Abstract:

This dataset was generated from a study conducted in the Chandeleur Islands, a chain of barrier islands off the coast of Louisiana which were subjected to a gradient of oil deposition following the spill. Starting in May 2017, we collected sediments from plots dominated by black mangrove and smooth cordgrass at a lightly oiled site and a moderately oiled site subjected to a range of oiling to determine the impact of woody encroachment and oiling on sediment denitrification and dissimilatory nitrate reduction to ammonium (DNRA).

Suggested Citation:

Behzad Mortazavi. 2019. Salt marsh vegetation-mediated response of nitrogen cycling to the Deepwater Horizon spill, Chandeleur Islands, Louisiana from 2017-05-30 to 2018-06-13. Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/Z5MG1AFS

Purpose:

The purpose of this study was to determine the long-term impacts of oiling and woody encroachment on salt marsh ecosystem function (i.e. denitrification and DNRA) as well as drivers of ecosystem function (i.e. environmental conditions).

Data Parameters and Units:

N Cycling Data Date: month, day and year, Site number: represents the two sites that were visited on each date Denitrification potential: measured in micromoles of nitrogen per kilogram dry sediment per hour (µmol N kg-1 h-1) at 0-5 cm depths, Dissimilatory nitrate reduction to ammonium (DNRA) potential: measured in micromoles of nitrogen per gram dry sediment per hour (µmol N kg-1 h-1) at 0-5 cm depths, Anaerobic ammonium oxidation (anammox) potential: measured in micromoles of nitrogen per gram dry sediment per hour (µmol N kg-1 h-1) at 0-5 cm depths, Porewater extractable NH4 (porewater ammonium): measured in micromoles of nitrogen per gram dry sediment (µmol N g-1) at 0-5 cm depths, Total iron (Fe(II+III): measured in micromoles of iron per gram dry sediment (µmol g-1) at 0-5 cm depths, C:N (sediment molar carbon to nitrogen ratio): unitless, Chlorophyll a (sediment chlorophyll a inventory): measured in miligrams of chlorophyll a per meter squared (mg m-2), Belowground biomass (plant belowground biomass): measured in kilograms plant belowground biomass per square meter (kg m-2), A. germinans height: measured in meters (m) Water Quality Data Temperature: measured in degrees Celsius (°C), Salinity: measured in parts per thousand (ppt), NO2+NO3 (water column nitrite plus nitrate): measured in micromoles of nitrogen per liter water (µmol N L-1), PO4 (Water column phosphate): measured in micromoles of phosphorus per liter water (µmol P L-1), NH4 (water column ammonium): measured in micromoles of nitrogen per liter water (µmol N L-1),

Methods:

We established two sampling sites subjected to a range of oiling along the western shore of the Chandeleur Islands (LA, USA). Site 1 was the southernmost point (29.863750º 88.841466º). Site 2 (29.895448º -88.827780º) was located 3.8 km north of Site 1. Six 1 x 1 meter plots were established at each site: 3 in A. germinans dominated sediments and 3 in S. alterniflora dominated sediments. Samples were collected from each plot at both sites on five dates (30 May 2017, 03 July 2017, 02 August 2017, 07 May 2018, and 13 June 2018). Denitrification and anammox potential rates were determined by calculating the production of 29N2 and 30N2 using isotope pairing technique (IPT). Two cores (0-5 cm) were collected from each plot and homogenized in polyethylene bags. Following an overnight incubation at ambient water temperature, roughly one half of each sample was slurried with artificial sea water (ASW) adjusted to the average salinity of the sitewater at the time of sampling. Slurries were bubbled with dinitrogen (N2) gas to produce anoxic conditions and siphoned into Exetainer vials. Following an overnight incubation at sitewater temperature to remove ambient NO3- and oxygen (O2), slurries were spiked to ~50 µM Na15NO3 (99 atom %; Cambridge Isotope Laboratories, Inc.). One half of the slurry tubes were immediately spiked with zinc chloride (ZnCl2, 50% W/V) to stop biological activity. The other half were incubated for ~6 hours at ambient water temperature then spiked with ZnCl2. Denitrification and anaerobic ammonium oxidation (annamox) were measured based on the concentration of 29N2 and 30N2 concentrations in slurry water using a membrane inlet mass spectrometer (MIMS) outfitted with a copper reduction column to remove excess oxygen. DNRA was measured based on 15NH4 production using a hypobromite reduction. Briefly, DNRA tubes were bubbled with N2 to purge 29N2 and 30N2 produced by denitrification and anammox. Samples were then amended with 200 µL sodium hypobromite, which converts NH4+ to N2. The resulting 29N2 and 30N2 concentrations were measured on the MIMS. Following analysis, sediments in tubes were dried to a constant weight to calculate NO3- reduction rates as µmol N kg dry weight-1 h-1. Two cores (0-5 cm) were collected from each plot to determine porewater extractable NH4+ and total iron (Fe) concentrations. Porewater NH4+ was extracted with 2N potassium chloride (KCl) on a shaker table. After 24 hours, the slurries were centrifuged and the supernatant was filtered (0.45 µm nylon membrane filter) and frozen until analysis. Porewater NH4+ was determined. Total Fe concentrations were determined photometrically following extraction with 0.5M hydrochloric acid (HCl). One core (0-5 cm) was collected per plot determine the molar carbon to nitrogen ratio (C:N) of sediments. Cores were oven dried at 60°C, ground with a mortar and pestle, then fumigated with 12 N HCl overnight to remove carbonate. Following fumigation, samples were re-oven dried and ground prior to analysis on a Costech 4010 CHN analyzer. Sediment chl-a samples were collected from each habitat from the top 2 cm using a 10 mL syringe core. Filters and sediments were freeze-dried, dry weight was recorded, and chl-a was extracted with 90% acetone for 24 hours. Chl-a concentrations were determined fluorometrically on a TD-700 fluorometer (Turner Designs). Sediment cores (5 cm x 9.5 cm i.d.) were collected to determine plant belowground biomass for all dates but 30 May 2017 and 03 July 2017. Roots were separated from sediment by rinsing with tap water through a 2 mm sieve. The remaining plant material was oven dried at 60°C to a constant weight and belowground biomass was determined on an areal basis. A. germinans height was measured on all dates but 30 May 2017 and 03 July 2017. Average plant height (m) for each plot was determined by measuring 6 points along a transect perpendicular to the shoreline. Site water measurements were taken roughly 10 m from the edge of the marsh platform at each site. Point measurements of water column temperature and salinity were taken in the field with a 556 multiprobe (YSI). Site water was filtered (0.45 µm nylon membrane filter) for DIN (NO2+3-, NH4+) and phosphate (PO43-). NO2+3 concentrations were determined microphotometrically via vanadium(III)chloride reduction on a Tecan Sunrise absorbance microplate reader. PO4 concentrations were determined photometrically on a Genesys 10S UV-Vis spectrophotometer (Thermo Scientific) with a 4.95 cm path length. NH4 was determined fluorometrically on a Trilogy fluorometer outfitted with a CDOM/NH4 module (Turner Designs).

Instruments:

Costech CHN analyzer model 4010 was used to measure sediment molar C:N Genesys 10S UV-Vis spectrophotometer was used to measure sitewater PO43- Membrane Inlet Mass Spectrometer (MIMS) was used to measure dissolved 29N, 30N, O2, Ar and N2 Tecan Sunrise absorbance microplate reader was used to measure sitewater NO2+3 Trilogy fluorometer outfitted with a CDOM/NH4 module was used to measure sitewater and porewater NH4+ YSI 556 multiprobe was used to take point measurements of water column temperature and salinity

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