For nearly two decades, the Bay has been whipsawed by weather extremes. It has bounced from some of the wettest years on record to some of the driest—sometimes with dire consequences for the cleanup effort.

Freshwater flows into the Bay during wet years tend to carry huge amounts of nutrients and sediment, fouling Chesapeake water quality. Dry years tend to have better water quality, but have consequences of their own, such as increased salinity levels, which allow for the spread of oyster diseases and can kill freshwater grasses.

For a long time, scientists and agency officials have wondered just how the Bay would fare if it ever had an “average” rainfall year.

Last year, they found out—sort of.

Recent data from the U.S. Geological Survey shows that—at first glance—2005 appears to have been the most “average” year for the amount of freshwater flowing into the Bay since record keeping began in 1937.

Total river flow to the Chesapeake, which is extrapolated from USGS monitoring on major tributaries, show that average river flow in the Bay last year was 78,616 cubic feet per second. The long-term average is 78,431 cfs, but flows have ranged from a high of 137,289 cfs in 1972 to a low of 47,776 cfs in 1941.

Yet if 2005 was so average, why did some water quality measurements plunge last year? The amount of anoxic water in the Bay—an oxygen-starved “dead zone” lethal to most aquatic life—was the third greatest on record.

The reason is that while 2005 appeared average overall, a closer look at monthly data shows it was anything but normal. In fact, USGS figures reveal that only four months fell within what its scientists consider a “normal” range. Eight months were either wetter or drier than normal.

(The USGS defines “normal” flow to be a middle range which covers half of the variability observed over the years. The top 25 percent of wet years [or months] and bottom 25 percent of dry years [or months] are considered outside the normal range.)

Wetter than normal months early in 2005 poured fresh water and nutrients into the Bay, which set up conditions for poor dissolved oxygen levels that persisted all summer. Yet drier than normal summer months led to clearer surface waters, contributing to a surge of underwater grasses in upper parts of the Bay.

But the question of what constitutes “normal” conditions for the Bay is even more complex. River flows into the Chesapeake vary not only from month-to-month and year-to-year, but also from decade-to-decade and even century-to-century as various wide-scale climate patterns exert their influence over the region.

The issue of what is “normal” for the Bay—and whether “average” conditions actually exist—is not just academic.

Although nutrients are the main culprit in worsening Chesapeake water quality, their impact is exacerbated by climatic factors. “It’s not just man’s actions that have caused the degradation that we have seen,” said Scott Phillips, Chesapeake Bay coordinator for the USGS. “Human activities are the principle ecosystem stressors, but river-flow variability is another major factor in the condition that we see in the Bay in a particular year, or over a longer period of time.”

The amount of nitrogen washing into the Bay is strongly related to rainfall and river flows because it easily dissolves in water. The amount of sediment and phosphorus—which often binds with sediment—increases when high river flows related to strong storms transport sediment down river corridors and into the Bay.

When those nutrients reach the Bay, they spur excess algae growth, which clouds the water, blocking sunlight to underwater grass beds, one of the most important habitats in the Chesapeake. When the algae die, they sink to the bottom and decompose in a process that removes oxygen—needed by almost all aquatic life—from the water.

But river flows present a sort of double-whammy for Bay water quality, especially dissolved oxygen concentrations in deepwater areas. That’s because as strong river flows enter the Bay, they create a barrier, known as a pycnocline, between fresh water near the surface, and ocean water near the bottom. Typically, the higher the freshwater flows into the Chesapeake, the stronger the stratification between the top and bottom, which reduces mixing. Strong river flows not only carry more nutrients, but result in a stronger pycnocline, making it difficult for bottom oxygen to be replaced—setting up conditions that lead to a “dead zone.”

When the Bay Program established nutrient and sediment reduction goals to meet new water quality standards for the Chesapeake, they were based on average flow conditions for a 10-year period spanning 1985 through 1994. The new goal would limit nitrogen inputs to 175 million pounds a year, phosphorus to 12.78 million pounds and sediment to 4.15 million tons.

The standards are aimed at ensuring that various places in the Bay have enough oxygen to support the types of fish, shellfish—even bottom-dwelling worms—that would normally be found in those areas. They are also intended to make sure water is clear enough from algae and sediment to support healthy underwater grass beds.

Right now, it’s estimated that roughly 275 million pounds of nitrogen would enter the Bay under “normal” conditions. But that can swell when it gets wet: Bay scientists have estimated that about 500 million pounds of nitrogen reached the Chesapeake in 2004, a wetter than normal year for most months.

Whether cleanup goals based on “normal” conditions will meet the new water quality standards is unclear: USGS figures show that since 1993, abnormal conditions have ruled the Bay. From 1993 through 2005, only three years fell into what the USGS defines as the long-term normal range.

