Dog owners are urged to be aware of deadly algae found in the water at the nearby Gladhouse Reservoir.
Local man Lee Simpson shared images of the blue-green substance spotted around the edges of the reservoir, which is close to Penicuik in Midlothian.
The area is popular with runners, swimmers and dog walkers, and people are being urged to keep their pets well clear of the water if they come across the affected area.
The Scottish SPCA say anyone who finds algae such as this should report it to the relevant authorities.
Chief superintendent Mike Flynn said, “Blue-green algae can be toxic to dogs. Any dog owners concerned that their dog has come into contact with it should consult their vet immediately.
“The algae looks like foam and will usually be found in lakes or ponds at the edge of the water.
“If you find algae this should be reported to the local authority environmental services.”
After spotting the substance in the water at Gladhouse, local photographer Lee Simpson posted: "At the moment there is none at all in the area opposite the main island. But, speaking to some paddleboarders, there is quite a bit drifting from near the small island to the centre of the reservoir.
"It is clearly visible in just one area near the shore. It potentially can cause illness and a skin rash. It can also be fatal to dogs."
The British Veterinary Association (BVA) have previously issued warnings about the toxic substance after it was found in water bodies in and around the Scottish capital.
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Symptoms can appear in pets within hours or even minutes depending on the toxin. These can include vomiting, diarrhoea, drooling, trouble breathing and seizures.
Algae is known to grow in waterways, reservoirs and rivers, particularly during warmer summer temperatures.
