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https://doi.org/10.5194/bg-2020-335
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-335
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Sep 2020

29 Sep 2020

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This preprint is currently under review for the journal BG.

Warming and ocean acidification may decrease estuarine dissolved organic carbon export to the ocean

Michelle N. Simone, Kai G. Schulz, Joanne M. Oakes, and Bradley D. Eyre Michelle N. Simone et al.
  • Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, 2480, Australia

Abstract. Estuaries make a disproportionately large contribution of dissolved organic carbon (DOC) to the global carbon cycle, but it is unknown how this will change under a future climate. As such, the response of DOC fluxes from microbially dominated unvegetated sediments to individual and combined future climate stressors of warming (from Δ−3 °C to Δ+5 °C on ambient mean temperatures) and ocean acidification (OA, ~2 times the current partial pressure of CO2, pCO2) was investigated ex situ. Warming alone increased sediment heterotrophy, resulting in a proportional increase in sediment DOC uptake, with sediments becoming net sinks of DOC (3.5 to 8.8 mmol-C m−2 d−1) at warmer temperatures (Δ+3 °C and Δ+5 °C, respectively). This temperature response changed under OA conditions, with sediments becoming more autotrophic and a greater sink of DOC (1 to 4 times greater than under current-pCO2). This response was attributed to the stimulation of heterotrophic bacteria with the autochthonous production of labile organic matter by microphytobenthos. Extrapolating these results to the global area of unvegetated subtidal estuarine sediments, the future climate of warming (Δ+3 °C) and OA may decrease the estuarine export of DOC by ~80 % (~150 Tg-C yr−1) and have a disproportionately large impact on the global DOC budget.

Michelle N. Simone et al.

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Michelle N. Simone et al.

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Short summary
Estuaries are responsible for a large contribution of dissolved organic carbon (DOC) to the global C-cycle, but it is unknown how this will change in the future. DOC fluxes from unvegetated sediments were investigated ex situ subject to conditions of warming and ocean acidification (OA). The future climate shifted sediment fluxes from a slight DOC source to a significant sink, with global coastal DOC export decreasing by 80 %. This has global implications for C-cycling and long-term C-storage.
Estuaries are responsible for a large contribution of dissolved organic carbon (DOC) to the...
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