Dead fish and green water turned up in Baltimore’s Inner Harbor this week, but experts say the phenomenon is related to temperature changes, not necessarily poor water quality.
The region has experienced cool nighttime temperatures over the past several days. When colder, denser water sinks from the harbor’s surface to the bottom, it pushes bottom-dwelling sulfur bacteria to the surface, explained officials from the National Aquarium.
This process, called thermal inversion, causes bacteria to be exposed to sunlight and perform anoxygenic photosynthesis, meaning photosynthesis without oxygen production.
The resulting algal blooms give the water a “pistachio green” color and funky smell. The algae also consume dissolved oxygen in the water without replacing it. Without enough oxygen in the water, fish and other aquatic animals suffocate.
In addition to last night’s pistachio tide, aquarium officials said the harbor also experienced a mahogany tide in recent days.
Nitrogen and phosphorus runoff into the water, combined with several warm and dry days that leave the water undisturbed by wind or rain, results in a mahogany brown algal bloom.
The algae blocks sunlight from reaching deep under the water’s surface and can cause dissolved oxygen levels in the water to drop. Without enough oxygen, animals like small fish and blue crabs die off.
“Pistachio and mahogany tides are not necessarily an indicator of poor water quality,” aquarium officials said in a news release. “Rather, they are naturally occurring, often temperature-related seasonal phenomena which happen one-to-four times per year, lasting several days at a time after which water quality improves.”
The National Aquarium’s conservation team has recently noticed more small animals sheltering in the aquarium’s new “Harbor Wetland” exhibit, which has an aeration system that increases dissolved oxygen levels in the water.
