Blue-green algae, also called cyanobacteria, can gather in still fresh water during warm weather and release toxins that act fast in dogs. The danger is practical rather than abstract: a dog who swims in, drinks or licks contaminated water off his coat can become ill within a short time, so the owner's whole job is recognition, avoidance and a quick, correct response if contact happens anyway. No owner can judge safety by testing a handful of water; if the water looks suspect, the answer is to keep him out.
Prepare Before You Reach the Bank
Pack for water walks as deliberately as for any outing: a short lead you can control him on near edges, a bottle of his own drinking water with a bowl so thirst never tempts him towards the lake, and a towel plus a container of clean water kept in the car for rinsing. Check local notices before you set out in summer, since councils and land managers post warnings for particular lakes and ponds when blooms are reported, and a closed lake saves you a wasted journey with an excited dog.
Plan the route so the walking does not depend on a swim. Know which stretches have shallow, scummy margins where dogs slip in while sniffing, and which have clear gravel shores or running streams where a paddle is a reasonable judgement on the day.
Reading the Water
Learn the look of a bloom: water coloured bright green like spilled paint, a surface film or scum, streaks and clumps of green or blue-green, mats floating at the edge, or a whole pond turned the colour and thickness of pea soup. Blooms gather downwind, so one sheltered corner can be thick with scum while the open shore looks clear; check the part of the water he can actually reach, not the view from the car park. Algae can also be present without an obvious scum, which is why posted notices outrank your eyes.
When in doubt, stay out. A missed swim costs him an afternoon; a wrong call can cost far more, and there is no way to make suspect water safe by letting him in briefly, keeping his head up or rinsing him as an afterthought to a decision you already doubted.
Keeping Him Out, Minute by Minute
Shorten the lead before the shore, not at it, and walk him past access points with his attention on you and a reward ready. Do not throw balls or sticks into lakes or ponds in bloom season, because a retrieving dog cannot be recalled out of a habit mid-flight, and do not let him drink from the edge, graze on shoreline mats or roll in scum washed onto the bank. Offer his own water at every stop so the lake is not the only drink on offer.
If another stretch of the same water looks clear, read it on its own evidence, but keep the lead short until you have seen the whole margin he could reach, including the downwind corner where the film collects.
If Contact Happens Anyway
Lead him out at once and keep him from licking his coat, using the lead and your hands to interrupt grooming. Rinse him thoroughly with the clean water you carried, working it through the coat, and dry him with the towel; wash your own hands afterwards. Ring your vet promptly, say plainly that blue-green algae exposure is possible, and describe the place, how long he was in and what he did in the water. Signs such as vomiting, drooling, wobbling, unusual tiredness, pale gums or collapse after water contact mean he goes in urgently, without waiting to see whether they pass.
Write the exposure down the same day: lake or pond name, the spot, what the water looked like and any signs that followed. Kept on his record through KinsLe, that dated entry in Pet Passport travels with him to any out-of-hours clinic, where a clear account of where and when he swam is part of the information the team will ask for first.