The Charles River is having its first major bloom of cyanobacteria in years, the Massachusetts Department of Public Health (DPH) stated last week.

Exposure to cyanotoxins released by these bacteria can cause serious health issues, especially for pets and young children.

The bloom is concentrated between the Longfellow and Zakim Bridges. DPH has deemed the water there unsafe and has advised the public against swimming or boating in it, as cyanobacteria can sometimes produce cyanotoxins, which range from skin irritants to neurotoxins depending on the bacteria’s species.

“Ingestion is the primary concern since ingesting small amounts of cyanobacteria or cyanotoxin can cause gastrointestinal symptoms while larger amounts may cause liver or neurological damage,” the DPH website states. “Small children and pets are more susceptible to the effects of cyanotoxins than adults.”

Visitors to the river will see that the water has turned bright green.

“[Cyanobacterial blooms] often results in the waterbody turning bright green or blue-green and forms a surface scum or a discoloration of the water column,” the DPH website states. “They often look like pea soup or spilled, blue or green paint. However, the color can also vary.”

Such blooms can also negatively impact wildlife in and along the river. The algae sucks oxygen out of the water, preventing fish and other aquatic life from breathing.

Cyanobacteria is the scientific name for blue-green algae, a naturally occurring microorganism in rivers and lakes. It is usually present at minimal levels and does not have health effects at such low concentrations. According to the Charles River Watershed Association (CRWA), the algae grows excessively when the water meets certain conditions.

“Cyanobacteria grow rapidly when bodies of water experience high phosphorus levels, hotter temperatures and stagnant conditions,” the CRWA said in a press release. “In the Charles River, cyanobacterial blooms are a symptom of excess stormwater pollution and degraded ecology.”

Stormwater pollution has been a consistent problem for the city bound sections of the Charles, including the part that meets Back Bay and Beacon Hill, as well as the Muddy River tributary, which runs through the Fenway.

Last month, the CRWA released its annual report on the water quality of the Charles, based on metrics like cyanobacterial concentration and E. coli levels. The lower basin, which encompasses Back Bay and Beacon Hill, received a B-, while sections farther away from the city scored in the A range.

Emily Norton, the association’s executive director, said at an event at the time that the lower basin’s grade meant it was swimmable between 50 and 75 percent of the time. Some parts of the river near the Longfellow Bridge were swimmable nearly all of the time, she said. “This bloom is particularly troubling as it comes amid Boston’s longest dry spell in 25 years, with no significant rainfall for more than three weeks,” the CRWA press release stated. “The decreased precipitation has led to lower water levels, which, coupled with the heat and increased phosphorus from stormwater runoff, creates ideal conditions for these toxic blooms.”