Samantha B Joye

Estimating denitrification rates in estuarine sediments : A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m−2 d−1. We determined the apparent Michaelis constant (Kapp) and the maximum potential rate (Vmp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the Kapp and Vmp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our ‘kinetic-fix’ adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m−1 d−1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m−2 d−1. Average denitrification rates obtained using the ‘kinetic-fix’ acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our ‘kinetic-fix’ adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.

Analyses of Water Samples From the Deepwater Horizon Oil Spill: Documentation of the Subsurface Plume

Monitoring and M A Record-Breakin Geophysical Mon Copyright 2011 b 10.1029/2011GM Surface and subsurface water samples were collected in the vicinity of the Deepwater Horizon (DWH) wellhead in the Gulf of Mexico. Samples were extracted with dichloromethane and analyzed for a toxic component, polycyclic aromatic hydrocarbons (PAHs), using total scanning fluorescence (TSF) and by gas chromatography/mass spectrometry (GC/MS). An aliquot of fresh, floating oil from a surface sample was used as a DWH oil reference standard. Twelve of 19 samples collected from 24 May 2010 to 6 June 2010 on the R/V Walton Smith cruise contained TSF maximum intensities above background (0.7 μg L 1 based on 1 L sample size). These 12 samples had total petroleum hydrocarbon (TPH) concentrations as measured by quantitative gas chromatography flame ionization detector (FID) ranging from 2 to 442 μg L . Quantitative GC/MS analysis of these 12 samples resulted in total PAH concentrations ranging from 0.01 to 59 μg L . Low molecular weight, more water-soluble naphthalene and alkylated naphthalene dominated the PAH composition patterns for 11 of the 12 water samples. Sample 12 exhibited substantially reduced concentrations of naphthalenes relative to other PAH compounds. The total PAH concentrations were positively correlated (R = 0.80) with the TSF maximum intensity (MI). TSF is a simple, rapid technique providing an accurate prediction of the amount of PAH present in a sample. TSFderived estimates of the relative contribution of PAH present in the oil provided evidence that PAH represented ~10% of the higher molecular weight TPH. The subsurface oil plume was confirmed by the analyses of discrete water samples for TSF, TPH, and PAH.

An improved chromatographic method to measure nitrogen, oxygen, argon and methane in gas or liquid samples

Abstract An improved gas chromatographic method for measuring dinitrogen (N 2 ), oxygen (O 2 ), argon (Ar) and methane (CH 4 ) in gas or liquid samples has been developed. By employing a zeolite, calcium chabazite, as column packing material, optimum separation of O 2 and Ar was achieved at 303 K with a carrier gas flow rate of 33.5 ml min −1 . Dinitrogen and CH 4 were eluted at slightly higher column temperatures of 363 and 438 K, respectively. The coefficients of variation (CV, %) for the various gases in liquid (~ 1% air) and gas (10% air) standards were: N 2 = 0.48–0.82, O 2 = 0.50–0.76, Ar = 0.50–0.76 and CH 4 = 0.23. The contamination of liquid samples by air bubbles was easily corrected by using Ar as an internal standard. The small amount of sample required (2 ml), the short analysis time (12 min) and the high resolution data obtained makes this method ideal for a variety of applications, including the study of dissolved gas biogeochemistry in ground water, the water column or in sediments. To evaluate the performance of the system, sediment incubation experiments were performed. Rates of aerobic respiration (3.6–33 mmol m −2 d −1 ), denitrification (0.21–4.5 mmol m −2 d −1 ) and methanogenesis (5.5–10 mmol m −2 d −1 ) were easily quantified using this approach.