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Councils can organise for testing of algae samples to see if they are toxic - the presence of cyanobacteria resembling toxic species does not necessarily mean there is a toxicity issue. However, there have been numerous reports of dog deaths attributed to contact with river margin growths of filamentous cyanobacteria belonging to the order Oscillatoriales, likely to be Oscillatoria or Phormidium (pictures below). Cyanobacteria often have an unpleasant musky smell (you may have noticed a similar smell if you’ve cleaned out an old fish tank). Some dogs seem to be attracted to the smell produced by Oscillatoria/Phormidium as growths become exposed along stream/river margins during periods of low flow.
A few of the cyanobacteria taxa that form blooms in ponds and lakes can be toxic. Among these are species of the chain-forming Anabaena and the colonial Microcystis. Both can form bright green or blue-green surface scums, and these tend to be most obvious during relatively calm spring or summer conditions.
Remember to avoid skin contact with water or sample material if you suspect you may be dealing with something toxic. If samples are required for toxicity testing, leave the sampling to the council or experts with the required sampling gear. If the council considers the cyanobacteria presents a potential health risk, their initial response might be to erect signs warning people to stay away from the water until further notice.