Bay ‘dead zone’ average in early August
Oxygen conditions in the Chesapeake, which started the summer better than average only to spike to among the worst “dead zones” on record for late July, settled into the normal range for early August, according to an update released by the Maryland Department of Natural Resources.
Scientists said the area of low-oxygen water, which in late July had stretched from near the mouth of the Patapsco River into Virginia’s portion of the Bay, had retreated into Maryland’s portion of the Bay by early August. Also, the so-called dead zone, where there’s almost no oxygen for fish to breathe, was more narrowly restricted into the Bay’s deep-water channel in monitoring done early this month.
Early August monitoring showed that about 14 percent of the Bay’s mainstem, or 1.15 cubic miles, suffered from low-oxygen water, according to the DNR. That was close to the long-term average of 1.13 cubic miles for this time of year, based on Baywide monitoring that began in 1985. It was also in line with the 12 percent prediction scientists had made at the beginning of the summer.
Still, that was much worse than late June, when just 0.42 cubic miles of low oxygen, or hypoxic, water was reported, the second smallest amount observed for that time of year. By late July, though, the amount of hypoxic water grew to 1.65 cubic miles, or about 20 percent of the Bay, which was the seventh largest dead zone for that time of year.
DNR scientists have said that higher than average temperatures, and higher salinities caused by low rainfall, may have contributed to the worsening conditions since early summer.
Poor dissolved oxygen is typically driven by nutrients that spur excessive algae growth in the Bay. When the algae die, they sink to the bottom and are consumed by bacteria in a process that draws oxygen out of the water. Unless mixed with oxygen-rich water near the surface of the Bay, the bottom waters can degrade into an oxygen-starved “dead zone.” Improving oxygen levels in the Bay is a key goal of the Chesapeake restoration’s nutrient reduction efforts.
Hypoxic water has less than 2 milligrams of oxygen per liter of water, and is harmful to most aquatic life in the Bay, which usually need significantly higher amounts of oxygen to thrive. Striped bass, shad, white perch, yellow perch and hard clams, for instance, all prefer at least 5 milligrams per liter of oxygen in the water.
On the positive side, early August monitoring continued to find no anoxic water — areas with essentially no oxygen. Scientists had predicted that 2 percent of the Bay would be covered with anoxic water by late summer.
- Category: Pollution
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