Geochemical and microbiological methods were used to evaluate sources of nutrient enrichment, nutrient cycling, and groundwater residence times in two springs that discharge water from the karstic Upper Floridan aquifer to the Santa Fe River in northern Florida. Various chemical tracers (delta ¹⁵N and ¹⁸O isotopes of nitrate; dissolved gases, major ions; ³H/³He, chlorofluorocarbons (CFCs), SF₆, selected pesticides and degradates; and organic wastewater compounds) were analyzed in water samples collected from Hornsby Spring (flow >2.8 m³/s) and Poe Spring (flow 1.4-2.8 m³/s), at three separate times during varied hydrologic conditions. Additionally, C and N stable isotopes were measured in algae, selected invertebrates, and fish from spring pools and runs.  Nitrate-N concentrations were lower in water samples from Poe Spring (0.16-0.31 mg/L) than Hornsby Spring (0.45-0.52 mg/L).  Denitrification likely has occurred in both spring systems, indicated by enriched delta ¹⁵N-NO₃ values (15.7-23.9‰), excess N₂ (1-2 mg/L), low dissolved oxygen (<0.5 mg/L), and elevated dissolved organic carbon (2.3-7.6 mg/L).  ³H/³He, CFCs, SF₆ concentrations were consistent with binary or exponential mixing models and indicated shorter groundwater residence times for Poe Springs (25 years) than Hornsby Spring (40 years).  Fecal coliforms and human enterovirus sequences were detected in April 2004 samples compared to non-detects in previous samples (September and December 2003), although no corresponding differences were measured for chemical indicators.  Delta ¹³C and ¹⁵N values for filamentous algae, invertebrates, and fish exhibited significant spatial and seasonal differences within and between spring systems. Algae delta ¹³C and delta ¹⁵N values were consistently depleted relative to water, ranging from -39 to -48‰, and 6 to 16‰, respectively.  Invertebrates and fish were enriched in both ¹³C and ¹⁵N relative to algae, with the large variability in consumers expressed in ¹³C from Hornsby Spring and in ¹⁵N from Poe Spring.