ATMOSPHERIC DEPOSITION AND HYDROGEOLOGIC FLOW OF NITROGEN IN NORTHERNFLORIDA WATERSHEDS

Atmospheric wet and dry deposition (''acid rain'') appears to be the p rincipal source of nitrogen in twelve northern Florida watersheds that range from Pensacola to Gainesville (Escambia to Alachua Counties). T he study was based on statistical analysis of chemical concentrations measured for more than ten years in weekly rainfall samples of the Nat ional Atmospheric Deposition Program, NADP, and more than twenty years of river water samples of the US Geological Survey, USGS. River fluxe s of total dissolved nitrogen average close to the atmospheric deposit ion fluxes of nitrate and ammonium ions. Factor analysis was applied t o the datasets to resolve principal components: (1) in atmospheric dat a, that distinguish air pollution nitrate and sulfate from sea salt so dium and chloride, and (2) in surface water data, that distinguish gro und water Ca, Mg, and silica from meteoric water nitrate and sulfate. Relationships within the sets of measured concentration data suggest t hat, following atmospheric deposition, inorganic nitrogen undergoes bi ogeochemical transformation within the watersheds, which results in in organic nitrogen being transformed to organic forms. River concentrati on 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 w atersheds but not retained, so that it could eventually flow to the co astal zone where N may be a limiting nutrient for marine plants. Chemi cal interactions of meteoric water within the watersheds depend on geo logic, hydrologic, and biogeochemical processes and are certainly comp lex. However, in one watershed that is geologically the simplest, sepa rate statistical analyses of river water composition during high and l ow flow conditions show nitrate and sulfate to be correlated during hi gh flow, but not during low flow, providing further evidence for an at mospheric nitrogen source and watershed transformation after depositio n.