Direct and indirect coupling of primary production and diel nitrate dynamics in a subtropical spring-fed river

We used high-frequency in situ measurements of nitrate (NO {3) and dissolved oxygen (DO) from the spring-fed Ichetucknee River, Florida, to derive multiple independent estimates of assimilatory nitrogen (N) demand, and to evaluate the short-term dependence of heterotrophic assimilation and dissimilation (e.g., denitrification) on gross primary productivity (GPP). Autotrophic N assimilation estimates derived from diel DO variability and GPP stoichiometry agreed closely with estimates based on integration of diel variation in NO3- concentration, although the correspondence of these metrics depended on the method used to estimate NO3- baselines. In addition, day-to-day changes in nocturnal NO3- concentration maxima were strongly negatively correlated with day-to-day changes in GPP. Diel temperature variation in the Ichetucknee River indicated that this pattern could not be explained by hydrologic dispersion, while relationships between N assimilation and O-2 production at hourly intervals indicated minimal physiological lags. The estimated magnitude of heterotrophic assimilation was small, indicating that the relationship between changes in GPP and changes in nocturnal NO3- maxima reflects sensitivity of denitrification to variation in exudation of labile organic matter by primary producers. We estimate that similar to 35% of denitrification may be fueled by the previous day's photosynthesis; this result is consistent with the broader hypothesis that the magnitude of autochthonous production in aquatic systems influences the fate of N via both direct and indirect mechanisms.