Testing has begun on blue-green algae growing on Bradley Stoke Lakes to determine whether it is responsible for the death of some water birds.
Environment Agency tests have shown positive for oscillatoria - an algae which is known to be toxic to wildlife.
The algae can cause dermatitis, asthma, eye irritation, rashes, blistering around the mouth and nose, abdominal pain, diarrhoea and nausea.
Tests carried out on the dead birds by Defra for avian flu came back negative.
A spokesman for the Environment Agency said once algal numbers are high, the bloom is unlikely to decline until winter.
Dog walkers in South Gloucestershire are being warned not to allow pets into the water until further notice.