State officials are investigating what killed thousands of fish in Marley and Furnace creeks in northern Anne Arundel County, but suspect they suffocated after an algae bloom sucked the oxygen out of the water, a Maryland Department of the Environment spokesman said Tuesday.

Investigators saw an estimated 6,000 dead and apparently dying fish Monday, mostly in Marley Creek but some as well in adjoining Furnace Creek, said MDE spokesman Jay Apperson. There were at least nine different species of fish involved, including Atlantic menhaden, silversides, silvery minnows and sunfish, he said.

The fish apparently suffocated, according to Apperson; investigators measured almost no dissolved oxygen in the water at the time.

The low oxygen levels were likely a byproduct of the algae blooms seen since March along the upper western shore of the Chesapeake Bay and in tributaries, such as the Patapsco River and Marley Creek, Apperson said.

The MDE spokesman said investigators identified Prorocentrum minimum in the water, a type of algae often associated with mahogany or red tides that discolor the water. Though not considered harmful to humans, they can deplete the water of oxygen as they die and decay. The bloom in Marley appeared to be dying back, Apperson said.

About 30 dead fish were reported near City Dock in Annapolis, Apperson said. Investigators suspect those were blown there from the Severn River, he said, which has also been experiencing an algae bloom.

Algae “bloom,” or grow like crazy, when the water is chock-full of the plant nutrients nitrogen and phosphorus, which get into the bay from sewage discharges and leaks, from storm runoff and from air pollution falling out of the sky.

The bay received a near-record infusion of nutrients late last summer when Tropical Storm Lee soaked the region. The lower Patapsco took another hit in late March when 50 million gallons of untreated sewage spilled from a Baltimore County pumping station. About that time, with warm weather favoring algae growth, scientists first noticed blooms there and elsewhere around the bay, and have been following their growth since.

Bacteria levels from the sewage spill just recently dropped back to acceptable levels, so that health officials on Monday lifted their warning against swimming or water contact.

Story: Md. state officials investigate what killed thousands of fish