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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-468
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-468
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 02 Mar 2020

Submitted as: research article | 02 Mar 2020

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This preprint is currently under review for the journal BG.

Stable isotopes track the ecological and biogeochemical legacy of mass mangrove forest dieback in the Gulf of Carpentaria, Australia

Yota Harada1, Rod M. Connolly1, Brian Fry2, Damien T. Maher3,4, James Z. Sippo3, Luke C. Jeffrey3, Adam J. Bourke5, and Shing Yip Lee6 Yota Harada et al.
  • 1Australian Rivers Institute – Coast and Estuaries, and School of Environment and Science, Griffith University, Gold Coast, Queensland, Australia
  • 2Australian Rivers Institute, Griffith University, Nathan, Queensland, Australia
  • 3Southern Cross Geoscience, Southern Cross University, Lismore, New South Wales, Australia
  • 4School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, Australia
  • 5College of Engineering, Information Technology and the Environment, Charles Darwin University, Northern Territory, Australia
  • 6Simon F.S. Li Marine Science Laboratory, School of Life Sciences, and Earth System Science Programme, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China

Abstract. A combination of elemental analysis and stable isotope analysis (SIA) was used to assess and monitor C, N and S cycling of a mangrove ecosystem that suffered mass dieback of trees in the Gulf of Carpentaria, Australia in 2015–16, attributed to an extreme drought event. Three field campaigns were conducted over a period from 2016 to 2018, at 8, 20 and 32 months after the event. Samples including invertebrates, mangroves, and sediment were analysed for CNS elemental and isotopic compositions including compound-specific stable isotope analysis (CSIA) of amino acid carbon. Samples collected from the impacted ecosystem were enriched in 13C, 15N and 34S relative to those from an adjacent unimpacted reference ecosystem, likely indicating lower mangrove carbon fixation, lower nitrogen fixation and lower sulfate reduction in the impacted ecosystem. For example, invertebrates representing the feeding types of grazing, leaf feeding, and algae feeding were more 13C enriched at the impacted site, by 1.7–4.1 ‰ and these differences did not change over the period from 2016 to 2018. The CSIA data indicated widespread 13C enrichment across five essential amino acids and all groups sampled (except filter feeders) within the impacted site. Mangrove seedling and sapling populations increased substantially from 2016 to 2018 in the impacted forest, suggesting recovery of the mangrove vegetation. Recovery of CNS cycling, however, was not evident even after 32 months, suggesting a biogeochemical legacy of the mortality event. Continued monitoring of the post-dieback forest would help to predict the long-term trajectory of ecosystem recovery. In such long-term monitoring programs, SIA that can track biogeochemical changes over time can help to detect underlying biological mechanisms that drive changes and recovery of the mangrove ecosystem. To gain further insight, our use of CSIA can help show feeding dependencies in mangrove food webs and their response to disturbances.

Yota Harada et al.

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Yota Harada et al.

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Short summary
In 2015–16, an extensive area of mangroves along ~ 1000 km of coastline in the Gulf of Carpentaria, Australia, experienced dieback as a result of a climatic extreme event that included drought conditions and low sea levels. Multiannual field campaigns conducted from 2016 to 2018, show substantial recovery of the mangrove vegetation. However, stable isotopes suggest long-lasting changes in carbon, nitrogen and sulfur cycling following the dieback.
In 2015–16, an extensive area of mangroves along ~ 1000 km of coastline in the Gulf of...
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