Waterways like Marley Creek on Maryland’s Western Shore have been hit by algae blooms called mahogany tides, and it’s been killing thousands of fish. They’re dying from a lack of oxygen.We had a huge dinoflagellate bloom in the area where I work, which colored the water reddish brown for weeks, consuming CO2 and producing high levels of oxygen. In the past two weeks however, the blooms crashed (the dinoflagellates died and rotted), and oxygen levels have declined sharply and dissolved CO2 increased sharply. You can see some of the date here. I suggest looking at the past 3 months of data; look at chlorophyll (a measure of algae density), DO% (dissolved oxygen as percent of atmospheric) and pH (a measure of acidity, mostly CO2), and set the graph type to "stacked".
“Typically, it’s at the end of creeks where you get this algae blooming, and it depletes the oxygen from the water,” Pierre Henaart, a scientist with the Severn Riverkeeper Program, said.
I have not seen any fish kills here, and the oxygen graph above does not show dangerously low levels.
These fish kills are not quite the same as the ones we often see on my stretch of beach caused by the upwelling of low oxygen waters, which occur under some specific wind conditions in summer. The algae that fuel them are much the same, but the physics of the situations that cause them are different.