Browsing by Author "Martin, JB"

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    Hydrologie and biotic influences on nitrate removal in a subtropical spring-fed river
    (Limnology and Oceanography, 2010-01-01) Heffernan, JB; Cohen, MJ; Frazer, TK; Thomas, RG; Rayfield, TJ; Gulley, J; Martin, JB; Delfmo, JJ; Graham, WD
    We use a long-term chemical and hydrologic record in combination with longitudinal sampling and highfrequency nitrate (NO3-) measurements from in situ sensors to describe temporal and spatial patterns of nitrogen (N) inputs and removal in the spring-fed Ichetucknee River (Columbia County, Florida) and to determine the hydrological, geomorphic, and biological factors that influence those dynamics. Over a 20-yr period of record, NO 3-N removal averaged 118 kg N d-1 (0.77 g N m-2 d-1 ) over the upper 5 km of the Ichetucknee River. Three independent estimates of gross autotrophic N assimilation (from gross primary production, diel NO3- variation, and standing biomass) agreed closely but accounted for less than 20% of observed N removal. Longitudinal surveys indicate negligible or negative dissolved organic nitrogen and ammonium (NH4+) production, suggesting that denitrification is the predominant mechanism of N removal in this river. A positive relationship between discharge and the magnitude of NO3-N removal shows that interactions with the surrounding floodplain exert considerable influence at high flows, and longitudinal NO3- patterns indicate that N removal may be influenced by channel morphology. These results suggest a greater role for dissimilatory processes and hydrologic connectivity with hyporheic and floodplain sediments than has been previously recognized in highly productive spring-fed rivers of north Florida. While hydrologic variation is the primary determinant of variation in NO 3- removal within the Ichetucknee River, comparison across systems indicates that biotic characteristics can cause significant deviation from predictions based on purely physical models of relationships between river size and N removal. © 2010 by the American Society of Limnology and Oceanography, Inc.
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    Inference of riverine nitrogen processing from longitudinal and diel variation in dual nitrate isotopes
    (Journal of Geophysical Research: Biogeosciences, 2012-03-01) Cohen, MJ; Heffernan, JB; Albertin, A; Martin, JB
    Longitudinal and diel measurements of dual isotope composition (δ15N and δ18O) in nitrate (NO3-N) were made in the Ichetucknee River, a large (∼8m3 s -1), entirely spring-fed river in North Florida, to determine whether isotopic variation can deconvolve assimilatory and dissimilatory removal. Comparing nitrate concentrations and isotope composition during the day and night we predicted (1) daytime declines in total fractionation due to low assimilatory fractionation and (2) diurnal variation in dual isotope coupling between 1:1 (assimilation) and 2:1 (denitrification). Five daytime longitudinal transects comprising 10 sampling stations showed consistent NO3-N removal (25-35% of inputs) and modest fractionation (15ε total between -2 and -6‰, enriching the residual nitrate pool). Lower fractionation (by ∼1‰) during two nighttime transects, suggests higher fractionation due to assimilation than denitrification. Total fractionation was significantly negatively associated with discharge, input [NO3-N], N mass removal, and fractional water loss. Despite well-constrained mass balance estimates that denitrification dominated total N removal, isotope coupling was consistently 1:1, both for longitudinal and diel sampling. Hourly samples on two dates at the downstream location showed significant diel variation in concentration ([NO3-N] amplitude = 60 to 90 μg N L-1) and isotope composition (δ15N amplitude = -0.7‰ to -1.6‰). Total fractionation differed between day and night only on one date but estimated assimilatory fractionation assuming constant denitrification was highly variable and implausibly large (for N, 15ε = -2 to -25‰), suggesting that fractionation and removal due to denitrification is not diurnally constant. Pronounced counterclockwise hysteresis in the relationship between [NO3-N] and δ15N suggests diel variation in N isotope dynamics. Together, low fractionation, isotope versus concentration hysteresis, and consistent 1:1 isotope coupling suggests that denitrification is controlled by NO 3- diffusion into the benthic sediments, the length of which is mediated by riverine oxygen dynamics. While using dual isotope behavior to deconvolve removal pathways was not possible, isotope measurements did yield valuable information about riverine N cycling and transformations. Copyright © 2012 by the American Geophysical Union.