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Pelagic denitrification and methane oxidation in oxygen-depleted waters of the Louisiana shelf

Abstract

Anthropogenic nutrient inputs fuel eutrophication and hypoxia ([O2] < 2 mg L−1), threatening coastal and near shore environments across the globe. The world’s second largest anthropogenic coastal hypoxic zone occurs annually along the Louisiana (LA) shelf. Springtime loading of dissolved inorganic nitrogen (DIN) from the Mississippi River, combined with summertime stratification and increased water residence time on the shelf, promotes establishment of an extensive hypoxic zone that persists throughout the summer. We investigated the patterns of pelagic denitrification and methane (CH4) oxidation along the LA shelf. Microbial activity rates were determined along with concentrations of dissolved nutrients and greenhouse gases, nitrous oxide (N2O) and CH4, during summer in 2013, 2015, and 2016. We documented denitrification rates up to 1900 nmol N L−1 d−1 and CH4 oxidation rates as high as 192 nmol L−1 d−1 in hypoxic waters characterized by high concentrations of N2O (range: 1 to 102 nM) and CH4 (range: 3 to 641 nM). Ecosystem scaling estimates suggest that pelagic denitrification could remove between 0.1 and 47% of the DIN input from the Mississippi River, whereas CH4 oxidation does not function as an effective removal process with CH4 escaping to the atmosphere. Denitrification and CH4 oxidizing bacteria within the LA shelf hypoxic zone were largely unable to keep up with the DIN and CH4 inputs to the water column. Rates were variable and physiochemical dynamics appeared to regulate the microbial removal capacity for both nitrate/nitrite and CH4 in this ecosystem.

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Acknowledgements

We thank the captain and crew of the R/V Pelican for help in sample collection, and Lauren Gillies, Ariella Chelsky, and Matthew Rich for help with sample collection and onshore support. Guangchao Zhuang and Ryan Sibert provided help and guidance with the methane oxidation rate assays. This work was made possible by the generous support from Ecosystem Impacts of Oil and Gas in the Gulf (ECOGIG) to SBJ under The Gulf of Mexico Research Initiative, the National Science Foundation (awards OCE-1558916 and OCE-1151698 to FJS), and NOAA’s National Centers for Coastal Ocean Research (award NA09NOS4780204, NGOMEX #251) to NNR and RE Turner. Supporting data for the 2013 and 2015 hypoxia cruises are archived in NOAA’s National Center for Environmental Information. Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org (https://doi.org/10.7266/SQNEJDPR). This paper is ECOGIG contribution number 533.

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Correspondence to Samantha B. Joye.

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This paper is an invited contribution to the 35th Anniversary Special Issue, edited by Sujay Kaushal, Robert Howarth, and Kate Lajtha.

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Rogener, M.K., Hunter, K.S., Rabalais, N.N. et al. Pelagic denitrification and methane oxidation in oxygen-depleted waters of the Louisiana shelf. Biogeochemistry 154, 231–254 (2021). https://doi.org/10.1007/s10533-021-00778-8

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  • DOI: https://doi.org/10.1007/s10533-021-00778-8

Keywords

  • Gulf of Mexico
  • Hypoxia
  • Denitrification
  • Methane oxidation
  • Nitrous oxide
  • Methane