Climate-driven mangrove dieback in the Gulf of Carpentaria, Australia: using stable isotopes as a tool to assess and monitor ecosystem changes

Abstract

Extreme climatic events can trigger sudden but often long-lasting and irreversible changes in ecosystems by causing mortality of foundation (habitat-forming) species. The magnitude and frequency of such events are likely to increase due to human-induced climate change, but the dynamics of such extreme biological events remain poorly understood, with only a limited number of case studies reported in the past. In many cases, assessing the impact of rare, extreme biological events can be challenging because these events can be unexpected and sudden, often making pre-event sampling not achievable. In late 2015 to early 2016, an extensive area of mangrove forest along ~ 1,000 km of coastline in the Gulf of Carpentaria, Australia, experienced severe dieback as a result of a climatic extreme event that included extreme temperatures, drought conditions and lower than average sea levels. My research aimed to address a knowledge gap in the effects of extreme climatic events on intertidal coastal ecosystems by assessing the ecological impacts of mass mortality of mangrove trees on the intertidal ecosystem. This aim was achieved through the use of a comparative experiment of an impacted forest and an adjacent unimpacted forest using traditional ecological survey techniques combined with conventional bulk stable isotope analyses and a more novel compound-specific isotope analysis of amino acids. My research also offers significant insights into the use of the more novel compound-specific isotope analysis of amino acids to complement the conventional bulk stable isotope analysis in mangrove ecosystem trophic analyses. Firstly, I used a combination of traditional ecological survey techniques and bulk stable isotope analysis of carbon (C) and nitrogen (N) to measure the effects of mangrove forest mortality on benthic faunal communities, with a focus on functional aspects of food web dynamics. I tested if changes in benthic faunal assemblages would be evident due to mangrove mortality, and if food web structure was impacted by the mangrove mortality and alternations in available food resources. I found that in the forest that experienced tree mortality, there were fewer crabs that relied on mangrove litter as a food source but more crabs that fed on the microphytobenthos. As the microphytobenthos was largely unaffected by the die-back event, they provided a buffer to the food-web responses. The infauna, e.g. burrowing crabs, was also largely unaffected by the mortality effect. However, overall, the habitat value for mangrove ecosystem services could be decreased due to lower physical habitat complexity following tree losses. Secondly, the initial dieback and recovery of the impacted mangrove ecosystem were evaluated using a combination of bulk stable C, N and S isotopes and a more novel amino acid compound-specific isotope method. I tested if tree mortality changed the overall circulation of C, N and S elements, and if this change would be reflected in δ13C, δ15N and δ34S values of mangrove ecosystem components such as mangrove plants, soil and associated animals. I also tested if/how these isotopic compositions change over time with the recovery of mangrove vegetation. Stable isotope analyses confirmed significant changes to the circulation of C, N and S elements following tree mortality. Recovery of the mangrove vegetation was evident from increased numbers of mangrove seedlings and saplings in the impacted forest over the two-year survey, but recovery of CNS cycling was not evident even after 30 months, suggesting a long-lasting effect of the mortality event. Finally, the use of bulk stable CNS isotopes and more novel compound-specific stable C and N isotopes of amino acids were compared to evaluate which isotopic compositions are more conservative tracers of mangrove organic matter and suited for analyses of mangrove food webs. I tested if stable isotopic compositions in essential amino acids that cannot be synthesised by animals would be more conservative in food web links between consumers and mangrove organic matter. Isotopic compositions in essential amino acids effectively separated mangrove organic matter from the microphytobenthos and helped to trace mangrove organic matter in a mangrove food web. These more sophisticated tracing techniques complemented traditional bulk stable isotope analyses by providing improved resolving power in mangrove trophic analyses. The outcome of this research will be an important contribution to the emerging global body of case studies that show significant ecological impacts driven by extreme climatic events, and how changes in habitat forming species result in significant impacts on ecosystem community dynamics as well as biogeochemical processes including C, N and S cycling. My research also provides a framework for combining the use of conventional and novel stable isotope measurements with traditional ecological survey techniques in reporting difficult to measure impacts of extreme biological events. The results of this research may also be used for planning and future-proofing coastal wetlands from future impacts of climatic extreme events, and support wetland conservation and restoration efforts.