The Chesapeake Bay last summer had the best oxygen conditions ever reported in the 13-year history of the Bay Program's water quality monitoring effort.

While cautioning that it is difficult to draw conclusions from one good year, some officials believe it could be a sign that some cleanup efforts are beginning to show results. "It may be the signal that we've been looking for - that when nature keeps the flow conditions in the normal range, we're looking at substantial improvements," said Bill Matuszeski, director of the EPA's Bay Program Office.

The amount of "dissolved oxygen" has long been a key indicator of the health of the Bay. The link between oxygen levels in Bay water and nutrients led to the adoption of nutrient reduction efforts in the region.

Overall, there is no improving trend in the amount of oxygen observed in the Bay over the last 13 years, but that in part may be explained because so many recent years have had higher-than-average freshwater flows into the Chesapeake.

Water quality is often tied to the amount of water flowing into the Bay. Falling rain accumulates large amounts of sediment and nutrients as it runs off farm fields, lawns and streets and enters local rivers.

The sediments cloud the water and the nutrients spur algae blooms. When the algae die, they sink to the bottom and are decomposed by bacteria in a process that depletes water of oxygen in lower portions of the Bay. 

Wet years further worsen the situation because strong freshwater flows from the rivers create a physical barrier that prevents fresh surface waters from mixing with saltier water on the bottom. As a result, there is no way for the bottom water to be replenished with oxygen.

Last year's monitoring data show that the amount of water with levels of oxygen below 5 milligrams per liter (parts per million) of oxygen - the point at which it is generally considered stressful to fish and most other species - was the lowest since the monitoring program began in 1985.

Also, the amount of water with less than 2 milligrams per liter of oxygen - which is considered extremely stressful - was the second lowest since the monitoring program began. Upper layers of the Bay may contain 14 parts per million of oxygen.

While much of the improvement is probably due to last year's lower than normal flows into the Bay, the 1987 flows were not the lowest observed during the 13-year period. Three years during that period had similar or lower flows than 1997, but all had worse water quality.

"All in all, this is a hopeful sign," Matuszeski said. Noting that three-quarters of all the water entering the Bay comes from the Susquehanna and Potomac rivers, he said the improvements are the result of "a lot of work that has been done, particularly with the agricultural community in those areas."

Ironically, the good news came in a year where most attention was focused on the Bay's problems, particularly outbreaks of pfiesteria on the lower Eastern Shore. But that represented localized nutrient problems, Matuszeski said, not Baywide conditions.

Still Matuszeski acknowledged, the high flows that have ruled so far this spring are washing away hopes that last summer's results will be repeated. "So far this year, we are certainly facing an above average flow year, if not a record flow year, again," Matuszeski said. "So consequently, we can't conclude that we have a trend here."