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Study reveals long-term decline in Bay’s oxygen
Pictures worth 1,000 acres
In Bay food chain, little things can quickly add up
By taking a look back at the past two decades, Bay scientists believe they may be able to see the future of the Chesapeake—at least for the summer.
But the outlook in their first ever “ecological forecast” isn’t a pretty picture.
They predict the fourth largest area volume of anoxia—water with no oxygen—seen in the past two decades.
They also predict a harmful algae bloom on the Potomac River that will cover at least 10 miles starting in June and last about two and a half months.
Their prediction for grasses, the most uncertain of the forecasts, is a bit better. The scientists generally expect increases in the lower and upper Bay, with little change in the middle Bay.
The prediction was not so mystical as peering into a crystal ball. Instead, scientists turned their eyes toward 20 years of Chesapeake Bay water quality monitoring that has been collected by the Bay Program, generating about 10 million bits of data.
While it’s long been known that water quality—particularly dissolved oxygen levels—in the Chesapeake result from both weather and high nutrient inputs, it was the Bay Program’s first attempt to predict in advance what the summer could look like, rather than report on conditions after they occurred.
Like weather forecasts, the scientists acknowledged that their predictions are not a guarantee. “We could be wrong,” said Bill Dennison, a scientist with the University of Maryland Center for Environmental Science who is leading the forecast effort. “As well as we know this Bay, this is still an evolving state of science. This is not rocket science. It is a heck of a lot harder.”
Ecological forecasts are not widely done, although the size of the Gulf of Mexico “dead zone” and accompanying algal booms are predicted each year.
The Bay forecast is intended to help educate the public, challenge scientists, and help managers plan their activities.
Instead of being shocked by the poor water quality reports that are likely in coming months, the Bay Program hopes the forecast alerts people to the factors that lead to the conditions, and spurs interest in tracking water quality developments during the summer.
In addition, the forecast gives managers a chance to take action. For instance, officials along the Potomac River can plan early to warn citizens about potential impacts from the predicted bloom, and be prepared to monitor the bloom for toxins.
In the future, an expanded system of forecasts might be useful for planning where activities such as underwater grass plantings—which require good water clarity—should take place, and which areas should be avoided.
“Ecological forecasting really is something that is in its infancy,” Dennison said. “The Chesapeake Bay has a chance to play a leadership role in developing good forecasts that are disseminated in a public realm.”
By examining how close the the forecasts come to reality, scientists hope to improve their understanding of the Bay system, and ultimately fine-tune their predictive capability so they can make more specific forecasts in the future.
Dennison said future forecasts may include predictions for algae blooms in other areas, for different times of the year, and projections about the abundance fish, crabs and other “living resources.”
For now, the forecast was limited to anoxia, harmful algae on the Potomac and underwater grasses.
The anoxia forecast is based on the historic relationship between spring flows from the Susquehanna River, nutrient loads from the upper Bay, and the amount of anoxic water in the Bay. “This is a train that left the track staring in January and is going to roll right through until this summer, and there is very little we can do to stop that train,” Dennison said.
Strong flows help to create a barrier, known as the pycnocline, between fresh water on the surface, and salty ocean water on the bottom of the Bay. The nutrients fuel the growth of algae, much of which die and sink to the bottom where they are consumed by bacteria, which in turn use up oxygen. The pycnocline helps to prevent the bottom water from mixing with oxygen-rich water near the surface.
Anoxia is a true “dead zone” where almost nothing lives. And this year, scientists predict the anoxic area will cover 1.7 cubic kilometers of the deepest water in the upper and mid Bay. That’s 3.4 percent of the water volume of the mainstem of the Bay. (The mainstem does not include the Bay’s tidal tributaries.)
But the scientists said they were not able to forecast the amount of hypoxic water they expect this year. Hypoxic water—which occurs above the anoxic zone but below the pycnocline—contains oxygen, but at such low levels that it makes areas off limits for most fish and other aquatic life. In some years, the hypoxic zone has covered a third or more of the mainstem of the Bay.
It is actually hypoxic water, which can “slosh” from side to side in the Bay under the force of winds, which causes such problems as crab jubilees—where blue crabs leave the water to escape harmful conditions.
Scientists said they could not make a precise prediction for hypoxia because wind and other factors that affect water mixing play a larger role in determining the amount of hypoxia than is the case for anoxia. Nor could they forecast the amount of anoxia and hypoxia in tidal rivers, where most fish kills in the Bay take place.
But, said Dave Jasinski, a University of Maryland Center for Environmental Science analyst who helped to develop the dissolved oxygen forecast, “you would expect that if you are going to have severe anoxia, that would extend into hypoxia as well.”
Until the mid 1900s, anoxia rarely, if ever, occurred in the Bay. In recent years, an area of anoxic water has occurred every year except the unusually dry 2002, when fewer nutrients were washed into the Bay.
If the Bay Program’s nutrient reduction goals were achieved, computer models suggest that anoxia would once again become a rare event, showing up only in the most extreme weather conditions.
“While we can’t rely on Mother Nature to call off the rains and reduce the amount of runoff, we can reduce the amount of pollution in that runoff by better managing the lands that surround the Bay,” said Carlton Haywood of the Interstate Commission on the Potomac River Basin and chair of the Bay Program Monitoring and Analysis Subcommittee.
The Potomac River bloom prediction is based on a model that relates spring flow rates, along with the previous year’s flows, to when the bloom starts, how long it lasts and the area it covers.
Based on that model, scientists predicted a bloom starting in June that would cover more than 10 miles of the river and last about two and a half months. But factors such as high wind events or unusual temperatures could affect the size of the bloom.
Blooms often occur in other tributaries as well. The Potomac was singled out for the forecast because it has been closely monitored for a longer period of time. “This was a system where we were able to develop a set of relationships that we felt were fairly strong,” said Peter Tango, chief of Quantitative Ecological Assessment for the Maryland Department of Natural Resources. “We would like to extend that to other parts of the Bay.”
The Potomac has had a bloom of cynobacterium, Microcystis aeruginosa, most summers since the 1960s. Microcystis often contains toxins, and last year a huge bloom that covered more than 30 miles of the river caused several beach closures. Tango said all of the samples tested during the past two years contained toxins, although the amounts varied by location and time of year.
Bay scientists also attempted a forecast for underwater grass beds. It called for increased grass coverage in the Bay’s low-salinity waters, similar acreage in medium salinity areas and a small increase in high-salinity waters.
Dennison said the grass bed predictions had the highest margin of error because many weather factors during the summer can affect growth in particular places. But he said scientists generally believed the increasing trend of recent years in the upper Bay should continue, and that grass beds in the lower Bay—which were damaged by Hurricane Isabel in fall 2003—should rebound.
“It’s a positive picture,” he said. “We think this will be a good year for grasses in the Bay, but probably not the best.”
For information, and to keep track of field observations made by scientists this summer, visit the Bay Program’s Ecological Forecasting web page, www.chesapeakebay.net/bayforecast.htm
