4th UF Water Institute Symposium Abstract

Submitter's Name Bobby Hensley
Session Name Poster Session: Impact of changing drivers on water resources
Poster Number 4
Author(s) Robert Hensley,  University of Florida (Presenting Author)
  Matthew Cohen,  University of Florida
  Profile "smearing" and its effect on inferring nutrient removal pathways and kinetics
  Recent development of in-situ sensors has enabled detailed investigation of nutrient dynamics at high sampling frequencies. This has led to a better understanding of removal pathways and ecosystem response to increased nutrient loading. However in rivers with large residence time distributions, removal signals may become “smeared” as a result of temporally varying removal (autotrophic uptake as a function of sunlight for example) and dispersion/transient storage effects. This is true of data collected from a Eulerian (time series) perspective, where the mixture of day and night water prevents identification of a true nighttime baseline. It is also true of data collected from a Lagrangian perspective (longitudinal profiling) where a sampling craft travelling at the channel velocity overtakes water retained in transient storage zones, resulting in sampling of water which is a mixture of newer and older water. The decrease in profile variability (both Eulerian and Lagrangian) due to mixing inhibits our ability to differentiate removal pathway (i.e. assimilation versus denitrification). Also inhibited is our ability to infer removal kinetics (i.e. how the system will respond to increased nutrient loading) because these processes may influence the geometry of the profile to a much greater extent than reaction order. This has important management implications; for example a river where autotrophic uptake is a zero-order process (saturation) is capable of producing an exponentially declining longitudinal profile suggestive of first-order kinetics (nutrient limitation). Resolving these issues requires the development and implementation of a reactive transport model which can account for temporal variability in removal rates was well as dispersion and transient storage effects.