『Abstract
　Isotope analyses of nitrate and algae were used to gain better understanding of sources of nitrate to Florida's karst springs and processes affecting nitrate in the Floridan aquifer at multiple scales. In wet years, δ¹⁵N and δ¹⁸O of nitrate ranged from +3 to +9‰ in headwater springs in north Florida, indicating nitrification of soil ammonium as the dominant source. With below normal rainfall, the δ¹⁵N and δ¹⁸O of nitrate were higher in almost all springs (reaching +20.2 and +15.3‰, respectively) and were negatively correlated with dissolved oxygen. In springs with values of δ¹⁵N-NO3 and δ¹⁸O-NO3 greater than +10‰, nitrate concentrations declined 40-50% in dry years and variations in the δ¹⁵N and δ¹⁸O of nitrate were consistent with the effects of denitrification. Modeling of the aquifer as a closed system yielded in situ fractionation caused by denitrification of 9 and 18‰ for Δ¹⁸O and Δ¹⁵N, respectively. We observed no strong evidence for local sources of nitrate along spring runs; concentrations declined downstream (0.42-3.3μmol-NO3 L^-1 per km) and the isotopic dynamics of algae and nitrate indicated a closed system. Correlation between the δ¹⁵N composition of nitrate and algae was observed at regional and spring-run scales, but the relationship was complicated by varying isotopic fractionation factors associated with nitrate uptake (Δ ranged from 2 to 13‰). Our study demonstrates that nitrate inputs to Florida's springs are derived predominantly from non-point sources and that denitrification is detectable in aquifer waters with relatively long residence time (i.e., matrix flow).

Keywords: Florida; Karst springs; Isotope; nitrate; Algae; Denitrification』

Introduction
Materials and methods
　Synoptic and longitudinal surveys of springs
　Collection of algae and water
　Processing and analysis of algae and water
Results
　Regional patterns of spring water nitrate
　Potential fractionation factors for denitrification
　Relationship between the δ¹⁵N of algae and δ¹⁵N of spring water nitrate
　Variation in stable isotopes and algal stoichiometry along longitudinal gradients
Discussion
　Nitrate sources and processes affecting nitrate in the Floridan aquifer
　δ¹⁵N and nitrate gradients in spring-fed river runs
　Utility of δ¹⁵N measurements of algae
Conclusions
Acknowledgments
References