That blue-green scum that sometimes covers rivers and ponds in the summer months isn’t just ugly, it can be dangerous.

Fueled by nitrogen and phosphorous runoff from lawn fertilizer and sewage systems, cyanobacteria, also known as blue-green algae, appears in many Massachusetts water bodies in the summer, when the temperatures rise and the days are longest.

“It has become a common issue during the summer and early fall,” said Mike Celona, chief of the water toxics unit at the Massachusetts Department of Public Health. “There are approximately 20 cyanobacteria blooms at recreational water bodies in Massachusetts each year.”

As of Aug. 9, the DPH issued cyanobacteria advisories for nine bodies of water, cautioning the public that exposure to the toxic algae can cause conditions ranging from skin irritation to gastrointestinal illness. The advisories discourage swimming and other activities, such as kayaking, that could cause a person’s skin to come in contact with the water, even through incidental splashing.

 

“Our advice is to limit all water contact,” Celona said. “Health concerns from harmful algae blooms and their toxins vary depending on the type of exposure and the amounts and types of toxin present. Direct contact with algae can cause skin and eye irritation, and ingesting or inhaling small amounts are most likely to cause gastrointestinal symptoms. Animals, especially dogs, can get very ill from drinking water with algae or licking it off their body.”

Persistent skin rashes, eye irritation or gastrointestinal illness may require medical attention.

The cyanobacteria advisories, as of Aug. 9, cover the lower basin of the Charles River in Cambridge and Boston, Martin Pond in North Reading, Oldham Pond in Pembroke, Savery Pond in Plymouth, Schoolhouse Pond in Barnstable, Wampatuck Pond in Hanson, Hummock Pond in Nantucket, Lake Slog in Holland and Tully Lake in Athol and Royalston.

Julie Wood, director of projects for the Charles River Watershed Association, said the cyanobacteria blooms seem to have become more frequent on the Charles over the past decade. Before 2006, she said, they were less common. Since then, they’ve appeared nearly every summer.

“It’s pretty startling,” she said.

Wood advocates for taking steps to control stormwater runoff, including using unpaved barriers to help filter phosphorous and nitrogen.

“The best thing you can do is use nature,” she said. “One piece of that is as simple as getting rain water into the ground, and filter it through the ground. But we’ve paved everything and built on everything.”

While exposure to the algae can cause health issues for people and pets, cyanobacteria can also harm the ecological health of a water body. At the surface, it can block sunlight. It also consumes oxygen in the water, sometimes creating dead zones.

Rob Moir, executive director of the Cambridge-based Ocean River Institute, recalled an incident in Falmouth several years ago, when 15 striped bass and a horseshoe crab washed ashore dead near an algae bloom in a saltwater inlet known as Little Pond.

Since then, he said, the town has enacted measures to reduce the amount of nitrogen and phosphorous entering the water. A municipal ordinance in Falmouth, for example, limits the yearly use of lawn fertilizer to one pound per 1,000 square feet of lawn.

“When the algae blooms, it eats up all the oxygen,” Moir said. “Fish swim into that dead zone and roll up dead and wash up onto the shore.”

Pine DuBois, executive director of Jones River Watershed Association in Kingston, home to Silver Lake, described other man-made factors that can contribute to the spread of cyanobacteria.

After Brockton began drawing its water supply from Silver Lake decades ago, planners diverted water from nearby Monponsett Pond to replenish the lake. In the 1960s, DuBois said, they built dams on Monponsett Pond to keep its level high.

“That’s caused more nutrients from lawns and septic systems to go into Monponsett Pond, in addition to runoff from cranberry bogs,” DuBois said.

It’s been a challenge, she said, to manage algae blooms in Monponsett Pond and keep the cyanobacteria from being diverted into Silver Lake. In recent years, Monponsett Pond has been treated with an aluminum compound that causes the algae to sink to the bottom, DuBois said. She opposes that technique, saying it only offers a short-term fix.

“Monponsett Pond has been chronically afflicted with cyanobacteria,” DuBois said.

The DPH does not actively test for cyanobacteria, but will test if it receives reports or photographs indicating a body of water may be experiencing an algae bloom.

“What can we do about it?” Wood pondered. “One of the biggest things is controlling stormwater runoff.”