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November’s warm gloom brought unusual blooms and peril to Twin Cities lakes and waterways.

The beaches aren’t open this time of year, but the Minneapolis Park and Recreation Board sent written warning that people and their pets should be wary near the water.

The board reported toxin-producing blue-green algae blooms at Cedar, Nokomis and Brownie lakes. A similar algae bloom in Lakewood Cemetery’s pond caused the water in the Roberts Bird Sanctuary to run a milky tan.

Fall algae blooms used to be uncommon but have become more typical as the region’s seasonal temperatures rise. Blue-green algae, which are actually bacteria, make the water look like split pea soup and can create a swampy odor and a telltale green scum along the shoreline.

The end isn’t imminent. The National Weather Service forecast predicts a continued stretch of above-freezing temperatures — and overcast skies — well into December.

Most blooms aren’t harmful, but it’s not possible to tell from looking which are the toxin-producing blooms and which are safe. The Park Board recommends that people and pets stay out of the water where blooms or algae scum are visible.

The warning came from Minneapolis, but the need for concern stretches throughout the Twin Cities and beyond. Wisconsin officials have reported similar blooms across that neighboring state.

Harmful algae thrive in calm, shallow, nutrient-rich lakes. They will often be found on the downwind side of a lake or in a secluded bay or shoreline — the sorts of spots a pet might go for a sip.

Neither pets nor humans should drink from the waters. The Minneapolis board also warns against playing in the waters that appear to have a blue-green bloom. Those who enter the water should wash off immediately afterward, the board warned.

Humans and animals can get sick if they swallow, breathe or have skin contact with the toxins. Symptoms usually begin within two hours of exposure and include vomiting, diarrhea, rashes, eye irritation, cough, sore throat and headache.

Toxins also can stay in the water after the dangerous blooms are gone. The Minnesota Pollution Control Agency warns, “when in doubt, stay out.”

The Minneapolis board monitors lakes throughout the year and posts algae updates.

Dogs are not allowed in Minneapolis waters, according to city ordinance.

Rochelle Olson • 612-673-1747

Twitter: @rochelleolson