Pistachio Tides: What we know (and what we’re learning)
While we’re getting used to seeing pistachio tides in the fall, this summer we experienced a big one in July. We’re seeing them more and more frequently in the Harbor, and they’re not a straightforward phenomenon, so we wanted to offer some background based on what we know right now.
Our collective understanding is evolving, and this is an emerging issue for our Harbor. This is cutting-edge science that many partner organizations are working on with us. Stay tuned!
Although they are similar to algae blooms, pistachio tides are caused by a combination of two naturally occurring bacteria. Both of these bacteria live in anoxic environments, where there is little to no oxygen in the water.
sulfate reducing (SR) bacteria that thrive in the low-oxygen, mucky sediments at the bottom of the Harbor, where they make the hydrogen sulfide
green sulfur (GS) bacteria that live in the middle, where they can access both the sunlight and the hydrogen sulfide they need to photosynthesize
The bottom of the Harbor is a deep, dark, oftentimes oxygen-depleted place, where SR bacteria eat organic matter through a process called microbial sulfate reduction. This means they use sulfate to “eat”, rather than oxygen like us, and live in anoxic (low-oxygen) environments.
In the process of “eating”, SR bacteria release hydrogen sulfide (H₂S) that builds up in the bottom waters and sediment. Hydrogen sulfide causes the pistachio tide’s characteristic rotten egg smell.
When hydrogen sulfide finds its way to the sunlit portion of the water near the surface through strong winds, currents, or boat tides, GS bacteria use it to photosynthesize the way that plants use water. These bacteria produce the green color we see in a pistachio tide.
This is a process that’s natural and happens all the time. When there’s plenty of oxygen at the top of the water column, the process happens deeper and doesn’t affect the animals that live higher up (or us).
We see a pistachio tide when the typical layering of oxygen and hydrogen sulfide in the water column gets disturbed, and bacteria multiply in the wrong place. There are two ways that a lot of hydrogen sulfide can make its way to the surface:
First, “Thermal Inversion“: when air at the surface suddenly cools, it also cools the top layer of water, making it more dense and causing it to sink and the entire water column flips upside-down. This process brings up the hydrogen sulfide gas, which was produced at the bottom by SR bacteria. It also brings up cold, low-oxygen water from the bottom of the water column, allowing the GS bacteria to multiply close to the surface.
Second, “Party at the Surface”: too many nutrients in the water, usually from sewage and polluted stormwater runoff, make the surface water a better environment for both bacteria. Other organisms consuming these nutrients use up all the dissolved oxygen, allowing the SR bacteria to rise into the low-oxygen upper layers and produce hydrogen sulfide right where the green sulfur bacteria can immediately use it for photosynthesis.
Unlike a harmful algae bloom, the green sulfur bacteria that color the water do not cause any harm to people or animals. In fact, when they photosynthesize, they use up the hydrogen sulfide that smells so bad.
Unfortunately, both a thermal inversion and an oversupply of nutrients are bad for aquatic life that need the oxygen that’s no longer available. GS bacteria also don’t produce oxygen when they photosynthesize, unlike algae and other plants. The fish, crabs, and other animals we see on the surface during a fish kill have usually suffocated.
We’re continuing to work with our collaborators, including The National Aquarium, the University of Maryland Center for Environmental Science – Institute of Marine and Environmental Technology, Waterfront Partnership of Baltimore, and Johns Hopkins University, to research the patterns and additional triggers for pistachio tides and algae blooms.
We’re always learning new things about Harbor ecology. What we do know for sure is that a healthy, thriving Harbor ecosystem would be better able to handle seasonal weather fluctuations without mass die-offs of aquatic life.
And we know that a healthy Harbor starts upstream, with responsible development and functioning infrastructure. As we found in our long-term data analysis, polluted stormwater is consistently dragging down ecosystem health, and while sewage spills are improving, they’re still a major issue.
That’s why Blue Water Baltimore takes a holistic approach to restoring our waterways alongside our communities. What’s bad for the fish is bad for the rest of us, too.
Thanks to the following partners for contributing to our understanding of pistachio tides and this article:
Maya Gomes, Earth and Planetary Sciences, Johns Hopkins University
Charmaine Dahlenburg, National Aquarium
Adam Lindquist and Allison Blood, Waterfront Partnership of Baltimore
Eric Schott, UMCES and IMET
