Atmospheric Nitrate Deposition: a Large Nutrient Source in North Florida Watersheds

Dry deposition of nitrate, estimated from a box model based on NO_{x} emissions and rain chemistry monitoring data over the contiguous 48 states, accounts for about half of the total US NO_{x} emissions, a deposition flux twice that of measured wet deposition. Thus, total atmospheric nitrate deposition is roughly three times wet only deposition. Ten subregions of wet only nitrate depositions were delineated by EOF analysis from the entire U.S.A., in which each has a narrow range of annual deposition flux and exhibits unique seasonal variation. The study was based on statistical analysis of chemical concentrations measured for more than 10 years in weekly rainfall samples of the National Atmospheric Deposition Program, NADP, and more than 20 years of river water samples of the U.S. Geological Survey, USGS. NO _{x} emissions appear to regulate the annual average total deposition fluxes while in the subregions rainfall characterizes the seasonal and shorter term variations in wet only depositions. Atmospheric wet and dry deposition ("acid rain") appears to be the principal source of nitrogen in twelve northern Florida watersheds that range from Pensacola to Gainesville (Escambia to Alachua Counties). River fluxes of total dissolved nitrogen average close to the atmospheric deposition fluxes of nitrate and ammonium ions. Factor analysis was applied to the data sets to resolve principal components: (1) in atmospheric data, that distinguish air pollution nitrate and sulfate from sea salt sodium and chloride, and (2) in surface water data, that distinguish ground water calcium, magnesium, and silica from meteoric water nitrate and sulfate. River concentration ratios N/P in the watersheds are high, averaging twice the Redfield mole ratio N/P = 16 for aquatic plant nutrients. The results indicate that excess dissolved nitrogen could be temporarily recycled in the watersheds but not retained, so that it could eventually flow to the coastal zone where N may be a limiting nutrient for marine plants. Hydrologic conditions, which exhibit variations on seasonal and longer time scales, play an important role in the transport of nutrients and other species in the rivers.