[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-P08] Coral archives at remote Tromelin reveal the interplay of upwelling and thermal stress in the southwestern Indian Ocean

*Saori Ito¹, Miriam Pfeiffer¹, Takaaki K. Watanabe¹, J. Henrich Bruggemann², Jens Zinke³, Nicolas Duprey⁴, Mireille M.M.M. Guillaume⁵, Lars Reuning¹, Dieter Garbe-Schönberg¹, Daniel A. Frick¹, Îles Éparses 2017-2020 Research Consortium (1.Christian-Albrechts-Universität zu Kiel, 2.University of La Réunion, 3.University of Leicester, 4.Max Planck Institute for Chemistry, 5.Muséum National d'Histoire Naturelle)

Keywords:thermal stress, coral, upwelling, Indian Ocean Dipole, El Niño-Southern Oscillation

In the western Indian Ocean (WIO), thermal anomalies have been frequently reported in the past decades, and they are attributed to El Niño-Southern Oscillation (ENSO) and/or the positive phase of the Indian Ocean Dipole (pIOD). Nevertheless, coral reefs may tolerate severe thermal stress due to an upwelling system in the Seychelles Chagos Thermocline Ridge (SCTR), where cold, nutrient-rich water is transported from the deep sea to the warm surface ocean in the summer months. Here, we investigate the response of corals to thermal stress in the WIO and possible mitigation by summer upwelling in the SCTR by analysing coral cores from Tromelin, a remote island in the WIO located at the edge of the SCTR upwelling area. It is an ideal setting for investigating the influence of sea surface temperature (SST) variability associated with thermal stress, the cooling effect of upwelling and their consequent impacts on coral growth. Terrestrial runoff, pollutions and anthropogenic disturbance should have negligible impact as the island is uninhabited, small and has a flat topography with very limited vegetation/freshwater and no coastal development. As a part of the consortium research “Îles Éparses 2017-2020,” two Porites coral colonies were drilled on opposite sides of the island in April 2019. We measured the coral skeletal strontium-calcium ratio (Sr/Ca) as a SST proxy, along with skeletal growth parameters and bioerosion observations. Both coral colonies experienced two ‘strong’ thermal events in 2010 and 2016, while ENSO and pIOD warmed SST in the WIO and suppressed summer upwelling. As a result, these climatic phenomena led to ocean heat accumulation and triggered the ‘strong’ thermal events in 2010 and 2016. The Tromelin corals recorded these thermal events in their skeleton with high-density stress bands, partial mortality, bioerosion, encrustation by crustose coralline algae, a temporary reduction in skeletal extension rates and a breakdown in seasonality in the skeletal Sr/Ca record. These stress signals were more pronounced during the 2016 event compared to the 2010 event, as the simultaneous occurrence of ENSO and pIOD strongly forced upwelling suppression and intensified the warming effect in 2016. We conclude that the cooler SST resulting from open ocean upwelling buffers against heat accumulation on corals but that interannual climate anomalies suppressing upwelling may locally lead to coral bleaching or mortality. However, both colonies recovered from bleaching in 2010 and 2016 and continued calcification and skeletal extension along the main growth axis, albeit with increased variability compared to before. We hypothesize that the regular summer upwelling may play an essential role in supporting coral resilience to thermal stress in this remote site where corals are thriving today.