Oxygen-starved areas of the Chesapeake Bay appear to have grown steadily since the 1950s, and the situation has not improved in the last decade despite efforts to control nutrient inputs, according to a new scientific study.
The paper, which has been well-received in the Bay scientific community, shows that low oxygen conditions became more severe and affected more of the Chesapeake as nutrient inputs sharply rose after the late 1960s.
But in a surprise finding, the study also found that the largest areas of severe hypoxia—low oxygen—occurred since 1990, even as nutrient loads into the Bay have leveled off or decreased.
That suggests other factors may have recently compounded the Bay’s chronic summertime hypoxia problem, said Jim Hagy, a scientist with the EPA’s National Health and Environmental Effects Research Laboratory in Florida.
“A simple explanation that ‘X’ amount of nitrogen results in ‘X’ amount of hypoxia does not hold up as completely adequate to explain what we see,” he said. “There is something different about how the system is responding.”
The study was published in the August issue of the journal Estuaries and grew out of Hagy’s graduate work at the University of Maryland’s Center for Environmental Science in which he sought to establish a long-term relationship between dissolved oxygen concentrations and nitrogen loads to the Bay.
Almost all of the organisms living in the Bay require at least small amounts of dissolved oxygen to survive. If the oxygen is not available, they are either squeezed into other, potentially less desirable, habitats or—if they can’t move—they may die.
Trying to improve water quality conditions has been one of the top cleanup goals for the state-federal Bay Program partnership, which last year established sharp nutrient reduction goals to accomplish the job.
Scientists generally agree that nutrients have worsened the Bay’s dissolved oxygen situation, but it’s been unclear to many exactly how important a role nutrients play compared to other factors, such as river flow.
Strong river flows cause the Bay to stratify between warmer, less-salty water near the surface, and cooler, saltier water along the bottom.
Nutrients fuel the growth of phytoplankton and other material which—when produced in greater quantities than can be consumed by predators—die and sink to the bottom where they are decomposed by bacteria. The bacteria have high metabolisms and therefore consume large amounts of oxygen. When stratification is strong, the bottom layer of water does not mix with the surface layer (which can get oxygen from the atmosphere) and gradually becomes oxygen-starved.
Piecing together a historical picture of oxygen trends is difficult because no comprehensive water quality monitoring program existed for the Chesapeake until one was launched by the Bay Program in 1984.
As a result, Hagy had to scour through dissolved oxygen data collected as parts of different studies over the years by different researchers at various institutions. He was also able to find long-term measurements of nitrogen concentrations from the Susquehanna River.
Because it delivers more than two-fifths of the nutrients and half of the water entering the Bay, the Susquehanna is the single most important river affecting Chesapeake dissolved oxygen levels. The figures showed that nitrogen concentrations in the river increased threefold from 1945 through 1989.
Meanwhile, the dissolved oxygen data pieced together for the Bay showed that prior to 1968, anoxic (areas with no oxygen) and severe hypoxic conditions were limited to high river flow years when stratification was strong. Even then, according to Hagy, anoxic conditions were confined to the deepest part of the Bay in Maryland, a relatively small area. By the late 1960s, dissolved oxygen conditions were noticeably worsening. Summertime oxygen concentrations in deep waters were decreasing, and the area of affected water was expanding.
By the late 1980s, a clear relationship between increasing nutrient loads and decreasing dissolved oxygen concentrations was evident, according to Hagy. Regardless of the amount of river flow entering the Bay, anoxia and hypoxia were worse than earlier years with similar flows.
“It used to be that there was so little data that you were splitting hairs to say it wasn’t just flow causing low dissolved oxygen,” Hagy said. “But when you combine the two data sets in the way it’s been done here for the first time, it is simply not possible to make that argument. That theory cannot be supported by the data any more.”
What surprised Hagy is what the data showed after 1990. Figures from the Susquehanna River showed a slight decrease in nitrogen concentrations starting in 1989. Yet the 1990s had some of the worst low oxygen conditions on record, with severe low oxygen conditions stretching well into Virginia’s portion of the Bay on several occasions. Those conditions could not be explained by either river flow or nitrogen loads.
