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Influence of labile dissolved organic matter on nitrate reduction in a seepage face

Abstract

Seepage faces, the outer rim of subterranean estuaries, are an important reaction node for SGD-borne nitrate (NO3) on a global scale. Labile dissolved organic matter (DOM) has been suggested to be a key factor constraining the NO3 removal rate in aquifer systems. To determine whether and to what extent the availability of labile DOM affects benthic NO3 reduction in seepage faces, a series of flow-through reactor (FTR) experiments with sandy sediment collected from a seepage face was conducted under oxic conditions. Experimental results revealed that the addition of labile DOM (glucose) to porewater did not trigger a significant enhancement in NO3 reduction rate. In contrast, the aerobic respiration was boosted from ca. 50 to 90 μmol dm−3 sediment h−1 by glucose amendments, accounting for approximately 70% consumption of the labile DOM pool. This rapid consumption may increase the NO3 reducing capability within the sediment, but only indirectly. Together with fluorescent DOM (FDOM) analyses, it can be inferred that NO3 reducers tend to choose sediment organic matter the prime electron donor under the experimental conditions. As a result, enrichment of DOM in seepage faces, depending on composition, might only stimulate aerobic respiration and nitrification, thus promoting the increase of ensuing NO3 fluxes to adjacent coastal waters.

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Abbreviations

R red1 :

\( \mathrm{Maximum}\ {\mathrm{NO}}_3^{-}\ \mathrm{reduction}\ \mathrm{rate}\ \left(\upmu \mathrm{mol}\ {\mathrm{h}}^{-1}\right) \)

K halfnitrate :

\( \mathrm{Half}-\mathrm{saturation}\ {\mathrm{NO}}_3^{-}\ \mathrm{reduction}\ \mathrm{constant}\ \left(\upmu \mathrm{mol}\right) \)

R red2 :

\( \mathrm{Maximum}\ {\mathrm{NO}}_2^{-}\ \mathrm{reduction}\ \mathrm{rate}\ \left(\upmu \mathrm{mol}\ {\mathrm{h}}^{-1}\right) \)

K halfnitrite :

\( \mathrm{Half}-\mathrm{saturation}\ {\mathrm{NO}}_2^{-}\ \mathrm{reduction}\ \mathrm{constant}\ \left(\upmu \mathrm{mol}\right) \)

R com :

Maximum DOC consumation rate (μmol h−1)

K halfDOC :

Half − saturation DOC consumtion constant (μmol)

K ads :

First order glucose adsorption rate constant (h−1)

[Glu]eq :

Total glucose adsorbed in the reactor at equilibrium (μmol)

[Glu]ads :

Total glucose adsorbed in the reactor (μmol)

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Acknowledgements

The authors would like to thank the editor and anonymous reviewers whose comments significantly improved the manuscript. Technical support during the laboratory analyses by Dr. Tara Kelly, Dr. Dannielle Senga Green, Dr. Yue Lu and Mr. Mark Kavanagh at Trinity College Dublin is gratefully acknowledged.

Funding

Funding for this study was provided by the Portuguese Foundation for Science and Technology (FCT), the EU (FEDER) and the Portuguese Government through project NITROLINKS - “NITROgen loading into the Ria Formosa through Coastal Groundwater Discharge (CGD) - Pathways, turnover and LINKS between land and sea in the Coastal Zone” (PTDC/MAR/70247/2006).

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Correspondence to Shan Jiang.

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Responsible editor: Philippe Garrigues

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Jiang, S., Ibánhez, J.S.P. & Rocha, C. Influence of labile dissolved organic matter on nitrate reduction in a seepage face. Environ Sci Pollut Res 25, 10654–10667 (2018). https://doi.org/10.1007/s11356-018-1302-1

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  • DOI: https://doi.org/10.1007/s11356-018-1302-1

Keywords

  • Submarine groundwater discharge
  • Subterranean estuaries
  • Nitrate removal
  • Organic matter
  • Seepage faces
  • Remineralisation