Atmospheric nitrogen inputs to the Delaware Inland Bays: the role of ammonia

A previous assessment of nitrogen loading to the Delaware 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 a 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, biweekly-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 at the 13 sites ranged from <0.5 μg NH3 m−3 to >6 μg NH3 m−3, with a mean of 1.6 ± 1.0 μg NH3 m−3. 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 and published NH3 deposition rates to water surfaces (5–8 mm s−1), the direct atmospheric deposition of gaseous NH3 to the Inland Bays is 3.0–4.8 kg ha−1 yr−1. This input, not accounted for in previous assessments of atmospheric loading to the Inland Bays, 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 at Riverdale on the Indian River Estuary, approximately 21 km southwest. While the volume-weighted mean precipitation NO3− concentrations did not differ significantly between sites, the NH4+ concentration observed at Riverdale (26.3 μmoles L−1) was 73% greater than at Lewes (15.2 μmoles L−1). More recently, a NADP site was established at Trap Pond, DE (DE 99), which was intentionally located within the region of intense poultry production. A comparison of the initial two years (6/2001–5/2003) of precipitation chemistry data from Trap Pond with other nearby NADP–AIRMoN sites (Lewes and Smith Island) reveals fairly homogeneous NO3− wet deposition, but significant spatial differences (∼60%) in the NH4+ wet flux. Overall, these results suggest that local emissions and below-cloud scavenging provide a significant contribution to regional atmospheric N deposition.