title: Nitrogen cycling in natural waters using in situ, reagentless UV spectrophotometry with simultaneous determination of nitrate and nitrite.
abstract: Reliable, high temporal, and spatial resolution data is essential for enhancing our understanding of aquatic nitrogen biogeochemical cycling. This paper describes a novel UV spectrophotometric sensor (ProPS, TriOS GmbH, Oldenburg, Germany) for the real time, in situ, high resolution simultaneous mapping of nitrate/nitrite (linearity 0.01 - 6 mg N L(-1), RSD's NO3-N 4-10%, NO2-N 7-14%) in fresh and estuarine waters. Good agreement (t test at p = 0.05) was found with MOOS-1 certified reference material and with reference segmented flow analysis data. River Taw deployments identified a diurnal cycle for NO3-N (0.22-0.63 mg L(-1), RSD 3.9%) and for NO2-N (0.01-0.28 mg L(-1), RSD 12.4%) with the photo-oxidation of dissolved organic nitrogen a source of diurnal nitrate/nitrite, and a large cyclical amplitude (30-62% of mean nitrate/nitrite). In situ Tamar Estuary nitrate/nitrite concentrations, mapped through the salinity gradient, were strongly correlated with suspended particulate material and inversely correlated with dissolved oxygen and pH, indicating midestuarine, bacterially mediated nitrification/denitrification, with the raised estuarine nitrite also significantly correlated with particulate organic nitrogen. Such previously unquantified inputs have important implications for N loadings calculated from coarse scale sampling and laboratory analysis, pollution assessment, and our understanding of the biological rhythms of aquatic organisms.
authors.affiliation: Biogeochemistry and Environmental Analytical Chemistry (BEACh) Group, School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, England. rsandford@plymouth.ac.uk
authors.forename: Richard C
authors.initials: RC
authors.lastname: Sandford
authors.forename: Alexandra
authors.initials: A
authors.lastname: Exenberger
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Worsfold
mesh_terms: D009566:Nitrates / Q000032:analysis*; D009573:Nitrites / Q000032:analysis*; D009584:Nitrogen / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D014867:Water / Q000737:chemistry*
chemical_list: D009566:Nitrates; D009573:Nitrites; D014867:Water; D009584:Nitrogen