When it comes to freshwater flows, it’s been a roller-coaster year for the Chesapeake.

After peaking in December and January, river flows into the Bay were below normal all spring, before once again rising to record and near-record levels in June and July as torrential rains flooded the watershed.

But when it was all averaged out, the 2006 “water year”—which runs from October 2005 through September 2006—was about as normal as it could get: averaging 77,800 cubic feet per second—which is 98 percent of the long term average of 78,600 cfs.

(A water year begins in October because that is when stream flow typically begins to increase after the summer.)

That made it one of the most “average” years since the U.S. Geological Survey began calculating flows into the Bay in 1937.

In reality, it was anything but average. It had the third highest July flow on record (110,000 cfs), and flows for January, June and September were all among the top 10 for those months since 1937.

On the flip site, March, April and May were each were among the 10 lowest for those months.

It was a sharp reversal from 2005, which also had total flows in the normal range, but a far wetter than average spring followed by a drier than normal summer.

The wet spring conditions of 2005 led to an extensive oxygen-starved dead zone that was the fourth-worst on record. But the dry summer resulted in clear water that helped underwater grass beds flourish—grass bed coverage increased 7 percent from the previous year.

This year, dissolved oxygen conditions were much better, but grass coverage is thought to have declined.

The contrast between the two years illustrates the complex interaction between nutrients, the amount and timing of river flow, and their combined impact on Bay water quality.

Nutrients spur the growth of algae blooms, which block sunlight to underwater grass beds that provide important food and habitat for fish, shellfish and waterfowl. When the algae die, they sink to the bottom and are decomposed by bacteria in a process that removes oxygen from the water. Sediment also harms grass beds by clouding the water, and also buries bottom-dwelling creatures, such as oysters.

Reducing the amounts of nutrients and sediment entering the Bay is the main goal of the Chesapeake cleanup effort.

But their exact impact each year depends on the timing of rainfall, which flushes nutrients and sediment into waterways, and ultimately the Bay.

Scott Phillips, Chesapeake Bay coordinator for the USGS, said the impact of years with high spring flows like 2005 tends to be compounded because the water also has higher nutrient concentrations. That’s because spring rains wash nutrients off the land before crops and lawns begin growing and absorbing nutrients.

“We need to focus on trying to reduce those concentrations during that time, whether it is through winter cover crops or manure management or the decreased application of fertilizer on suburban lawns,” he said. “This illustrates why that is so important.”

In addition, actions that slow water flow, such as stormwater systems that promote infiltration into the ground instead of discharges into the stream, can to some extent reduce the peak flows that compound the nutrient problems.

In contrast to 2005, this year’s low spring flows helped to make it the seventh best summer for anoxia since Baywide monitoring began in 1985.

On average this summer, the volume of anoxic water—water with no oxygen—in the Chesapeake’s mainstem was 0.92 cubic kilometers, or 1.77 percent of the its volume. That was very close to the 1.08 cubic kilometers scientists had predicted earlier this year.

“We were pleasantly surprised,” said Dave Jasinski, data analyst with the University of Maryland. “We had speculated that conditions would worsen as the Bay responded to nutrient loads from the late June flood.”

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.

Anoxia is a true dead zone where almost nothing lives. Scientists do not attempt to predict the larger volume of “hypoxic” water, which has oxygen, but less than most Bay-dwelling fish and shellfish need, because it is affected by more variables.

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. When oxygen is used up on the bottom during algae decomposition, the pycnocline helps to prevent the bottom water from mixing with oxygen-rich water near the surface.

The unusually high flows that come in June and July had little impact on anoxia, though.

Bill Boicourt, an oceanographer with the University of Maryland Center for Environmental Science, said after the pycnocline is set up by spring flows, it essentially remains in place for most of the summer. If disrupted by an unusual event like this summer’s high flows, it “heals itself” rapidly.

In fact, he noted, 1972 had low flows during the spring, and even the record high June flows resulting from Hurricane Agnes did not cause a particularly bad year for dissolved oxygen.

Rich Batiuk, associate director for science with the EPA’s Bay Program Office, said the fact that scientists can predict—and explain—the varying impacts on the Bay each year illustrates the extent to which understanding of the ecosystem has improved over the years.

“It helps us learn about how the system will respond,” he said. “From a management perspective, it is very helpful.”

Studying year-to-year variations also provides evidence that the magnitude of the nutrient and sediment reductions sought by the Bay Program will produce the desired improvements. When dry years send less of those pollutants into the Bay—as was the case during the drought of 2001-02—water quality improved. But those improvements were washed away when flows increased the following years, brewing a torrent of nutrients.

“It gave us a glimpse to say that we were pretty much on the right track,” he said.

But the key is to reduce nutrient concentrations without having to rely on droughts.

That’s because dry spells bring consequences of their own. What was good for oxygen levels this year, for instance, was not necessarily good for other resources.

While there were exceptions, biologists making field observations of underwater grass beds this year generally reported declines from levels observed in 2005, although a final analysis of aerial photos of the grass beds will not be completed until next spring.

Bob Orth, a seagrass expert at the Virginia Institute of Marine Science who conducts the annual Baywide survey, said many of the declines were not the result of high June flows, as many suspected, but because of low spring flows.

Low freshwater flows meant higher salinities in the Bay, which in some areas knocked back beds of freshwater grasses. “I think that many of the freshwater plants, in particular the milfoil that we saw in areas last year, took it in the ear well before June,” Orth said. “That is my gut feeling.”

Spring’s dry weather was also blamed for promoting the growth of oyster-killing parasites in Virginia’s portion of the Bay by increasing water salinities, which are conducive to the growth of both MSX and Dermo.

When oyster boats cruised the James River on opening day of the oyster season in October, most came back empty-handed.

Billy Belvin, of Gloucester County, scoured the river for five miles below Menchville, pulling his hand scrape across oyster beds where oysters were thriving as late as this summer.

He was hoping to meet his limit—30 bushels a day. He and his crew ended the day with one bushel. “I don’t know what we’re going to do,” Belvin said.

Ironically, early harvests were better in Maryland. The reason? Biologists suggested that the high June flows were just enough to push the diseases out of many of the oyster bars in the upper half of the Chesapeake Bay.

— The Associated Press contributed to this report