Abstract:

Humans have more than doubled the amount of reactive nitrogen (N) in the environment, contributing to marine eutrophication. Salt marshes provide important areas of N removal through microbial mediated N-cyling. However, oil spills may alter or reduce their functional capacity by reducing plant biomass or altering sediment microbial communities. The objective of our study is to determine the effects of the Deepwater Horizon oil spill on nitrogen (N) cycling in salt marsh sediments. A study was 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 June 2016, we collected sediments from three sites subjected to a range of oiling to determine the legacy effect of the oil spill on sediment denitrification and nitrogen fixation (N2-fixation). Denitrification capacity, which was determined using the isotope-pairing technique, was highest in September (63.9±30.2) µmol N m-2 h-1).

Suggested Citation:

Corianne Tatariw, Nikaela Flournoy, Alice Kleinhuizen, Derek Tollette, Edward Overton, Patricia Sobecky, Behzad Mortazavi. 2018. Spatial heterogeneity of nitrogen cycling in Deepwater Horizon-impacted salt marshes from May 2016-February 2017. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7251GJ3

Publications:

Tatariw, C., Flournoy, N., Kleinhuizen, A. A., Tollette, D., Overton, E. B., Sobecky, P. A., & Mortazavi, B. (2018). Salt marsh denitrification is impacted by oiling intensity six years after the Deepwater Horizon oil spill. Environmental Pollution, 243, 1606–1614. doi:10.1016/j.envpol.2018.09.034

Purpose:

The purpose of this study was to determine the long-term impacts of oiling on salt marsh ecosystem function (i.e. denitrification and nitrogen fixation) as well as drivers of ecosystem function (i.e. microbial community composition and functional abundance, environmental conditions).

Data Parameters and Units:

Sampling date: month, day and year, Site number: represents the three sites that were visited on each date, Denitrification capacity: measured in micromoles of nitrogen per meter squared per hour (µmol N m2 h-1), Denitrification potential: measured in micromoles of nitrogen per gram dry sediment per hour (µmol N g sediment-1 h-1) at 0-2 and 5-7 cm depths, Nitrogen fixation (N2-fixation) potential: measured in micromoles of nitrogen per gram dry sediment per hour (µmol N g sediment-1 h-1) at 0-2 and 5-7 cm depths, NO2+NO3 Flux (nitrite plus nitrate flux): measured in micromoles of nitrogen per meter squared per hour (µmol N m2 h-1), PO4 Flux (phosphate flux): measured in micromoles of phosphorus per meter squared per hour (µmol P m2 h-1), NH4 Flux (ammonium flux): measured in micromoles of nitrogen per meter squared per hour (µmol N m2 h-1), Porewater NH4 (porewater ammonium): measured in nanomoles of nitrogen per gram dry sediment (nmol N g sediment-1) at 0-2 and 5-7 cm depths, C:N (sediment molar carbon to nitrogen ratio): unitless, Chlorophyll a (sediment chlorophyll a inventory): measured in micrograms of chlorophyll a per gram dry sediment (µg g sediment-1), Belowground biomass (plant belowground biomass): measured in kilograms plant belowground biomass per square meter (kg m-2), TPH (total petroleum hydrocarbons): total milligrams of TPH per gram of sediment (mg kg-1), Porosity: fraction of void space in sediment (unitless) at 0-2 and 5-7 cm depths, 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), NPOC (non-purgeable organic carbon): measured in micromoles of carbon per liter (µmol C L-1), Chlorophyll a (water column chlorophyll a concentration): measured in micrograms chlorophyll a per liter (µg L-1) N.D. = not determined, B.D. = below detection limit Sampling Site Latitude (decimal degrees) Longitude (decimal degrees) 1 29.863750 -88.841466 2 29.895448 -88.827780 3 29.927599 -88.829308

Methods:

