Greenish, cloudy, and with a smell like rotten eggs, “pistachio tides” are causing concern among residents who live along the Magothy River.
Magothy River Association member Bob Moyer said he went out to photograph a particularly large pistachio tide on Cattail Creek in October, and was so overwhelmed by the sulfur smell that he collapsed to his hands and knees and had to crawl off the pier.
Pistachio tides occur when bacteria in the river produce hydrogen sulfide, which depletes the oxygen in the water and creates a rotten smell. The bacteria produce a bright green color, which is where the name comes from.
Working with the river association, Johns Hopkins University is researching the bacterial booms in Cattail and Old Man creeks this summer to identify where and when they occur. Pistachio tides usually develop between August and October, and Hopkins started collecting water samples in early July to get ahead of the blooms.
Paul Spadaro, president of the Magothy River Association, is concerned about people breathing in the sulfur, especially on Cattail Creek, where recreational activity often takes place. Without any monitoring or warning system, kayakers may paddle into a pistachio tide.
“I do think people need to know that when you smell that sewer gas, it’s time to move away,” Spadaro said.
Part of Hopkins’ research this summer is to determine if there is a danger to wildlife or humans. Sulfur bacteria are naturally occurring, but stormwater and fertilizer runoff entering the creek can increase the pervasiveness of these bacteria by depleting oxygen.
In October, Spadaro said 380 fish in the Town Neck region of Cattail Creek were found dead during the pistachio tides. In September, a bloom in Baltimore’s Inner Harbor killed 24,000 fish. The Department of Natural Resources deploys continuous monitoring of these blooms in the Inner Harbor.
“It’s the sulfur bacteria’s appetite for oxygen that makes it dangerous to fish, crabs, eels, and other aquatic life that need dissolved oxygen to breathe,” said DNR program manager Cathy Wazniak.
However, Wazniak said that without long-term data on pistachio tides in the Chesapeake Bay, she could not say whether any trends exist.
“We’re in here for the pistachio tides, because they’re kind of falling under the radar,” said Sarah Preheim, an associate professor at Hopkins who is leading the research project.
Hopkins received a $312,000 grant in 2024 from the National Science Foundation to research microbial processes in the Chesapeake Bay. About $2,000 is going toward pistachio tide research on the Magothy River.
Preheim said rising overall temperatures and nutrient pollution contribute to pistachio tides because warmer water holds less oxygen, based on monitoring data from the Inner Harbor in the past decade.
Moyer, a Berrywood resident, hopes Preheim’s research will help educate residents about the blooms and encourage them to rethink where the rainwater on their property goes. In the Berrywood neighborhood, storm drains open right into the creek.
He says homeowners should fertilize their lawns late in the fall, if at all, when colder water holds more oxygen and is less susceptible to the damage caused by the fertilizer chemicals. He also wants more waterfront residents to consider putting native plants as a buffer between their lawn and the creek.
Preheim said she is focusing this summer on collecting water samples to help determine what environmental factors are causing the blooms, so that next year she can create a predictive model to help warn people when these pistachio tides might occur.
One possibility is to provide sulfur measuring strips to citizen scientists, such as those at the river association, so they can test the water themselves for sulfur levels.
“The people need to know if it’s really toxic,” Spadaro said.
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