3rd UF Water Institute Symposium Abstract

Submitter's Name Robert Hensley
Session Name Posters - Nutrient dynamics and enrichment impacts in aquatic ecosystems 2
Category Nutrient dynamics and enrichment impacts in aquatic ecosystems
Poster Number 43
Author(s) Robert Hensley,  University of Florida (Presenting Author)
  Matthew J. Cohen,  UF, School of Forest Resources and Conservation
  James B. Heffernan, Florida International University, Department of Biological Sciences and Southeast Environmental Research Center
  New Methods for Estimating Riverine N Removal Rates
  Previous studies seeking to quantify riverine nitrogen uptake rates have primarily relied on 15N isotope addition. While that approach yields process-specific inference of N fluxes, it is entirely impractical for larger river systems, and yields a single synoptic estimate of uptake, failing to account for diel, seasonal and spatial variation. Here we present a new approach using a deployable Submersible Ultra-violet Nitrate Analyzer (SUNA, Satlantic Corporation, Halifax Nova Scotia) that is scalable and inexpensive enough to permit repeated or prolonged deployment. These optical nitrate sensors are accurate and precise, and permit data acquisition at very high temporal resolution (up to 0.5 Hz). We utilized these sensors to collect both Eulerian and Lagrangian time series of nitrate concentrations in ten spring-fed rivers in north central Florida. First, we deployed sensors at a stationary point for several days to collect Eulerian diel concentration dynamics. Second, we used a Lagrangian approach along the length of each river to collect longitudinal concentration profiles. We used both these data sets, coupled with measured channel geometry and flow data, as two alternative methods of inferring both the assimilatory and dissimilatory nitrogen removal rates. The results of these two methods for calculating N removal rates compare favorably, and indicate that these river systems act as significant permanent sink for nitrogen because the dominant removal pathway is denitrification. Total removal rates ranged from 0.13-1.25 g N/m2/day, and denitrification was typically between 75 and 90% of this total. In the course of this investigation we also identified that an accurate understanding of the hydrology of these systems is a prerequisite to differentiating between actual N removal and dilution. These two methods provide a promising new approach to estimating riverine N removal and allow us to observe temporal and spatial heterogeneity which previously could not be detected.