Baltimore’s smelly pistachio tide seems to be happening earlier and getting worse

It’s a familiar experience along Baltimore’s waterfront: parts of the harbor turn a milky green and emit the stench of rotting eggs.

This phenomenon, which is known locally as a “pistachio tide,” has taken over Baltimore’s central waterway this week. It seems to be happening earlier — in the heat of July instead of early fall — and experts say it’s getting worse.

Harbor watchers think the widespread and prolonged pistachio tide that took over the harbor in late September and early October last year was likely triggered by the change of seasons. A drop in temperatures with the start of fall caused waters to turn over, bringing up an oxygen-deprived layer from the bottom. With it came sulfur bacteria, which multiplied in the sunlight and turned the harbor green and smelly.

But Maya Gomes, a geobiologist at Johns Hopkins University, said the event this week probably had a different trigger. During the heat dome in early July, algae bloomed and then promptly died off when heavy rains moved in soon after. Gomes said that dead algae remained near the surface of the water — already oxygen deprived from a hot summer — providing a feast for anaerobic bacteria that converted naturally occurring sulfates into toxic hydrogen sulfide.

Just like that, the harbor was teal green again, and the waterfront was drenched in a sulfuric stench.

Whatever the cause, Gomes pointed to satellite images of the harbor from the last five years that suggest these pistachio tide events have gotten worse, enveloping more of the harbor.

Experts think a persistent drought in Maryland may be exacerbating the harbor’s problem, while Gomes said climate change is probably another culprit.

Gomes studies bacterial ecosystems dating back to the Earth’s Precambrian times. She said phenomena like the one happening in Baltimore’s harbor have gotten more common during periods of high levels of carbon dioxide in the atmosphere.

As off-putting as the sickly green may be, Gomes said the color isn’t the problem. The green sheen, she said, comes from the process of photosynthesis that is oxidizing hydrogen sulfides, restoring dissolved oxygen to the system and bringing the harbor back to life.

Even so, Gomes doesn’t expect the problem to dissipate quickly. She said greenish color and stench of rotten eggs will likely linger in parts of the harbor at least until another rain can cleanse the system.

The pistachio tide that took over the waterway at the start of last fall — the worst local environmentalists can recall — got so extreme that Waterfront Partnership Vice President Adam Lindquist likened it at the time to an ecosystem “heart attack.” The event came on the heels of a series of major fish kills. This month’s pistachio tide also came with a fish kill that wiped out several thousand fish in the harbor.

Lindquist, who has helped lead a movement to swim in the Baltimore harbor, and other harbor experts have struggled to understand why that episode was so bad.

He’s noticed the periods where dissolved oxygen “flatlines” in the harbor have become more common in the last few years. Dissolved oxygen is necessary for fish and underwater life to survive. Tracking by the National Aquarium and the Maryland Department of Natural Resources shows the number of days with near-zero dissolved oxygen in the harbor jumped from none in 2022 to more than 50 in 2025, culminating in last year’s event.

Historic tracking shows that a similar prolonged period of depleted oxygen hit the harbor in September 2016, although Lindquist said it’s not clear whether that event was triggered by an inversion of water, an algae bloom or something else.

Decades of industrial pollution, sewage and urban runoff have depleted oxygen and fed sulfur bacteria in the harbor’s depths, creating the conditions for pistachio tides. Alice Volpitta, a water quality watchdog with the environmental group Blue Water Baltimore, said this week looks like a warning.

“I’m very concerned about the trajectory that we’re on and what we could be facing later this year,” she said.