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Inner Harbor Experiencing a Severe Water Quality Event

An algal bloom followed by a thermal inversion is changing the way the harbor looks and smells, and could potentially cause a fish die-off.

If you’ve been near the Inner Harbor in the past few days, you may have noticed things look and smell seriously off.

Based on initial water quality readings, National Aquarium Director of Field Conservation Charmaine Dahlenburg says the harbor has experienced an intense chlorophyll bloom, followed by a thermal inversion.

The bloom was triggered by the recent stretch of dry, warm days, which allowed algae to rapidly grow in the water. The thermal inversion occurred when warm water at the surface was quickly cooled by dropping air temperatures, making the water dense. In an inversion, this “heavier” water sinks to the bottom of the harbor, pushing deep water that contains low levels of dissolved oxygen to the surface.

The algal bloom causes the harbor to turn a chocolate brown color, called a mahogany tide. The strong odor linked to this event is likely from either decaying matter brought to the surface and exposed to air, or the beginning of a nearby fish kill sparked by low dissolved oxygen. The bloom and thermal inversion are likely to continue to alter the way the harbor looks and smells over the next few days.

Algal blooms are caused by excess nutrients, like nitrogen and phosphorus, entering the water through human activities. This rapid growth in algae creates a chain reaction leading to extended periods of low dissolved oxygen. Unfortunately, aquatic communities struggle to survive in these conditions.

The Aquarium team partners with the University System of Maryland’s Institute of Marine and Environmental Technology, Maryland Department of Natural Resources, Blue Water Baltimore and others to monitor water quality in the Inner Harbor.