Mahogany Tides: An Explainer

If you've ever noticed the water in Baltimore's Inner Harbor turning a rusty, chocolate-brown color, you've witnessed a mahogany tide.

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Baltimore's Inner Harbor commonly experiences two types of water quality events that turn the water unusual colors, can change the way it smells and negatively impact wildlife. We recently published information about one type, a pistachio tide. Now, we're taking a look at the science behind the other type—a mahogany tide—to explain what it is, why it happens, how it impacts wildlife and what we can do to prevent it.

Graphic Shows How Excess Nutrients in the Water Cause an Algal Bloom Known as a Mahogany Tide

What is a mahogany tide?

A mahogany tide is an algal bloom. Excess nutrients like nitrogen and phosphorus in the water can cause algae in the water to grow out of control. Prorocentrum minimum, a chlorophyll-rich dinoflagellate, is the algae species responsible for mahogany tides.

What does a mahogany tide look like?

The name mahogany tide stems from the characteristic chocolate to rusty-brown color of the water during these blooms. There is a wide range of mahogany tide intensities. Algal blooms impact water clarity—how far down light can penetrate through the water column. Normal algae concentrations, measured in micrograms per liter, are about 15 micrograms per liter. We've seen readings as high as 700 micrograms per liter in the Inner Harbor during a mahogany tide.

When do mahogany tides occur?

In the mid-Atlantic region, mahogany tides tend to occur during stretches of warm, dry days in spring and summer. Heavy springtime rains also contribute to these blooms by flushing excess nutrients into waterways. These nutrients can come from runoff containing fertilizers from yards and farms, as well as from sewage overflows.

How does a mahogany tide affect wildlife?

The Prorocentrum minimum algae that cause mahogany tides generally aren't toxic. Still, substantial amounts of nitrogen and phosphorus in the water can cause too much of these algae to grow too fast and then die off. This process, called eutrophication, uses up oxygen and produces carbon dioxide, causing the water's dissolved oxygen levels to plummet. Aquatic animals like anemones, fish and blue crabs that rely on dissolved oxygen to breathe then struggle to survive.

Are there any solutions?

Restoring more green spaces and reducing the amount of excess nutrients entering waterways are key to limiting the occurrence and intensity of mahogany tides. In 2010, the Aquarium started experimenting with floating wetland technology in the Inner Harbor to reintroduce wetland habitat and promote healthy water. Since then, the team has been evaluating results and refining the Aquarium's floating wetland prototype to mitigate algal blooms in the harbor. The aeration system on the floating wetland prototype—which taps into the aeration system inside the Aquarium—moves the water and mixes it with atmospheric air. This can benefit animals by disrupting the formation of algae and increasing localized dissolved oxygen levels, which is important when the harbor experiences a negative water quality event. Previous surveys have shown the shallow water channel of the Aquarium's floating wetland to be more favorable during low dissolved oxygen events when compared to the extended harbor. Because of these positive results, aeration will be an essential component of the new National Aquarium Harbor Wetland presented by CFG Bank, which will open later this year.

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.

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