Environ. Sci. Technol. 2002, 36, 3242-3249
Atmospheric Nitrogen Deposition to Estuaries in the Mid-Atlantic and Northeastern United States MARK S. CASTRO* Appalachian Laboratory, University of Maryland Center for Environmental Science, 301 Braddock Road, Frostburg, Maryland 21532 CHARLES T. DRISCOLL Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244
The purpose of this work was to determine the contribution made by atmospheric nitrogen (N) deposition to the total N input to 10 estuaries on the east coast of the United States. We estimated the amount of N fixed by human activities in the watersheds (N fertilization, biotic N2 fixation by legumes and pastures, atmospheric N deposition, and net food and feed import of N) of these 10 estuaries and used a land-use specific approach to estimate the N available for transport to the estuary from different watershed N sources (runoff from agriculture, urban areas and upland forests, point sources, and atmospheric deposition). Total atmospheric N inputs (watershed runoff plus direct deposition to the surface of estuary) accounted for 15-42% of the total N inputs to these 10 estuaries. Direct deposition to the surface of the estuary was an important atmospheric N source for four estuaries, accounting for 35-50% of the total atmospheric N inputs. Simulated reductions of atmospheric N deposition by 25% and 50% of current deposition rates reduced the contribution made by atmospheric N deposition to the total N loads by 1-6% and 2-11%, respectively. Largest reductions occurred in estuaries with direct atmospheric N deposition contributions >35% of the total atmospheric N input. Results from our simulated reductions suggest that considerable reductions (>25%) in atmospheric N deposition will be needed to significantly reduce the contribution made by atmospheric N deposition to the total N loads to our study estuaries. In addition, reductions in atmospheric N deposition will first be detected in estuaries with relatively high direct deposition inputs of atmospheric N deposition.
Introduction Over the past several decades, N inputs to estuaries in the United States have dramatically increased. Total N inputs to Chesapeake Bay are now 6-8-fold greater than during precolonial times (1), annual nitrate fluxes to estuaries in the northeastern United States have increased 3-8-fold since 1900 (2), and nitrate fluxes to the Gulf of Mexico from the Mississippi River have increased 3-fold since the 1970s (3). These elevated N inputs are of great concern because N often * Corresponding author e-mail:
[email protected]; phone: (301)689-7163; fax: (301)689-7200. 3242
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
controls primary production in N-limited estuaries (4-8). Numerous studies have already documented that chronic additions of N to N-limited estuaries accelerate primary production and eutrophication, leading to many undesirable responses, such as increased frequency of algal blooms, hypoxic (1.5 persons ha-1 (Table 1). Massachusetts Bay had the highest human population density of 8.1 persons ha-1. The northern (Casco Bay) and two southern most watersheds, Chesapeake Bay and Pamlico Sound, had the lowest population densities (
