Atmospheric Deposition of Inorganic Nitrogen to Hog Island

Various studies suggest that the deposition of atmospheric N significantly influences primary production in coastal ecosystems. Between March 1990 and the present, we measured, on a weekly basis, the concentrations of ammonium (NH₄⁺) and nitrate (NO₃^-) in precipitation on Hog Island and calculated the corresponding wet deposition fluxes of inorganic N to the system. Precipitation samples were treated with chloroform on site to prevent microbial activity and associated changes in chemical composition between collection and analysis. Samples were subsequently analyzed at the University of Virginia for major inorganic ions (H⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄⁺, NO₃^-, Cl^-, SO₄^2-, and CH₃SO₃^-).

The average annual rate of N deposition via precipitation was 33 mmol m^-2 yr^-1 (0.46 g m^-2 yr^-1). NO₃^-, derived primarily from fossil- fuel combustion, accounted for about two thirds of this total; NO₃^- concentrations and rates of wet deposition did not vary significantly by season (Fig. 1). NH₄⁺, derived primarily from agricultural sources, accounted for the remaining measured deposition and exhibited a marked seasonal variability; maximum concentrations and depositions occurred during mid summer (Fig 1). Relative to remote regions, N concentrations (Figs. 2 and 3) and depositions (Fig. 4) to Hog Island were enhanced by factors of 5 to 10 due to anthropogenic influences.

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Dry-deposition rates of inorganic N to coastal areas are poorly constrained but probably rival or exceed those of wet deposition. Reactive N in aged, polluted, continental air is typically dominated by nitric acid (HNO₃), which rapidly deposits to the surface (deposition velocities are in the range 0.6 to 1.0 cm sec^-1). When such air masses mix with marine air, HNO₃ is efficiently scavenged by super-micron, alkaline, sea-salt aerosol and subsequently deposited to the surface. This process enhances dry-deposition rates of inorganic N to coastal zones relative to those in adjacent continental and marine regions. The deposition of organic N to coastal zones may be significant; recent measurements at other sites on the U.S. east coast indicate that organic N accounts for 20% to 30% of total N deposition. Investigations to quantify the dry deposition flux of inorganic species and deposition rates of organic N are currently being planned.