The Jones Falls turned a milky green Thursday where it empties into the Inner Harbor, and hundreds of fish turned belly up in what authorities described as a bacterial event that suffocated the fish and released noxious sulfur from the bottom muck.
About 200 fish, mostly menhaden, appeared to have died in the channel between Pier 5 and Pier 6 where the Jones Falls emerges from beneath downtown streets, according to Jay Apperson, spokesman for the Maryland Department of the Environment. Tests found the water almost devoid of the dissolved oxygen fish need to breathe, he added.
Investigation of the cause continues, but Apperson said preliminary indications are that the water was discolored by a "bloom" or growth of bacteria that feed on sulfur.
The sulfur is released from bottom sediments when there is no oxygen in deeper water, he explained. There appeared to have been an algae bloom in the area earlier, he said, and that depleted oxygen in the bottom of the channel as the microscopic plants died, sank and began decaying.
Tests found little ammonia in the water, according to Jeff Raymond, spokesman for the city Department of Public Works, indicating that sewer overflows that have troubled the Jones Falls upstream did not play a role in this incident.
Harbor water-quality watchdogs said the lack of oxygen in the water should not be considered natural, that algae blooms and fish kills stem from pollution washing into the streams and storm drains that empty into the harbor.
"Everyone needs to remember that the root of this event in the harbor is nutrient pollution, mostly from polluted stormwater discharges," said Tina Meyers, Baltimore Harbor Waterkeeper.
"Water turns over in healthy bodies of water all the time and it's no big deal," said Adam Lindquist, Healthy Harbor coordinator for the Waterfront Partnership, a group of businesses, nonprofits and municipal agencies working to restore the harbor.
"What makes it so noticeable in the Jones Falls and the harbor is that polluted stormwater runoff has contributed too many nutrients to the water," he added, "which is why there is no oxygen, especially lower down in the water column."