CHESTERTOWN — A large harmful algae bloom has been active for about two weeks now in the Sassafras River.

Speaking Tuesday, July 28, ShoreRivers Sassafras Riverkeeper Zack Kelleher said the bloom — which has claimed the life of a dog thus far — stretches from Turner’s Creek up past Foxhole Landing.

“And it’s not just on the main stem. It’s also coming up into the creeks off the main stem of the river,” Kelleher said. “People should be avoiding or limiting water contact, especially young kids and pets.”

ShoreRivers, a clean water advocacy organization headquartered in Easton, has been sending water samples from the Sassafras to the Maryland Department of Natural Resources. The DNR has confirmed the presence of the bloom on its site tracking locations of harmful algae in the Chesapeake Bay and its tributaries.

Kelleher said the National Oceanic and Atmospheric Administration also has helped track the bloom with satellite imaging.

The Kent County Health Department has been notified of the bloom, Kelleher said, and a cautionary advisory has been issued for the entire river.

One unit of measure to determine the concentration of a chemical or toxin present in water is “parts per billion” or “ppb.” Kelleher said the safe threshold for water contact and recreation with this type of algae is 0.8 ppb. He said samples collected in the Sassafras River are showing levels of 300 to more than 500 ppb.

“Not only were the cell concentrations really high, but then the toxin results came back and confirmed that the toxins present were very high and well over the threshold,” he said. “The bloom is still very much active and subsequent samples have shown that the cell concentrations are remaining the same or increasing.”

Kelleher said those most vulnerable from exposure are children, pets and adults with underlying health conditions. He said healthy adults also could end up with chronic health issues due to exposure.

Some popular swimming sites farther down the Sassafras like Still Pond and Betterton Beach are still safe, Kelleher said. He said those areas have a lot of tidal flushing to keep algae at bay.

Still, Kelleher said he has seen some algae come up in the Betterton area. He said the bloom also is present in some popular raft-up spots for boaters along the Sassafras.

Blooms are present in other local rivers, though not to the extent currently seen in the Sassafras.

Chester Riverkeeper Tim Trumbauer said there also is a bloom in the Chester River, primarily in the shallower areas with warmer water. He said there is a significant bloom in Rosin Creek, a Chester tributary.

“Fortunately, none of the algal blooms we have sampled in the past several years on the Chester River have been toxic, but there are always inherent risks in open water swimming and recreation. In the interest of caution, we always recommend that people avoid contact if the water looks green or brown or smells bad and always rinse off after swimming,” Trumbauer wrote in an email Tuesday, July 28.

Kelleher said the algae blooms will go away.

“We really need just the right amount of rainfall to flush it out of the creeks. But if we get too much rainfall, that’s just going to bring a lot of fresh nutrients into the system and trigger another bloom,” he said.

The bloom also could die off from the algae consuming all the oxygen in the water and ultimately suffocating itself, he said.

While algae is naturally occurring, Kelleher said the causes of the larger toxic blooms are from within our watershed — excess lawn fertilizer running off, poorly maintained septic systems and issues at wastewater treatment plants. He said climate change is a factor as well, with blooms starting earlier, lasting longer and becoming more toxic.

All of this has ShoreRivers conducting more community education.

“There are everyday changes that all of us can make on our own properties, in our towns and in our neighborhoods and communities that will reduce the occurrence and the severity of the algae blooms,” Kelleher said.

In an effort to reach community members, Kelleher will be hosting a Facebook live event at 5 p.m. today to discuss the algae bloom in the Sassafras River and take questions from viewers. Find him at www.facebook.com/sassafrasriverkeeper.

ShoreRivers maintains an online swimming guide as well that monitors bacteria levels in waterways. It covers popular swimming areas from the Sassafras to the Choptank River. Go to www.theswimguide.org/affiliates/shore-rivers.

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