Pistachio Tides: An Explainer

Baltimore's Inner Harbor commonly experiences a type of water quality event that turns the water a weird green color, changes the way it smells and negatively impacts wildlife. We're taking a look at the science behind what's known as a pistachio tide—what it is, why it happens, how it impacts wildlife, and what we can do to prevent it.

What is a pistachio tide?

A pistachio tide is a type of sulfur bacterial bloom. These blooms occur in Baltimore's Inner Harbor and impact the quality of the water.

What does a pistachio tide look and smell like?

The chemical reaction between the sulfur bacteria and sunlight turns the water a bright green—or pistachio—color. The waste product of this process is hydrogen sulfide, which has an unpleasant, rotten egg smell.

When do pistachio tides occur?

These bacterial blooms tend to happen in the Inner Harbor during stretches of dry, precipitation-free, sunny days followed by sharp temperature fluctuations. In Baltimore, this typically happens in the fall. When it's hot during the day and cold at night, that drastic swing can bring naturally occurring sulfur bacteria living on the bottom of the harbor to the surface, a phenomenon known as thermal inversion.

Why does thermal inversion happen?

Water's density varies with temperature. Thermal inversion occurs when warm water at the harbor's surface cools quickly as air temperatures drop, increasing the water's density. This "heavier" water sinks to the bottom of the harbor, displacing the water at the bottom and pushing it to the surface. Sulfur bacteria—which live on the bottom of the harbor where there's little sunlight and no oxygen—come along for the ride. The next day, exposed to sunlight, the bacteria perform anoxygenic photosynthesis—or photosynthesis without the production of oxygen. It's this process that changes the color of the water and the way it smells.

How does this affect wildlife?

Along with changes to the water that we can see and smell, this anoxygenic photosynthesis process also causes the water's dissolved oxygen levels to drop. As a result, aquatic animals that rely on dissolved oxygen to breathe struggle to survive. The harbor is home to a wide range of species that need dissolved oxygen, including anemones, fish and blue crabs.

Are there any solutions?

In 2010, the Aquarium started experimenting with floating wetland technology in the Inner Harbor with the goals of reintroducing wetland habitat and promoting healthy water. Since that time, the team has been evaluating results and refining the Aquarium's floating wetland prototypes. The aeration system on the floating wetland prototype—which taps into the system used inside the Aquarium—moves the water and mixes it with atmospheric air. This can benefit animals when dissolved oxygen levels are low during water quality events. To evaluate the aeration component of the wetland, the team is conducting a study to compare water quality data from the center channel of the floating wetland with data from another part of the harbor. Results from 2021 showed that when the harbor experienced dangerously low dissolved oxygen levels, the shallow water channel within the floating wetland maintained dissolved oxygen levels that meet the standards to support aquatic wildlife 64% of the time.

As part of the Maryland Department of Natural Resources' Eyes on the Bay project, the Aquarium continuously collects and reports core water quality parameters in the harbor. This 24/7 monitoring provides important, near real-time data about water quality events as they are happening.