Westerly breezes take the wind out of Bay cleanup’s sails
Wind's direction tied to worse hypoxia since the 1980s
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Bay cleanup efforts may have, literally, been fighting a headwind almost from the day they started.
For years, scientists have been confounded by a Bay restoration paradox. Although the amount of nitrogen entering the Chesapeake has declined somewhat, the extent of low-oxygen waters-so-called "dead zones"-has increased.
New research offers a surprising explanation. Malcolm Scully, an assistant professor at Old Dominion University, suggests that a multi-decadal climate cycle has altered the region's wind patterns during spring and early summer months in a way that exacerbates the Bay's dissolved oxygen problems.
Scully's work shows that since the early 1980s, spring and summer winds over the Bay have shifted to blow more from the West compared with the previous several decades, when prevailing winds were more commonly from the South.
Winds help to improve dissolved oxygen conditions in the Bay by mixing oxygen-starved bottom water with oxygen-rich water near the surface. "But it seems you get the least amount of ventilation in response to a west wind," Scully said. "It is the least effective direction in terms of supplying oxygen to the bottom."
Scientists have long linked nutrient pollution to chronic low-oxygen concentrations in deep water during the summer months. Nutrients fuel algae production, and when there's more algae than can by consumed by predators such as oysters, zooplankton and certain fish, the excess dies and sinks to the bottom.
At the bottom, the dead algae are consumed by microscopic bacteria which, because of their high metabolism, rapidly draw oxygen from the water. In deep water, that oxygen is not easily replaced because of a barrier, known as the pycnocline, which separates saltier, heavier water on the bottom from lighter fresh water on the surface.
As a result, water below the pycnocline becomes hypoxic (having little oxygen) or anoxic (having no oxygen at all) creating the so-called "dead zone," a large area that contains too little oxygen for fish and many other aquatic species.
One of the key goals of the Chesapeake cleanup effort has been to reduce nutrient pollution to stem algae production, thereby boosting oxygen levels.
Bay Program monitoring, though, has shown that dissolved oxygen conditions have generally worsened in the Chesapeake over the years, even as nitrogen pollution has gradually decreased. A paper led by Jim Hagy, a former graduate student at the University of Maryland Center for Environmental Science, showed that since the early 1980s, a given amount of nitrogen was resulting in an increased amount of hypoxia compared with previous decades. (See "Study reveals long-term decline in Bay's oxygen," October 2004.)
For years, scientists have been trying to explain what they sometimes call the "Hagy phenomenon." Two research projects-one funded by the National Science Foundation and one by the National Oceanic and Atmospheric Administration-are aimed, at least in part, at understanding the discrepancy.
Scully's work appears to explain the majority, though not all, of the difference.
"I find it quite compelling," said Michael Kemp, a professor at the University of Maryland Center for Environmental Science, who has been involved with other projects working on the issue. "I'm not suggesting that there is a single bullet here, but his explanation makes a lot of sense."
Scientists have known since the mid-1980s that winds can affect oxygen levels below the pycnocline. Strong winds "slosh" water around in the Bay, sometimes pushing oxygen-starved bottom water onto shoals near the shore.
This sudden intrusion of low-oxygen waters into shallow areas causes the "crab jubilees" sometimes observed in the summer when blue crabs are forced out of the water.
As water sloshes into shallow areas, especially if it comes in contact with the surface, it picks up oxygen. So when the pycnocline sloshes back into its normal position, deep areas have more oxygen than before.
But Scully found that not all winds are created equal. Southerly winds-those that blow from South to North-are significantly more effective at supplying oxygen to hypoxic regions than winds blowing from the West.
This is mainly caused by how the water sloshes back and forth in response to the wind. The Coriolis force, caused by the rotation of the Earth, tends to transport water to the right of the wind direction. So when the wind blows from the South, the surface water is deflected right and piles up on the east side of the Bay, forcing the low-oxygen bottom water up onto the Bay's broad western shoals, where it is readily "ventilated" with oxygen directly from the atmosphere.
In contrast, when winds blow from the West, the Coriolis force tends to push water up and down the main axis of the Bay, rather than onto the shoals where it can gain oxygen
Scully examined long-term wind records from the Naval Air Station Patuxent River and found that since the early-1980s, the frequency of westerly winds has increased significantly in the spring and early summer. Prior to that, winds from the South, which are more effective at mixing Bay water, were more dominant.
The reason for this shift isn't totally clear, Scully and others said, but it appears to be related to the strength of the North Atlantic Oscillation. The NAO is the dominant atmospheric feature over the North Atlantic during the winter months, influencing both winds and storm tracks in the region. The strength of the NAO is measured by the difference in air pressure between Iceland and the Azores. The overall strength of the NAO fluctuates from year-to-year, and over multi-decadal time periods.
Since the early 1980s, the NAO has been in a phase in which it's had more of an impact on Eastern North America. Ironically, that's also the time Bay cleanup efforts began. The state-federal Bay Program was created in December 1983 and began promoting nutrient reduction efforts in the watershed.
The impacts of the NAO are generally felt in winter, and Scully said one of those impacts is an increased prevalence in westerly winds. When the NAO is strong, as has been the case for much of the past quarter century, Scully said it appears to cool the ocean surface and tends to weaken the Bermuda High, a dominant weather feature over the Atlantic during the summer. As a result, the NAO continues to influence the region's weather, and winds, well into the summer.
Scully's work appears to explain the majority of the discrepancy between nitrogen loadings and dissolved oxygen loadings, but not everything. "The total variability is not explained, so obviously there are other things going on," he said.
Some other physical factors affecting Bay anoxia may also be related to the NAO, such as changes in river flows into the Bay, or changes in salinity at the mouth of the Bay. In addition, other climate-change processes, such as temperature increase and sea-level rise, could also be affecting hypoxia.
Ecological process may also be involved in the shift to present conditions where more hypoxia is generated by the same nutrient loading. For example, research by Kemp's graduate students, Jeremy Testa and Jen Bosch, suggests a "vicious cycle" where hypoxia-induced changes in nutrient chemistry may reduce worm and clam abundance on the bottom which could, in turn, cause more hypoxia.
"There are quite a few details that need to be worked out before we have that final explanation," Kemp said.
It appears that the influence of the NAO is gone by midsummer. After mid-July, "the long-term trends in hypoxia have actually been following the long-term trends in nitrogen loads," said Rebecca Murphy, a graduate student at Johns Hopkins University, who has been analyzing long term Bay data as part of the NSF-funded project.
"This is really good news," Murphy said. "We are actually seeing a long-term pattern with the response of oxygen in the Bay to reductions in nutrient loads since the 1980s."
Scully and Kemp also said there's another hint of good news to come. If past history serves as a guide, the strength and location of the NAO is due for a shift. In fact, Scully said, there's a sign a shift may have taken place this year, but it's too early to say.
"It could be that it is flopping back into another phase," Scully said, "and maybe we will have atmospheric conditions that are less conducive to hypoxia in the next couple of decades."
In which case, Bay restoration efforts may finally have the wind at their back.
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