About Them

You’ve probably heard of blue-green algae, especially in summer when you might see the bright green presence on the lake or in warnings to advise lake users, but what exactly is algae, why does it bloom, and should we be worried? To find out, we spoke to Dr Sian Davies, a Water Scientist in the Environment Agency’s Chief Scientist Group, who has spent her career studying algae in lakes including Windermere.

What is algae—and why do we need it?

Algae are plant-like organisms that use sunlight and nutrients from the water to grow—just like plants on land. They’re the base of the aquatic food chain, providing food for tiny water creatures like zooplankton, which in turn feed fish.

“There are over 6,000 identified species of algae in Britain and Ireland,” says Dr Davies. “They range from large plants like seaweed to microscopic, single-cell organisms invisible to the naked eye. In lakes, there are three main types: filamentous algae (stringy plants that cling to rocks), phytobenthos (the algae that make rocks feel slimy), and phytoplankton—tiny, free-floating organisms that include blue-green algae.”

Blue-green algae: the good, the bad, and the bloom

Blue-green algae (technically known as cyanobacteria) are a natural part of lake ecosystems. But sometimes, when conditions are just right—usually in warmer months—they can multiply rapidly and form ‘blooms’.

These blooms can block light from reaching other aquatic plants and reduce oxygen levels in the water. In some cases, they may release toxins that can be harmful to animals or people who come into direct contact with the water.

“The key is balance,” explains Dr Davies. “Like everything in nature, algae are beneficial in the right amounts. It’s when their growth goes unchecked that problems arise.”

What causes an algal bloom?

During spring, Windermere typically sees an initial bloom of harmless diatom algae, which do an important job by absorbing nutrients left over from winter. As summer arrives and the lake warms, a different group of algae—including cyanobacteria—can take advantage of the calm, nutrient-rich conditions.

These species have evolved clever ways to stay afloat in the water column, using gas-filled bubbles to rise or sink. “On a calm day, these bubbles can push them to the surface where they’re blown into bays and gather into visible blooms or scum,” says Dr Davies. “These are the blooms people tend to notice—and worry about.”

Are blooms becoming more common?

Unfortunately, yes—and climate change is a key reason. “Even with the same amount of nutrients, warmer temperatures create more favourable conditions for blooms,” says Dr Davies. “That means we’re likely to see more frequent and longer-lasting blooms in the future.”

What can we do about it?

Reducing the nutrients that algae feed on—especially phosphorus—is one of the most effective ways to help.

That’s why local organisations, landowners, and partners across the Love Windermere partnership are working together to improve water quality through better land and wastewater management.

At the same time, scientists are improving how we monitor and respond to algal blooms. But public reports remain vital.

“If you see a possible bloom, you can report it via the Environment Agency’s 24-hour hotline (0800 807060) or the free Bloomin’ Algae app,” says Dr Davies. “Your report helps us monitor the lake and keep people informed.”

Looking ahead

Algae have been around for billions of years and are essential to lake ecosystems like Windermere. While we can’t eliminate them (and wouldn’t want to!), we can work together to limit the conditions that allow harmful blooms to grow.