We established three 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, and Site 3 (29.927599º -88.829308º) was 3.6 km north of Site 2. Samples were collected from each site on five dates (22 June 2016, 20 September 2016, 19 October 2016, 15 November 2016, and 23 February 2017) and from Sites 1 and 3 on one date (24 May 2016). A tar mat was present within 5 cm of the marsh surface at Site 2 in November and February. Denitrification capacity rates were determined by calculating the production of 29N2 and 30N2 using isotope pairing technique (IPT). Three intact sediment cores (33 cm x 9.5 cm i.d.) were collected from each site. On the day of collection, the cores were submerged in site water in a darkened environmental chamber at in situ temperature. After a 16—18 h period, cores were capped and set up in a continuous-flow (~2.5 ml min-1) system which sent 0.7 µm filtered site water spiked with ~50 µM Na15NO3 (99 atom %) to the 5 cm of water overlying the sediment surface. Overlying water was homogenized with magnetic stir bars attached to the core caps. Following a 24 h equilibration period, triplicate inflow and outflow samples were collected for dissolved gas and nutrient analysis by overflowing 12 ml Exetainer vials (Labco) by 2 tube volumes. Samples were preserved with 50% (v/w) ZnCl2 and were stored under water in the environmental chamber until dissolved gas analysis by membrane inlet mass spectrometer (MIMS) equipped with a copper reduction column set at 600ºC to remove oxygen (O2). Benthic nutrient flux samples (12 mL) were filtered (0.7 µm) and immediately frozen until being analyzed for DIN (NO3-, NO2-, NH4+) and phosphate (PO43-) as described for sitewater. Triplicate sediment cores (9.5 cm i.d.) were collected from each habitat and partitioned for 0-2 cm and 5-7 cm depths for denitrification and N2-fixation potential rates. Denitrification potential (DNP) rates were determined using the acetylene block method on sediment slurries. The slurries were enriched with KNO3, purged with N2 gas to produce anoxic conditions, then injected with acetylene (10% v/v). After a 1 h incubation, gas samples were injected into pre-evacuated 12 ml Exetainer vials and measured on a Shimadzu gas chromatograph (GC-2014) with an electron capture detector (GC-ECD). Potential N fixation (NFP) rates were determined using the acetylene reduction method. Slurries were bubbled with N2 gas to produce anoxic conditions and injected with acetylene (10% v/v). Following a 24 h incubation in the dark, headspace samples were injected into pre-evacuated 12 mL Exetainer vials and measured on a Shimadzu gas chromatograph (GC-2014) with a flame ionization detector (GC-FID). 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 (NO3-, NO2-, 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). Water column chlorophyll a (chl-a) samples were collected by filtering 60 ml of site water through a 0.7 µm GF/F glass filter, and 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). Belowground biomass was collected in triplicate in each habitat (10 cm x 9.5 cm i.d.). Roots were separated from sediment by rinsing with tap water through a 2 mm sieve. An additional 20 ml of sitewater was filtered into ashed glass vials for non-purgeable organic carbon (NPOC) measurements on a Shimadzu TOC-VS equipped with an ASI-V autosampler. Porewater NH4+ was extracted from sediments sampled at 0-2 and 5-7 cm depth increments with 50% (w/v) sodium chloride (NaCl) 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+ for samples collected in May and June was determined with a Skalar San+ autoanalyzer whereas samples collected on the remaining dates were analyzed fluorometrically as described for site water. The extracted sediments were oven dried at 45° C for until they reached a consistent weight to determine sediment porosity. Triplicate sediment samples for molar C:N analysis were collected from 0-2 cm using a 10 mL syringe core. Samples were oven dried at 65°C, ground with a mortar and pestle, then fumigated with 12 N HCl overnight to remove any carbonate. Following fumigation, samples were re-oven dried and ground prior to analysis on a Costech 4010 CHN analyzer. Triplicate sediment TPH concentrations were determined for 0-10 cm (9.5 cm i.d.). Samples were analyzed using Standard Method 5520F at Pace Analytical Services LLC (New Orleans, LA, USA).

Instruments:

Costech CHN analyzer model 4010 was used to measure sediment molar C:N Gas chromatograph with electron capture detector (GC-ECD) was used to measure nitrous oxide (N2O) gas concentration Gas chromatograph with flame ionization detector (GC-FID) was used to measure ethylene (C2H4) gas concentration Genesys 10S UV-Vis spectrophotometer was used to measure sitewater NO2 and PO4 Membrane Inlet Mass Spectrometer (MIMS) was used to measure dissolved 29N, 30N, O2, Ar and N2 Shimadzu TOC-VS equipped with an ASI-V autosampler was used to measure water column non-purgeable organic carbon 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 Skalar San+ autoanalyzer was used to measure porewater NH4 in May in June YSI 556 multiprobe was used to take point measurements of water column temperature and salinity

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