Exactly why that is happening, Hagy said, is unclear. But he said it’s possible that the Bay’s poor water quality has hampered natural processes that once helped to keep it clean.
For example, denitrification—a biological process that converts nitrate into harmless nitrogen gas—naturally takes place along the bottom of the Bay. Denitrification, though, requires both oxygen (for organisms that convert ammonium into nitrate) as well as anoxic conditions (to support organisms that convert nitrate into unusable nitrogen gas).
Anoxic conditions can be found in the sediment just a few millimeters below the surface almost anywhere. But when a large amount of the Bay’s bottom is covered with anoxic water, the oxygen needed to carry out the first part of the denitrification process is lacking. “Once you get a certain amount of anoxia, that process is reduced,” Hagy said, “so essentially the system retains more nitrogen longer.”
In addition, the loss of natural filtering organisms may result in more organic material making it to the deep areas of the Bay to be decomposed. Clams, mussels, oysters and other benthic-dwelling organisms may have filtered large amounts of phytoplankton and other organic material from the water before it reached deep areas of the Bay to die and decompose. But low dissolved oxygen conditions have taken a toll on those organisms as well. Benthic surveys over the past decade have shown that more than half of the Bay’s bottom fails to support healthy benthic communities, reducing the Chesapeake’s filtering capacity.
Also, extensive underwater grass beds once served as a buffer that trapped particles and nutrients in nearshore areas before they could make it to the mid-Bay, Hagy said. But the amount of those buffering grass beds remains far below historic levels.
Other factors could also be at play. In recent years, the ratio of nitrogen to phosphorus entering the Bay has changed. Phosphorus, the target of phosphate detergent bans in the Bay states and long regulated in wastewater discharges, has had greater reductions than nitrogen in the past two decades.
Phosphorus tends to spur algae growth in freshwater areas. Once the algae begin to grow, they start taking up more nitrogen. Nitrogen, by contrast, is more important in initiating algae growth—and spurring its continued growth—in saltier water. It’s possible, Hagy said, that phosphorus reductions have limited phytoplankton production in up-Bay areas, resulting in more nitrogen being transported down the Chesapeake. That could stimulate increased algae production over deep-water areas where there is less of a chance for them to be consumed before sinking to the bottom.
“Essentially, what that would do is move the phytoplankton production and its utilization of nitrogen into an area where it could have more of a negative affect in terms of hypoxia—into the stratified part of the Bay,” Hagy said.
Whatever the cause of the worsening dissolved oxygen is, Hagy said, the solution is greater nutrient reductions. Trying to restore a healthy benthic community without first improving dissolved oxygen, he said, is “putting the cart before the horse. You can’t restore oxygen conditions by making a healthy benthic community because the poor oxygen conditions is why you don’t have a healthy benthic community.”
The paper estimated that nitrogen reductions of 41 percent from mid-1980s levels are required to restore historic dissolved oxygen conditions—an estimate similar to the figure developed by the Bay Program.
Donald Boesch, president of the University of Maryland’s Center for Environmental Science, said the paper suggested that improvements in water quality may lag until enough nutrient reductions are achieved to allow the Bay’s natural buffering systems to expand. The bottom line, he said, is “we simply have to accomplish much more reduction in nutrient loading before we see reduced hypoxia.”
The study also bolstered earlier research that indicates the Bay has a short nutrient memory—that is, denitrified or exported, nutrient loads in a given year are quickly buried in sediment and are not available to fuel future algae growth.
“We couldn’t find any memory in the hypoxia or anoxia story,” said Walt Boynton, a scientist at the University of Maryland’s Center for Environmental Science and a co-author of the paper, whose past work suggests that unused nutrients in the Bay are usually lost within a few months.
The study found that bad dissolved oxygen years did not seem to impact water quality the following year, Boynton said. Nor did good water quality in a given year result in improved conditions the following year.
That means Bay water quality can show a quick recovery if nutrient levels are significantly cut. “If the loads are reduced by a factor of two, you don’t have to wait for a good response,” Boynton said. “it will be there posthaste.”