A series of hurricanes, followed by a wetter than normal fall carried a late-season wave of nitrogen into the Bay last year, making it the third worst year for nitrogen loads since watershedwide river monitoring began in 1990, according to figures from the U.S. Geological Survey.

Such a huge dose of nitrogen would normally be bad news for the Bay, but scientists say the nutrients hit the Chesapeake so late that their impact was greatly reduced.

“Timing is everything,” said Rich Batiuk, associate director for science with the EPA’s Bay Program Office. “We sort of squeaked by.”

Underwater grass beds expanded 14 percent Baywide last year, as the bulk of the growing season took place before the hurricanes hit. Early indications suggest grasses are also doing well so far this year.

But the nitrogen may have contributed to unusually low levels of dissolved oxygen seen in the Chesapeake last fall. Parts of the upper Bay had the lowest dissolved oxygen ever reported for October and November.

“It taught us a couple of things,” said Bill Dennison, vice president for science application with the University of Maryland Center for Environmental Science. “Number 1, there is not a whole lot of lag. If you turn the rain off, turn the nutrients off, the system rebounds. Second, if you put them in there, the system goes to pieces.”

Monitoring by the USGS, in partnership with the states, of the nine major nontidal rivers that enter the Bay—draining about 78 percent of the watershed—shows that they contributed 304 million pounds of nitrogen last year, compared with an average of 214 million pounds since monitoring of all nine basins began in 1990. That was exceeded only 1996 and 2003. The high nitrogen load in 2004 can be almost completely attributed to the Susquehanna River, because seven of the remaining rivers had flow and nitrogen loads very near long-term averages. The Mattaponi was the only other river with above average nitrogen loads in 2004.

But it was only the fifth highest year for sediment and phosphorus. Nontidal rivers contributed about 12.3 million pounds of phosphorus, and more than 4 million tons of sediment.

There were two primary reasons for the lower amount of sediment and phosphorus compared to nitrogen. First, by the time the heavy rains fell in late summer and early fall, vegetation was holding much of the sediment—and phosphorus which binds to sediment—in place, said Scott Phillips, Chesapeake Bay coordinator for the USGS.

Second, he said, even with above average flows in the northern part of the watershed, the reservoirs on the Susquehanna will trap some of the sediment and phosphorus before they reach the Bay.

Typically, the highest rainfall occurs in the spring, sending huge amounts of nutrients into the Bay where they cause algae blooms that cloud the water. When the algae dies, they sink to the bottom and decompose in a process that depletes oxygen from the water.

Last year, USGS figures show that river flows into the Bay were normal for the first six months. They began rising in July, and were above normal for the rest of the year as a series of hurricanes moved through the watershed in late summer and higher than normal rainfall fell on the watershed during the fall. In September, river flows were the highest ever reported for the month averaging 116.2 billion gallons a day.

The hurricanes initially brought some good news: The strong storms thoroughly mixed the water, increasing oxygen levels throughout the Bay as oxygen-starved areas on the bottom were stirred together with oxygen-rich water near the surface.

In the Upper Bay, oxygen levels sharply rebounded in September from summer lows. But that was reversed in October and November, as oxygen levels hit record lows for that time of year in parts of the Upper Bay as nitrogen—which spurs algae growth—and detritus flushed into the Bay took their toll. As the algae and detritus decayed, they used up much of the oxygen in the deep water.