There's a lot of fecal bacteria in Newburyport Harbor in the summer. Nobody knows why

There's something foul afoot in these waters — and the cause is unclear.

Repeated unsafe fecal bacteria levels have been recorded in Newburyport Harbor according to the Merrimack River Watershed Council. Newly released data from 2025 show a significant increase in summer bacteria spikes from the previous year — up to ten times higher in some cases.

Many Massachusetts rivers, lakes and harbors see a jump in dangerous fecal bacteria over the summer. The trigger is often heavy rain that leads to sewer overflows and stormwater runoff. Warm water then encourages bacterial growth.

But the bacteria spikes in Newburyport Harbor happen in both wet and dry weather, said Curt Rogers, executive director of the Merrimack River Watershed Council. He added that other data and a previous study indicate that the closest combined sewer overflow, in Haverhill, is unlikely to be the primary cause.

Rogers suspects that there are probably other culprits, like broken sewer lines, organic fertilizer or illegal dumping from boats.

“There’s a multitude of ways that fecal bacteria can get into the river,” he said. “We need to methodically move through each one of those and try to figure out which one — or ones — are contributing to what we're seeing.”

Swimming is not common in Newburyport Harbor. But the high summer bacteria levels can make the water unsafe for dogs, boating and other recreational use.

“I think if you're on a yacht and you're not really coming in contact with the water, you're pretty safe,” Rogers said. “If you're in a kayak, and your hand's going in the water and then you're touching your packed sandwich that you're about to eat, that's a different situation.”

Unsafe levels of fecal bacteria can cause gastrointestinal diseases and infections. Children, the elderly and people with compromised immune systems are at greatest risk.

The Merrimack River Watershed Council monitors the quality of the Merrimack River from Manchester, New Hampshire to Newburyport throughout the year, looking at factors like pH, salinity and temperature, as well as fecal bacteria. The full comprehensive 2025 Water Quality Report is expected to be released in late May.