Ponds, quarries and algae: where Cape Ann dogs should not swim

Blue-green algae is a bacteria rather than a plant, and it blooms when a pond is warm, still and rich in nutrients: on Cape Ann, the low, sunny ponds of July, August and September. A bloom can look like spilled green paint, pea soup, scum along the downwind shore or floating mats, and it can also be invisible. The ASPCA Animal Poison Control Center lists it among its emergencies because the toxins act within minutes to hours of a dog drinking or licking the water.

We name no pond as contaminated, because a bloom moves, clears and returns with the weather, and a page written in September cannot know what a pond holds the following July. What we can say is how to read one: no swimming in water with scum, paint-like color, mats or a strong smell, and no drinking from the shore. The state and the towns post advisories when a bloom is confirmed; a pond with no advisory is unknown, not safe.

The freshwater season asks three things of a dog. A leave it that works at the water's edge, so a thirsty dog does not drink before you have looked. A recall that stops a dog mid-charge toward a quarry rim or a scummy shore, trained on a long line and not assumed. And the habit of drinking from the bowl you carry. Our how-tos on drop it and on long-line recall cover the first two.

After any swim in fresh water, rinse the dog with clean water before it can lick its coat dry, and watch it for a few hours. Vomiting, drooling, weakness, wobbling, tremors or a dog that goes flat after a pond are reasons to call your vet or the ASPCA line at once. We offer no home treatment beyond that call; after hours, Massachusetts Veterinary Referral Hospital in Woburn takes emergencies.

The ocean is the safer swim, in its season. Gloucester's public beaches take dogs from October 1 to April 30 at the time of writing, Good Harbor on even-numbered days until its April 1 plover closure and Wingaersheek on odd days, and Rockport's outside the June 1 to September 15 ban; the beach calendar has every town. The beach hazards page covers salt water, hooks and jellyfish.

The leptospirosis page explains why a puddle and a marsh carry a second risk beyond algae, and the summer heat page covers the humidity that sends dogs into ponds in the first place. The seals, plovers and porcupines page includes the snapping turtles at a pond edge in June, and Halibut Point has its own page for the leash rule and the granite.