Presentation Information
[AOS25-P07]The possible contribution of marine heatwaves to the declining healthy coral reef cover in Nusa Dua, Bali
*Muhammad Azzam Hanif Prasetyo1,2, Anindya Wirasatriya4, Diah Permata Wijayanti1, Dwi Haryanti1, Toshio Suga2,5, Ardiansyah Desmont Puryajati2, Sayaka Yasunaka3,5, Syifa Fauziah4, Rizki Taqwa Putranto4, Hanani Adiwira5, Maria Vanessa B. Rodriguez6, Parichat Wetchayont7, Muhammad Rais Abdillah8, R. Dwi Susanto9 (1.Department of Marine Sciences, Faculty of Fisheries and Marine Science, