Pondering about danger

My dog is a Springer, but if it was only spring water she jumped in that would be fine. I've heard about the dangers of Algae and I have no idea what it is I'm meant to be looking for.
There's green stuff growing all over the surface of our pond, but it doesn't stop her drinking it and swimming in it.
How do I spot the dangerous stuff? Anything I can do to our pond to make sure we don't get it?
Jon James, High Wycombe

Alison Logan, vet, says...
The specific risk from algae is when there is a rapid proliferation of blue-green algae following warm or hot weather with little or no wind, generally in stagnant water. This is called an algal bloom. The windward parts of lakes and reservoirs are particularly affected with this blue-green scum and slime, which is toxic if swallowed or if groomed off the coat after swimming.
There are different types of algal bloom which have different toxic effects. There are three main toxins: one causes liver failure, the other two act on the nervous system. They can act very rapidly indeed, and all that can be done is to treat the clinical signs and wait for the toxin to leave the body. Sadly, there have been fatalities.
It is therefore wise - but often not possible – to check the water your dog has chosen for swimming before she launches herself into it when the weather has been calm and warm or hot. If you can see a blue-green scum or film on the water then I would put her on the lead and head off in the opposite direction.
The situation with your own pond is more difficult. Much will depend on the size and location, and whether there is circulation of the water, either naturally or with some artificial method. Avoiding stagnation is a key factor. Inspecting the water by eye in hot weather is advisable and it may well be possible to have samples analysed if you suspect the presence of blue-green algae.
In hot weather, a dog’s natural urge is to cool off with a dip. Swimming is a great form of exercise which avoids over-heating in hot weather, as well as being non-weight-bearing. If, on a particular day when weather conditions are warm and still, you suspect your pond may have been affected with a blue-green algal bloom then it would be safer and wiser to take her somewhere else.

Nick Thompson, holistic vet, says...
Cyanobacteria is the posh name for what you’re referring to here. They are called Blue-Green Algae and they are present in most brackish, still water, much less so in rivers. However, in the summer in hot, calm conditions especially, they can ‘bloom’, or ‘swarm’ across a pond, contaminating the water with hepato-toxins (affecting liver) or neuro-toxins, affecting the nervous system. It makes the water look a bit like spinach or watercress soup. You can see some good pictures if you look in Google Images
The toxins produced by the algae can cause very severe disease and even death, with some reports in the U.S. describing dogs being found dead at the edge of algae infested lakes. If your dog collapses or shows any extreme symptoms (collapse, salivation, breathing difficulty) after swimming in any still water during the summer, it’s probably worth mentioning this to the vet who can treat for toxicity as well as checking for heart problems or epilepsy etc.
Instances of these bloom problems are not that common in the UK at the moment, so please do not forbid your dog from swimming in lakes just yet.
The best advice is to be more careful with still water in the summer, especially late summer and to examine any ponds, lakes or non-flowing water, especially the windward shores, before your dog jumps in or even laps. It's a pain, but the alternative is to forbid all still water swimming, which is much worse as swimming is really important and a lot of fun!
Blue green (which can also be brownish greyish) algae over-grow in still, warm water where the balance of nutrients is high, the acidity of the water is abnormal. Normal maintenance of the pond, monitoring nutrient and pH parameters of the water and keeping the pump running well to oxygenate the water should be enough to stop algae blooming in a pond near you.