When those estimates, and their underlying assumptions, were closely examined, the report said the range of potential atmospheric nitrogen contributions to the Bay was 5 percent to 50 percent. Much has been learned about atmospheric impacts on water quality in the past decade, but much remains to be learned. Uncertainties in the assumptions used, scientists say, result in the impacts of nitrogen deposition being overcounted in some ways and undercounted in others. As a result, many researchers feel current estimates of nitrogen deposition on the Bay and are probably fairly good, but say areas of uncertainty need to be more closely examined so the numbers can be fine-tuned. Deposition is what lands on the Bay and its watershed.
Loads are what actually enter the Bay.
As a nutrient, nitrogen is taken up by trees or other plants. The excess that runs off and makes it to the Bay is the atmospheric load from the watershed to the Bay. When that is combined with the amount of deposition that lands directly on the Bay, the figure produced is the total atmospheric load to the Bay. The Bay Program models have estimated that total load, accounting for plant uptake and other variables, at about 25 percent. Using recent research information, Richard Valigura, of the National Oceanic and Atmospheric Administration's Air Resources Laboratory, has estimated that only about 5 percent to 9 percent of the atmospheric nitrogen deposition lands directly on the Bay.