About 24,000 fish and other sea creatures were killed in Baltimore’s Inner Harbor this week by a Pistachio Tide that depleted dissolved oxygen levels in the water.

Catfish, perch and blue crabs floated to the surface of the water.

Charmaine Dahlenburg, the director of field conservation at the National Aquarium, said the Pistachio Tide was caused by a hot day followed by a cool night.

The rapid change in temperature caused a thermal inversion, bringing bacteria from the bottom of the harbor to the top.

After being exposed to sunlight, the bacteria caused algae blooms, which quickly sucked up the oxygen in the water.

“This is a natural occurring event,” Dahlenburg said. “Normally what you'll see is this happening in ponds, because ponds are a more closed system. The water is very stagnant in a pond. And the harbor, unfortunately, acts like a pond at times because we dredged it so much.”

The Pistachio Tide usually happens about one to four times a year.

Dahlenburg said oxygen levels have crept up from zero milliliters per liter to two. However, an idea level is five milliliters per liter.

“At least we're seeing some sort of rebound. This event is going to be over very soon,” Dahlenburg said.

The harbor also experienced a Mahogany Tide recently. The Aquarium describes that phenomenon as “a type of brown algal bloom caused by nitrogen and phosphorous runoff found within the water combined with a stretch of warm, dry days with little water disruption from wind or rain.”

A Mahogany Tide can also cause oxygen issues.