In the mountains of Central Pennsylvania, Jim Lynch this winter resorted to something he has never done in the nearly four decades he has monitored streams in small, forested watersheds.
He had to hire a plow so he could get to the streams.
Usually, snow chains on a truck will do the job. But not when storm after storm has piled more than 3 feet of snow on the ground, as was the case by mid-February.
“It’s impossible to work in,” said Lynch, a professor of forest hydrology with Pennsylvania State University. “You can’t even walk on it. It’s up to my belt.”
One thing he has learned from monitoring the streams: The big accumulation of snow on the ground is likely to add up to bad news for the Bay this spring. Snow can accumulate large amounts of nitrogen—mainly from air pollution—which can be released en masse when it melts.
Last year’s heavier-than-normal snowfall and heavy spring rains combined to flush huge amounts of nutrients and water into the Chesapeake, contributing to some of the worst water quality conditions observed in the Bay.
This winter’s snowfall, Lynch said, is poised to send another large pulse of nitrogen toward the Bay, especially if there is a rapid melt that quickly flushes the water into streams before it can seep into the ground. “This is a big slug of nitrogen that is ready to come down, just like last year, and the same thing could occur,” he said.
If so, that means the health of the Chesapeake this year could hinge in large part on what happens to that snow in the next few weeks—particularly how quickly it melts.
This year’s situation illustrates how unusual episodes can provide a major shock to the Bay. Normally, forested lands like those that Lynch monitors are stingy with nutrients as trees and undergrowth soak up almost anything available. Acre for acre, forests, on average, leak fewer nutrients than any other land cover.
That’s good news for the Bay, as nearly 60 percent of its watershed is forested. During spring and summer growing seasons, hardly any nutrients escape from those lands unless they have been disturbed by insect defoliation, fire, logging or other activities. Most nitrogen that escapes forests usually does so during the winter and early spring when there is little biological activity to absorb the nutrient.
Exactly how much leaves the watershed during the winter is largely dependent on hydrology. Forest soils are like a sponge. As they dry, they can retain water and the nitrogen it carries. “But if you swamp that sponge with too much nitrogen and water too quickly, it can’t do anything with it, so it essentially ends up routing it out of the basin,” said Keith Eshleman, an associate professor at the University of Maryland Center for Environmental Science’s Frostburg Laboratory.
If much of the precipitation comes in a number of small rain or snow events, much of the water—and the nutrients associated with it—can be stored in the soil. A heavy rainfall that saturates the soil leads to more runoff.
A deep snowpack sets up conditions that can worsen the situation. That snow—often the product of several snowfalls—becomes a reservoir for water and nitrogen. It often melts when more moisture arrives in the form of warm rain. Then, the moisture and nutrients from several storms are released in one big gush.
These conditions do not occur every year because this region does not often accumulate large amounts of snow. But when it does, the results can be dramatic.
In March and April of 1993, Lynch and colleagues saw the effects of a 3-foot snowfall while monitoring small watersheds in Pennsylvania. Devices that automatically took water samples at two-hour intervals showed that during an eight-day period, when the snow melted after a warm rain, it carried with it more nitrogen than is typically lost from those watersheds over an entire year.
Likewise, after one snow melt in a forested watershed he monitored, Eshleman saw a quarter of the annual average nitrogen discharge run off in a single day. “Forests are still a small player [when it comes to nitrogen exports],” he said, “but under certain circumstances, they can be a big player.”
The snowpack also acts as an insulator that keeps the ground warm enough for bacteria to create nitrates as they break down leaves, twigs and other organic material that accumulated during the fall. With snow on the ground there is nothing to absorb the nitrate.
“It basically just sits in the soil and is easily mobilized in that spring flush of the snow melt,” said Douglas Burns, a hydrologist with the U.S. Geological Survey in Troy, NY, who has studied the process in the Catskill Mountains.
Without a melting snow—or heavy rain—to wash it away, that nitrogen would remain in the soil to be absorbed by trees, plants, fungi and other forest flora. But when the ground is saturated, it’s flushed out with the water. “That can be a huge amount of nitrogen that in some systems may even overwhelm what is contributed by the snowpack,” Eshleman said.
Most of the nitrogen, though, probably originates as air pollution. The watershed’s forests are mostly located in the mountains along its western edge. Those areas receive some of the highest amounts of acid rain in the nation. Nitrogen oxides—one of the pollutants that contribute to acid rain—are also the major source of nitrogen deposition.
Efforts to curb nitrogen oxide emissions have emphasized reductions during summer months, when the pollutants contribute to summertime smog problems. Lynch, who also conducts long-term air deposition monitoring, said nitrogen deposition has decreased during the summer but winter deposition has not declined significantly in recent years.
Rain and snow absorb the pollutants on their way to the ground. But when the precipitation is snow—sometimes called the “poor man’s fertilizer”—it has the potential to collect even more.
That’s because if it stays cold, snow remains on the ground and continues to collect “dry deposition”—particles of nitrogen and other pollutants that fall out of the sky when there is no precipitation.
Snow is particularly effective at removing particles from the air. Wind penetrates the top several inches of the porous snowpack, turning it into a big air filter, according to Dennis Lamb, a Penn State meteorologist.
“There is good contact between the air and the pollutants it contains and the surfaces of ice particles,” Lamb said. “That snow just sits there for days, so day in and day out it’s got the opportunity to be collecting or scavenging pollutants from the atmosphere.”
This winter has been particularly conducive to nitrogen accumulation. It turned cold in mid-December and stayed cold through mid-February. With no midwinter thaw, the snowpack increased in size with each storm. “We probably have 10 to 12 different storms lying on the surface up here and accumulating dry deposition,” Lynch said.
In contrast, the snow melt Lynch and colleagues monitored in 1993 melted shortly after it fell, giving it less chance to accumulate nitrogen “It is probably even worse today,” Lynch said. “The snowpack is fairly similar in depth, but it probably has a lot more nitrogen in it simply due to the fact that we’ve had such a long cold spell.”
As the snowpack begins to melt, it “ripens.” The snowpack compresses, and begins to melt, accumulating moisture at its bottom. At the same time, the accumulated nitrogen sinks to the bottom where it is ready to be flushed off with the melting snow before there is any significant biological uptake.
“It’s the long-term deposition, the accumulation, the ripening process and then the very, very rapid release,” Lynch said. “You get a tremendous amount of nitrogen coming off over a very short period of time.”
The same process would be true of other lands as well, but the forested mountains, being at higher elevations, are more likely to get more snow and hold it longer.
When the right conditions come together like this year, Lynch said, there is a “very real possibility” that air pollution could be the leading source of nitrogen entering the Bay during a given year.
On average, air pollution is thought to contribute about a quarter of the nitrogen to the Bay. Such averages, though, mask how extreme events can alter ecosystems, often with devastating results, Lynch said.
In the mountains, rapid snow melts quickly acidify streams, killing some fish and rendering streams unsuitable for others. In the Bay, high flows and nutrient loads set up conditions that cause low-oxygen conditions over large areas.
“People often look at annual averages and ignore these episodic things, which are huge,” Lynch said. “Nature doesn’t deal with annual averages.”
Several factors may still mitigate the impact of this year’s snow, Eshleman said. For instance, a long, slow melt would allow much of the moisture to soak into the soil where it will either be taken up by plants this spring, or slowly move out of the forests over a long period of time.
On the other hand, a warm rain could fall atop the snow causing a rapid melt and huge runoff, with little opportunity for nitrogen to be stored in the watershed. “That might be the worst-case scenario,” Eshleman said.
While conditions have been set for a bad year for the Bay, Mother Nature has yet to render her final verdict.