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

Abstract

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.

Reconstructing extreme climatic and geochemical conditions during the largest natural mangrove dieback on record

“…Lower water availability in the dead mangrove forest area was also evident in the lower plant growth rates and higher plant water use efficiency. Mangrove plant isotope data at the same sites from a study by Harada et al (2020) also show a similar trend: more enriched 13 C values in the dead mangrove zone.…”

“…Sippo et al: Reconstructing extreme climatic and geochemical conditions during mangrove dieback coincided with extreme drought and low regional sea levels. This extreme climatic event drove the largest recorded mangrove mortality event (∼ 1000 km coastline, ∼ 7400 ha) attributed to natural causes (Duke et al, 2017;Harris et al, 2017;Sippo et al, 2018) and led to extensive changes in the coastal carbon cycle (Sippo et al, 2019(Sippo et al, , 2020 and coastal food webs (Harada et al, 2020). Two other large-scale mangrove dieback events occurred at the same time: one in Exmouth (Lovelock et al, 2017) and the other in Kakadu National Park, Australia (Asbridge et al, 2019).…”

Gamma‐irradiation of common biological samples for stable carbon and nitrogen isotope and elemental analyses

“…While we consider the γ-irradiation treatment as a potential reason for these differences, there may be additional secondary factors that can exacerbate these changes, e.g., sample preservationwhere irradiation results in more thorough sterilisation than air drying alone. 27 Bacterial decomposition of analytical samples can lead to stable isotope fractionation and isotopic enrichment in heavier isotopes in both marine producers 28 and decomposing animal tissues. 29 The mechanism underpinning such changes is worthy of further investigation, as it has been shown to affect both nitrogen and carbon to a similar degree (i.e., see changes between fresh and decomposed mangrove leaves 28 ).…”

“…27 Bacterial decomposition of analytical samples can lead to stable isotope fractionation and isotopic enrichment in heavier isotopes in both marine producers 28 and decomposing animal tissues. 29 The mechanism underpinning such changes is worthy of further investigation, as it has been shown to affect both nitrogen and carbon to a similar degree (i.e., see changes between fresh and decomposed mangrove leaves 28 ). Nevertheless, γ-irradiation sterilises samples and prevents bacterial decomposition, thereby enhancing preservation of the original stable isotope signature.…”