A pistachio tide at Fells.

Alicia Crawford moved to Fells Point recently and was looking out her balcony window when she noticed the water looked really green. She tagged The Baltimore Banner in a post on Twitter hoping to get to the bottom of it.

“It just was contrasting so strongly with the sky, I figured there must be something going on,” Crawford said.

We investigated the issue and found a delightfully named answer: Crawford was seeing a “pistachio tide.”

Scientists say the milky green color comes from green sulfur bacteria, which are stinky microbes that dwell on the bottom of the Inner Harbor. They are associated with hydrogen sulfide, a toxic chemical that smells like rotten eggs. They are also anaerobic, meaning they don’t need oxygen to thrive in the oxygen-poor depths of the harbor.

A process known as thermal inversion, however, occasionally shuttles the smelly microbes to the surface. Here’s how it works: If a warm day follows a sharply cold night, the top layer of water becomes much colder than the water under it. Since cold water is denser than warm water, that top layer sinks, churning up the whole ecosystem and swirling any bottom-dwellers to the surface.

Thermal inversion is a normal process, but two things may make pistachio blooms more frequent: climate change and unabated water pollution. And more blooms spell danger for aquatic wildlife.

Makayla Stewart and Eric Schott examining a potential mahogany tide outside the Institute of Marine and Environmental Technology (IMET).

One of the greatest threats to the harbor’s wildlife is hypoxia, or a lack of oxygen in the water, said Eric Schott, associate research professor at the University of Maryland Center for Environmental Science (UMCES) Institute of Marine and Environmental Technology. Desperate for dissolved oxygen, fish rush to the surface of the water in hypoxic areas. If there’s not enough oxygen, they perish, resulting in the disturbing image of fish seemingly drowning in water.

Pistachio tides don’t immediately lead to hypoxia, but once those green sulfur bacteria die, decomposing microbes begin consuming the bacteria’s corpses. As the decomposing microbes feast, they suck up oxygen from the water, sometimes to dangerously low levels.

To cook up an algae or bacteria bloom you need nutrients, namely nitrogen and phosphorus. “Our water is so oversaturated with nutrients that we have constant low-level blooms of algae,” said Adam Lindquist of the Waterfront Partnership of Baltimore, a nonprofit dedicated to cleaning and protecting the city’s waterfront.

The city’s aging sewage infrastructure is a major source of excess nutrients in Baltimore’s harbor, which allows fecal matter to seep into the water, said Schott. Cars also contribute by spewing nitrogen oxides into the air. When it rains, that car exhaust condenses and falls into the water as a nitrogenous feast. Stormwater runoff is a third major source.

With all those excess nutrients, algae and bacteria populations start exploding as soon as it gets warm, Lindquist said.

“At first, I was worried it was a spill or something,” recalled Crawford. “So I thought there’d probably be someone that had an answer, and I posted the picture, tagged you guys. And sounds like you got to the bottom of it.”

Mahogany tides

Pistachio tides aren’t the only tides in town, either. Mahogany tides are caused by a dark reddish-brown algae, Prorocentrum minimum. “They’re almost a café au lait color,” said Schott. They aren’t directly toxic or oxygen-depleting, but like pistachio tides, their death and decomposition are a serious threat to underwater wildlife.

Makayla Stewart, an intern at the Institute of Marine and Environmental Technology (IMET), sampling the water for a potential mahogany tide in the Inner Harbor.

A more sinister curiosity in Baltimore’s water is Karlodinium veneficum. It’s an algae that sometimes acts like a plant, floating around photosynthesizing but at other times acting like a microscopic monster, hunting and killing other algae by deploying toxins. Those toxins linger in the water and destroy the gills of passing fish, said Allen Place, professor at IMET and UMCES.

These half-plant, half-predator oddities are not too common, because a parasite keeps their population levels low. “Basically, [the parasite] burrows inside them, eats the nucleus and then bursts out 40 copies of itself,” crashing populations, said Place. Still, their nutrient-bolstered presence is enough to threaten local fish.

So besides pistachio and mahogany, what does a normal harbor look like?

“I don’t think there is a normal color,” said Schott. But a healthy color might be “if you could see down a meter, and there’s a kind of green tinge, dark green like a plant,” instead of bright green like pistachios. That would be a sign of beneficial algae producing oxygen, he said. Another common but neutral color can be milky brown after rainfall, due to sediment.

The toxic algae Karlodinium veneficum, which sometimes acts like a plant and sometimes eats other algae.

Climate change threatens to make blooms more frequent as temperatures warm up and weather becomes more erratic, said Place. And the conditions that lead to algal blooms don’t only affect wildlife. They make the water unsuitable for humans to swim in, too.

In 2010, the Waterfront Partnership of Baltimore, which cooperates with government agencies, private groups and local communities to protect the waterfront, set the goal of making the harbor safe for fishing and swimming by 2020. That didn’t happen.

However, a report released by the organization in 2020 outlines the progress made so far. Levels of sewage-borne bacteria consistently declined that decade, and many areas of the harbor now meet the Maryland Department of the Environment’s guidelines for recreation — at least when the weather is dry.

The Waterfront Partnership is currently trying to expand water trails for recreation, such as paddle boarding and kayaking, said Linquist, and he remains hopeful that recreation will become even safer as fecal bacteria contamination levels continue to drop.

For now, it is not recommended for anyone to swim in the water. Reducing air pollution, improving sewage systems and mitigating climate change can make the harbor safer and less prone to blooms — benefitting both the wildlife and the people of Baltimore.