After being teased last year by a glimpse of what a clean Chesapeake might look like, 2003 provided a jolt of reality to the region as higher than average river flows delivered the Bay some of its worst-ever water quality.
In July, the Bay’s low-oxygen “dead zone”covered the greatest area observed since Baywide monitoring began nearly 20 years ago. The area of oxygen-starved water stretched more than 100 miles down the Bay, from north of the Bay Bridge in Maryland to the York River in Virginia.
The dead zone rarely extends into Virginia, and had never before been observed south of the Rappahannock River.
The bad news continued into the tributaries, many of which had dense algae blooms, some of which were formed by toxic species. Water clarity in many rivers was the worst on record.
Watermen told of pulling nets of dead fish out of lifeless patches of water, and in some places “crab jubilees” were reported as blue crabs fled the water to escape low-oxygen conditions.
The effects were evident to researchers from the Virginia Institute of Marine Science who were conducting a Baywide fish survey during July. While waters with normal oxygen levels would produce up to 2,000 fish during a 20-minute trawl sample, the scientists collected less than a fish per tow from oxygen-depleted deep waters.
“We saw tow after tow return without any fish across the affected area,” said Chris Bonzek, who headed the VIMS survey.
Problems were not confined to deep waters. Some tributaries this summer had the densest concentrations of algae on record. A number of fish kills, some involving thousands of fish, were reported in tributaries as blooms drew oxygen out of the waters.
In August, blooms of the blue-green algae Microcystis produced toxins in the Sassafras River, causing the Maryland Department of the Environment to issue a warning about swimming or allowing pets or another animals to enter the water. The toxin could cause skin irritations in humans and has been known to kill pets that ingest the water.
Earlier in the year, scientists found a dense bloom of Karlodinium micrum on the Lower Patuxent River, another algae that can form toxins. Thousands of fish died, but it was unclear whether it was toxins, or low-oxygen conditions, which caused the kills.
Poor water clarity, both from algae blooms and from large amounts of sediment flushed into the water, also posed problems for underwater grass beds, which need sunlight to survive.
Water clarity was so poor that it, along with ongoing cloud cover, hampered the annual Baywide aerial survey of underwater grass beds, said Bob Orth, a VIMS scientist who oversees the survey. But when results are available next year, Orth said he expects acreage to be down “drastically,” largely because beds of widgeon grass—which can fluctuate widely from year to year—appear to have sharply declined in the mid-Bay. “There’s no doubt that widgeon grass will be taking a major toll,” he said.
But the news was not all bad, he said. High freshwater flows were helpful to freshwater species in upper tributaries that had been hurt by drought-related high salinities the past two years. Also, the cool temperatures were helping extensive beds of eel grass withstand poor water clarity in the lower Bay. “If water temperatures are low, they can handle low-light conditions,” Orth said.
Much of the Baywide impact, he said, will likely depend on next year’s conditions. He said grasses seem better able to bounce back from a single high flow year than two back-to-back years, such as 1993-94, which caused a sharp drop in grass acreage.
This year’s poor water quality was driven by unusually strong freshwater flows into the Bay. Every month except February had higher than normal river flows, which flush large amounts of nutrients that fuel oxygen-consuming algae blooms into the Bay.
Freshwater flowing into the Bay is lighter than salt water, so the Chesapeake’s waters form two layers: a fresher layer on the surface, and a denser, saltier layer along the bottom. A barrier, known as the “pycnocline,” forms between the two. The greater the differences in salinity and temperature, the stronger the pycnocline becomes, preventing bottom waters from mixing with oxygen-rich water on the surface.
When algae and other organic material sink to the bottom, they are consumed by bacteria in a process that consumes oxygen. Unable to be recharged, oxygen on the bottom is gradually used up.
Scientists believe the conditions that made this year unusually bad began with the colder than normal winter that deeply chilled the Bay’s waters.
Because of the pycnocline, the salt water on the bottom remained colder than normal as the surface temperatures began to increase—the difference in temperature between the top and bottom made the pycnocline even more severe than normal.
Unrelenting high flows that continued through the summer kept the pycnocline strong, and poured more nutrients into the Bay.
The U.S. Geological Survey reported that July flows into the Chesapeake were the third highest for the month since record keeping began in 1937, with flows averaging 45.1 billion gallons per day—or 83 percent above average.
June had the second highest freshwater flow into the Bay, surpassed only by 1972, when Hurricane Agnes hit the Bay and its watershed.
Most types of fish become stressed when oxygen concentrations fall below 5 parts per million in the water. In July, 40 percent of the Bay’s water fell below that threshold, sharply reducing habitat for fish, shellfish and other creatures.
Part of that area was much worse. Hypoxic water—areas with less than 2 ppm of oxygen and can be lethal to many species—covered a quarter of the Bay. And about 6 percent of the deepest water was anoxic—having no oxygen at all. This was the worst observation for both hypoxia and anoxia in the Bay.
Scientists said by the end of July, conditions were relieved somewhat as more oxygen mixed into the water, ending anoxic conditions but still leaving a huge area of hypoxic and low oxygen water.
This year’s poor conditions followed some of the best conditions in recent history, which were observed during last year’s drought. The dry spring and summer of 2002 resulted in fewer nutrients being flushed into the Bay, which meant less algae blooms and sediment in the water.
The result was the clearest water in recent memory, and—although final figures are not available—it appears there was an increase in the amount of underwater grasses.
The low flows also resulted in a weak pycnocline, which meant better mixing and more oxygen on the bottom. In fact, 2002 was the first time in nearly two decades of monitoring that no areas of anoxia were detected in the Bay.
But the drought was likely partly to blame for this year’s problems, officials said. Because crops grew poorly, they absorbed less fertilizer, leaving more on the land that could be washed into the Bay this year.
The states in the Bay watershed have approved sharp nutrient and sediment reduction goals which, if achieved, should dramatically improve the Chesapeake’s water clarity and oxygen conditions.
Although those reductions would not eliminate poor water quality during years with extreme high flows like this year, conditions would be greatly improved, said Rich Batiuk, the EPA Bay Program’s associate director for science.
There were, he added, some glimmers of good news this year. Despite poor conditions, anecdotal evidence has not suggested large declines in underwater grass beds beyond those in the mid-Bay, which have been prone to wide fluctuations in the past.
“The system didn’t all of a sudden completely crash,” he said. “It was certainly an extraordinary event in terms of the amount of low dissolved oxygen, but we didn’t see the system take a huge step back on restoration.”
He said that may be a sign that such restoration efforts as streamside buffers, the planting of cover crops and wetland creation may be starting to have an effect—though much remains to be done to help the Bay withstand similar events in the future.
“Mother Nature is not an excuse for low dissolved oxygen,” Batiuk said. “We have to figure out how to learn from events like this and use that to continue to restore the watershed.”