Blooms in Baltimore's Harbor

Baltimore's Inner Harbor commonly experiences two types of water quality events that can turn the water unusual colors, give off a distinctive odor and have a negative impact on wildlife.

One type, an algal bloom fed by excess nutrients like nitrogen and phosphorus in the water, is often referred to as a "mahogany tide" due to its characteristic chocolate-brown color. These algal blooms are often observed over the summer months during stretches of hot, dry days.

The second type primarily happens in the early fall when Baltimore experiences sharp temperature changes between night and day, bringing sulfur bacteria living on the bottom of the harbor to the surface, a phenomenon known as a thermal inversion.

"Sulfur bacteria live happily on the bottom of the harbor where there's a tiny bit of sunlight and no oxygen," explains National Aquarium Director of Field Conservation Charmaine Dahlenburg. "When we experience a really hot day followed by a cold night, like we often see in September and October, it affects the density of the water. This causes surface water to sink, displacing the water at the bottom, which is then brought to the surface."

When the sun rises the next day, the bacteria harvest sunlight to perform anoxygenic photosynthesis—or photosynthesis without the production of oxygen. The waste product of this process is hydrogen sulfide, which has a characteristic smell of rotten eggs. The chemical reaction between the sulfur bacteria and sunlight also results in a bright green color, which is why this bloom is often referred to as a "pistachio tide."

Unfortunately, the outcome is a drastic drop of dissolved oxygen levels in the water, causing stress for aquatic wildlife in the harbor—including fish and blue crabs.

Since 2016, the Aquarium has partnered with the Maryland Department of Natural Resources to collect core water quality parameters in the harbor every 15 minutes, 24 hours a day, seven days a week. This allows us to continuously monitor dissolved oxygen levels as well as temperature, salinity, chlorophyll, turbidity and pH, make quick assessments when a water quality event occurs, and study short- and long-term patterns and trends.

Real-time data from the Aquarium's three devices, called sondes, can be found on the DNR's Eyes on the Bay site. Through this data, Charmaine and other Aquarium team members conduct and support research to better understand the harbor's water quality. This research is focused on seasonal water quality trends in the harbor, what triggers the cycle of algal and bacterial blooms, the role of water quality in biodiversity, and how the elements of the Aquarium's floating wetland contribute to harbor health.