HR: 1340h
AN: H13F-1376    [Abstracts]
TI: Assessing Diurnal Variability in Riverine Dissolved Organic Carbon and Nitrate with Discrete and High Frequency In Situ Monitoring
AU: * Pellerin, B A
EM: bpeller@usgs.gov
AF: US Geological Survey, 6000 J St., Placer Hall, Sacramento, CA 95819 United States
AU: Kraus, T
EM: tkraus@usgs.gov
AF: US Geological Survey, 6000 J St., Placer Hall, Sacramento, CA 95819 United States
AU: Downing, B D
EM: bdowning@usgs.gov
AF: US Geological Survey, 6000 J St., Placer Hall, Sacramento, CA 95819 United States
AU: Bergamaschi, B A
EM: bbergama@usgs.gov
AF: US Geological Survey, 6000 J St., Placer Hall, Sacramento, CA 95819 United States
AB: Understanding diurnal variability of nitrate (NO₃) and dissolved organic carbon (DOC) in surface water is important for characterizing water quality and quantifying constituent loads. However, measuring NO₃ and DOC at sufficiently high frequencies to detect diurnal patterns has historically been a challenge. Here we present results from a study examining high frequency variations of DOC and NO₃ in the San Joaquin River in California during a period without rainfall (July 2005). In situ optical instrumentation was used to quantify NO₃ concentrations and the intensity of chromophoric DOM at 30-minute intervals for a 5-day period. Discrete samples were also collected every 2 hours for 2 days of the study for lab analysis of bulk DOC and NO₃ concentrations, as well as spectral absorbance and fluorescence. Ancillary in situ data (electrical conductivity, pH, dissolved oxygen, chlorophyll fluorescence, and flow) were also measured. In situ and lab NO₃ concentrations were strongly correlated (r²=0.97) and varied diurnally by up to 25 % (0.6 mg/L). The general temporal pattern shows a daily minimum at night (ca. 2000 hr) and a daily maxima near noon. A second daily NO₃ peak occurred but was less predictable and may reflect changes in external sources such as agricultural return flow. Temperature, dissolved oxygen, and chlorophyll fluorescence exhibited clear diurnal signals with late afternoon maxima and early morning minima but were not correlated with NO₃ concentrations. Lab DOC concentrations ranged from 3.1-3.6 mg/L and showed no clear diurnal pattern during the 2-day sampling period. In contrast, high frequency CDOM values did show a clear diurnal variability with maxima at noon and minima at night. Differences between CDOM and lab DOC patterns may indicate diurnal changes in components of the bulk DOC pool (such as chromophoric material) or may reflect insufficient analytical precision of lab DOC analyses to detect small diurnal changes. Along with C and N isotopic data, observed high frequency data will provide important insights into the biological, physical and photolytic processes affecting NO₃ and DOC on diurnal timescales.
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0469 Nitrogen cycling
DE: 1871 Surface water quality
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005