Ammonia may cause foul smells near farms and wastewater treatment plants, but it does more than stink if it enters the Bay or other coastal waters.

Any form of nitrogen spurs algae growth when it enters salt water. And while algae like nitrogen in general, they love ammonium.

Algae have to convert nitrate - the main form of nitrogen in the water - into ammonium before they can use it. When ammonia hits in the water, it is ready to use. Algae don't have to spend any energy to convert it, so more of the nutrient is used for growth.

Simply put, a pound of ammonium will spur more algae growth than a pound of nitrate. So, unlike other nitrogen deposition, ammonia raises special concerns about where it comes down - especially if it is on salt water.

Excess algae fueled by too many nutrients is the largest water quality problem in the Bay. When there is more algae than predators can consume, the tiny, one-cell plants ultimately die, sink to the bottom and decompose in a process that depletes the water of oxygen needed by fish and other creatures.

Algae blooms also cloud the water, blocking light from reaching important underwater grasses that provide food and habitat for waterfowl, crabs, juvenile fish and a variety of other aquatic life.

Changing the type of nitrogen entering the water from nitrate to ammonia may also cause a fundamental change in the algae community - the building block for the aquatic food web. Species that best accumulate ammonia may be different from those that have thrived on nitrate for centuries.

"We know that adding too much of any kind of nitrogen source is probably going to enhance productivity," said Hans Paerl, a researcher with the University of North Carolina's Institute of Marine Sciences. "But it may turn out, depending on what the nitrogen source is, that productivity may be expressed in different kinds of algae."

"The one that can get it the fastest, and eat it the fastest, is going to grow the fastest."

Paerl is also concerned that the air is providing a new route to the water. Historically, rain would have carried little nitrate or ammonia, he said, and almost all the nitrogen would have washed out of rivers.

Atmospheric deposition bypasses that route, directly delivering nutrients - and with ammonia, a different kind of nutrient - to nitrogen-starved algae populations. Studies show that a large nitrogen "pulse" delivered during a storm can be consumed within a matter of hours. Today, Paerl said, atmospheric deposition may play a role in stimulating offshore blooms - including ones that become harmful.

"Because we have relatively large nutrient pulsing events as a result of manmade nitrogen sources being injected into the cycle, we're seeing more of these events where a good organism becomes 'bad' simply because it cannot be assimilated rapidly enough in the estuaries," he said.

That concern, though, is limited to the ammonium that falls directly onto the surface of coastal waters and estuaries.

Water quality monitoring typically shows that little - if any - ammonium runs off the land, except after heavy storms. That's an indication that most of it is being absorbed by plants on the land. But the appetite for nitrogen in the landscape is limited. "If the watershed is saturated with nitrogen, the ammonium can be discharged or converted to nitrate and discharged," said James Galloway, professor and chair of the University of Virginia's Environmental Sciences Department. "Even if the watershed is not saturated, some of the atmospheric nitrogen can be discharged."

In that case, deposition on the watershed adds to the total "load" of nitrogen that may reach the Bay, primarily in the form of nitrate.