Toxic blue-green algae have been spotted at fresh locations along the north coast, Northern Ireland environmental authorities have warned.
The bacteria were found at Portstewart and Castlerock beaches last week.
Red flag warnings were extended west to the beach at Downhill on Sunday, the Department of Agriculture, Environment and Rural Affairs (DAERA) said.
These have also been extended to Benone beach as a precautionary measure.
Blue-green algae is not actually an alga but rather a type of bacteria called cyanobacteria.
It can cause illness in humans but is particularly dangerous for pets and has been implicated in the deaths of several dogs in other locations where it has been detected.
Swimmers and dog walkers have been advised to adhere to any advice about getting into the water.
Scientists from the Agri-Food and Biosciences Institute are working with DAERA, assisting with modelling to predict how tidal flows will affect the algal blooms.
Boffins say the organism cannot survive in salt water, but it was still being washed from Lough Neagh down the River Bann to the coast.
The DAERA officials have encouraged people to get involved in citizen science and help monitor the presence of blue-green algae, via an app or by contacting the department directly.
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