Preprints
https://doi.org/10.5194/bg-2021-184
https://doi.org/10.5194/bg-2021-184

  09 Aug 2021

09 Aug 2021

Review status: this preprint is currently under review for the journal BG.

Nitrification in the oligotrophic Atlantic Ocean

Darren R. Clark1, Andrew P. Rees1, Charrisa Ferrera2, Lisa Al-Moosawi1, Paul J. Somerfield1, Carolyn Harris1, Graham D. Quartly1, Stephen Goult1, Glen Tarran1, and Gennadi Lessin1 Darren R. Clark et al.
  • 1Plymouth Marine Laboratory, Prospect Place, The Hoe, PL1 3DH
  • 2The Marine Science Institute, Velasquez St., University of the Philippines, Diliman, Quezon City 1101 Philippines

Abstract. The recycling of scarce nutrient resources in the sunlit open ocean is crucial to ecosystem function. Ammonium oxidation, the first stage of the nitrification process, directs ammonium derived from organic matter decomposition towards the regeneration nitrate, an important resource for photosynthetic primary producers. However, the technical challenge of making nitrification rate measurements in oligotrophic conditions combined with the remote nature of these marine systems means that data availability, and the understanding that provides, is limited. This study reports rate measurements of ammonium oxidation over a 13, 000 km transect within the photic zone of the Atlantic Ocean. These measurements, at relatively fine resolution (order 300 km), permit the examination of interactions with environmental conditions that may warrant explicit development and inclusion in model descriptions. At all locations we report measurable rates with significant variability between and within Atlantic provinces. This adds to evidence that nitrification is an important component of pelagic nitrogen cycling which modifies the inorganic nitrogen inventory of the sunlit ocean. Particular features of interest included a significant hemispheric difference in ammonium oxidation rate and elevated rates associated with mesoscale eddy features. Statistical analysis of potential links between ammonium oxidation rate and routinely measured ecosystem variables indicated significant correlative structure, explaining ~65 % of the data variability. Differences between sampling depths were of the same magnitude or greater than horizontally resolved differences along the transect length, identifying distinct biogeochemical niches between depth horizons. Principle component analysis demonstrated that the best overall match between ammonium oxidation rate and environmental variables involved a combination of chlorophyll-a concentration, the duration of the light phase and silicate concentration (which we argue to be a short-term tracer of physical instability). Results allude to an association between ammonium oxidation and potentially short-term product(s) of photosynthetic activity and subsequent degradation. Approximately 35 % of data variability was not explained, which may include descriptions of DOM pool dynamics.

Darren R. Clark et al.

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Darren R. Clark et al.

Darren R. Clark et al.

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Short summary
We undertook measurements of microbial processes in the sunlit open ocean during a cruise from the UK to Chili. These help us to understand how microbes maintain the function of remote ecosystems. We find that the microbes respond to changes in the environment. Our insights will aid the development of models that aim to replicate and ultimately project how marine environments may respond to on-going climate change.
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