The city of Worcester has begun treating some of its water bodies for blue-green algae harmful to humans and fish.

City officials have asked residents to stay out of Coes Reservoir as contractors spray it on Wednesday with a copper sulfate formula that kills blue-green algae, also known as cyanobacteria. Contractors sprayed Indian Lake with the same treatment earlier in June. It’s a preventative measure that may become increasingly common as climate change helps fuel algae blooms.

Algae growth is common in the summer as water temperatures rise, and low levels of it are considered normal and safe. Too much, however, can be toxic to humans and deadly for fish.

“So we really want to avoid these blooms from occurring,” said Jacquelyn Burmeister, a senior environmental analyst for the city of Worcester. “If we see it reaching a level capable of producing these toxins, we will respond with an algaecide treatment.”

Polluted rainwater runoff that carries nutrients like nitrogen and phosphorus fuel the algae blooms. On land, nutrient-rich substances like fertilizer and manure help plants grow. When they flow off lawns and streets in water bodies, they do the same for algae.

Some of the toxins produced by the cyanobacteria can affect people’s skin, liver and brain. When the algae eventually die and decompose, they suck up oxygen in the water, effectively suffocating fish to death.

Burmeister said city workers and volunteers continuously test water bodies for cyanobacteria levels. In recent years, the city has had to close Indian Lake multiple times to recreational use due to algae blooms.

“Cyanobacteria really do enjoy Indian Lake,” Burmeister said. “It’s a shallow lake. It’s highly developed.”

The city usually sprays the lake with the algaecide multiple times a year. Burmeister stressed the copper sulfate treatment poses no risks to people and animals as contractors only use minimal amounts approved by state and federal environmental agencies. However, it’s easier for workers to apply the treatment when swimmers and boaters aren’t in the way, Burmeister said.

Worcester’s other large water bodies, Lake Quinsigamond and Bell Pond, haven’t had elevated cyanobacteria levels and haven’t required the algaecide treatment in recent years. Still, Burmeister fears climate change will exacerbate algae growth in water bodies around the city and elsewhere in Massachusetts.

As intense downpours become increasingly common due to global warming, more polluted stormwater rushes into lakes, rivers and ponds. Rising temperatures also make waterways more welcoming to algae, which prefer warmer water.

“I feel like we’re going backwards” because of climate change, said Patricia Austin, president of the Tatnuck Brook Watershed Association, which works to protect several Worcester water bodies. “We’re not being proactive about it.”

Austin said the state and federal government should allocate more funding to help localities upgrade stormwater systems to prevent polluted runoff from flowing into waterways. Indeed, Worcester officials said they’re already trying to install rain gardens and greenspace around lakes and ponds to help filter pollutants out of rainwater before it reaches water bodies.

Burmeister added residents can also help prevent algae blooms by picking up nutrient-rich dog droppings, avoiding use of fertilizers on lawns and not feeding geese.

“Geese like to congregate where they’re going to get food, usually on the beaches. And when they do that, all their droppings get into the water and can also [cause] cyanobacteria blooms,” she said.