“During the 1990s through the present day, river-flow variability is the norm,” Phillips said. “Average conditions are pretty rare.”

To help account for the annual variation in flows, attainment of the new Bay water quality standards will be measured over a three year “window”—if the standards are not fully achieved in one year because of high flows, the Bay could still be considered cleaned up if it met the standards the other years.

That could still be tough. Since 1993, no three-year period exists in which two years were in the normal range. Yet there were five, three-year periods in which two of the three years in the “window” would have been wetter than normal—something that might make standard attainment difficult, even if all of the actions needed to achieve nutrient reduction goals are implemented.

Because of such variability, some scientists argue that efforts aimed at restoring coastal areas around the nation must place more focus on the role climate plays in water quality.

“Your ‘normal’ condition is going to vary depending on which decade you are in, whether you like it or not,” said Tom Cronin, a USGS geologist who has written numerous papers on the region’s climate history.

Nutrients began increasing in the Bay during the 1950s as the watershed’s population grew and the use of manufactured fertilizers on farms became widespread. Yet the decline of Bay water quality did not become clearly evident until the 1970s. Cronin said the reason is that the 1970s was one of the wettest decades this century, which worsened the impact on the Bay ecosystem caused by excess nutrients. The 1960s, by contrast, was one of the driest decades.

Long-term regional climate analyses by Cronin and others show that periods dominated by wet or dry conditions can persist for decades—even centuries.

Most of the 1800s, for instance, was at least as “wet” as recent decades, although sediment cores show those flows—accompanied by far fewer nutrients—did not result in the dead zones seen today.

In a soon-to-be-published essay in the journal, Climatic Change, Cronin and colleague Hal Walker contend “there is growing evidence that long-term ecosystem management and restoration efforts should integrate abrupt climate change, including human-induced change, into research and modeling programs.”

Major coastal restoration programs, such as those for the Chesapeake Bay and the Gulf of Mexico, are likely to take decades and cost billions of dollars, they note. Historic records suggest sharp climatic changes, which could affect those efforts, take place every few decades. Nonetheless, the scientists said, such climate variability “is usually not taken into account in coastal systems management.”

In addition to natural climate cycles, human impacts could affect future climate and the Bay. Modeling done by researchers at Pennsylvania State University suggest a likely regional climate change—as greenhouse gases build up in the atmosphere—toward more precipitation during the winter and spring.

“There is a lot of debate and uncertainty about exactly how the average flows will change, but there is a lot of reason to believe that the seasonality of the flow will more predictably change,” said Donald Boesch, president of the University of Maryland Center for Environmental Science.

In those projected scenarios, like last year, the overall flow to the Bay may be the same, but proportionately more would arrive in the winter and spring — when it has the greatest impact on the Bay — and less in the summer. “Even though the average flows don’t change that much, there will be seasonal shifts,” Boesch said. “That has consequences in terms of water quality issues and the health of the Bay.”

In that case, he said, “the hill is going to get steeper” to meet the Bay cleanup goals, he said. “That is going to require more aggressive controls.”

Some of those issues may get more attention over the next several years as state and federal officials plan a review of the Bay cleanup goals. With so many years being anything but average, many suggest that officials will have to re-examine whether their existing nutrient reduction goals will actually result in a clean Bay

Instead of looking at how computer models of the Bay respond to average conditions, officials may need to evaluate its response to a more realistic range of wet, average and dry years, said Rich Batiuk, the associate director for science with the EPA’s Bay Program Office.

“We’ve got to have confidence our nutrient allocations are going to support the new dissolved oxygen standards under a variety of wet and dry years,” he said.

One management option, Batiuk said, is for states to emphasize nutrient and sediment control practices that also can help reduce or slow water runoff. For instance, planting forest buffers or restoring wetlands not only absorb nutrients, but they can literally soak up water, helping to slow the rate of runoff.

If the states plant 40,000 miles of forest buffers, as called for in their nutrient reduction strategies, it would help improve oxygen conditions in the Bay by reducing both nutrients and water flow into the Bay—the two factors that set up low-oxygen conditions.

“That is helping us get at the dissolved oxygen issue, not only through the nutrient loads but also by slowing down runoff and promoting infiltration into groundwater,” he said. “That reduces the ‘flashiness’ of the runoff events so we could potentially reduce the strength of stratification.”

Likewise, Phillips, of the USGS, said efforts could be made to better target where nutrient control strategies take place. For instance, restoring wetlands and forests in areas with wide floodplains could be one of the most effective ways to control phosphorus and sediment, Phillips said, because those areas can trap sediment—and the phosphorus that binds to sediment—when they are flooded during high flow events.

“When you are starting to design management actions to implement the tributary strategies to meet the load allocation goals,” Phillips said, “you need to think about designing for those extremes—especially the high extremes—and not just designing strategies to meet nutrient and sediment load allocations for normal conditions.”