Atmospheric Nitrogen Inputs to the Delaware Inland Bays: The Importance of Ammonia(um)

A previous assessment of nitrogen loading to Delaware’s Inland Bays indicates that atmospheric deposition provides 15-25% of the total, annual N input to these estuaries. A large and increasing fraction of the atmospheric wet flux is NH₄⁺, which for most aquatic organisms represents the most readily assimilated form of this nutrient. Particularly noteworthy is the reported 60% increase in the precipitation NH₄⁺ concentration at Lewes, DE over the past 20 years, which parallels the increase in poultry production on the Delmarva Peninsula over this period (currently standing at nearly 585 million birds annually).

To further examine the relationship between local NH₃ emissions and deposition, weekly-integrated gaseous NH₃ concentrations were determined using Ogawa passive samplers deployed at 13 sampling sites throughout the Inland Bays watershed over a one-year period. Annual mean concentrations ranged from <0.5µg NH₃/m3 to >6 µg NH₃/m3. At most sites, highest NH₃ concentrations were evident during spring and summer, when fertilizer application and poultry house ventilation rates are greatest, and seasonally elevated temperatures induce increased rates of microbial activity and volatilization from soils and animal wastes. The observed north-to-south concentration gradient across the watershed is consistent with the spatial distribution of poultry houses, as revealed by a GIS analysis of aerial photographs. Based on the average measured NH₃ concentration (1.63µg NH₃/m3) and published NH₃ deposition rates to water surfaces (5-8 mm/s), the direct atmospheric deposition of gaseous NH₃ to the Inland Bays is 3.0-4.8 kg N/ha/yr. This input, not accounted for in this or most other previous assessments of atmospheric loading to coastal waters, would effectively double the estimated direct dry deposition rate, and is on par with the NO₃^-and NH₄⁺ wet fluxes.

A second component of this study examined spatial differences in NO₃^- and NH₄⁺ wet deposition within the Inland Bays watershed. In a pilot study, precipitation composition at the Lewes NADP/AIRMoN site (DE 02) was compared with that at a satellite site established on the Indian River Estuary, approximately 14 mi. southwest. While the volume-weighted mean precipitation NO₃^- concentrations did not differ significantly between sites, the NH4 concentration observed at Indian River (26.3 µmoles/L) was 73% greater than at Lewes (15.2 µmoles/L). More recently, a NADP site was established at Trap Pond, DE (DE 99), which was intentionally located in an area containing intense poultry production and other agricultural activities, typical of this region. A comparison of the initial 2 years (6/01-5/03) of precipitation chemistry data from Trap Pond with other nearby NADP sites (Lewes and Smith Island) indicates little (<10%) spatial variability in regional NO₃^- wet deposition, but fairly significant differences (50-60%) in the NH₄⁺ wet flux. These observations are in agreement with the pilot study, and the spatial trend in NH₄⁺ wet deposition is consistent with the airborne NH₃ concentration gradient described above.

Overall, these results suggest that local emissions and below-cloud scavenging provide a significant contribution to regional atmospheric N deposition. This impact is exacerbated during summer, when the bay surface water productivity is most severely limited by the availability of N, and seasonally diminished stream loadings are coupled with elevated atmospheric N inputs.


* Corresponding Author, Scudlark@udel.edu ; Telephone: (302) 645-4300