A popular bathing spot in Oxford has been affected by a thick layer of bright green duckweed following an unusually warm and dry summer.

The water at the River Cherwell in University Parks was initially believed to have been covered by an algal bloom, prompting warnings for visitors to avoid swimming.

One local resident described the water as looking “fluorescent”.

Bright Green Layer Covers the River

Signs were installed beside the River Cherwell warning people about the unusual growth.

The University of Oxford initially warned that a possible algal bloom could potentially be harmful to people and pets.

The warning came amid a period of unusually dry weather and low river levels.

However, further investigation by the university’s parks team suggested that the bright green layer was not algae but duckweed.

What Caused the Duckweed to Spread?

According to the university, several environmental conditions created ideal circumstances for the plant to multiply.

These included:

• Low water levels
• Warm temperatures
• Plenty of sunlight
• Slow-moving river water

Together, these conditions allowed duckweed to form a dense layer across sections of the water.

University Removes Precautionary Warning

The university later replaced the original warning signs after determining that the growth was not a potentially toxic algal bloom.

The parks team said it was confident the material covering the water was duckweed rather than harmful algae.

The original signs had been installed as a precaution while the nature of the unusual growth was being assessed.

Why Duckweed Can Spread Quickly

Duckweed is a small floating aquatic plant that can reproduce rapidly when conditions are favourable.

Warm, sunny weather combined with slow-moving water can encourage it to spread across the surface, creating the appearance of a bright green carpet.

While the sight may look unusual, the university’s assessment indicates that the current growth at the Oxford bathing spot is not a toxic algal bloom.

For swimmers, however, the episode serves as a reminder that unusual changes in water conditions should be checked before entering a river or other natural swimming location.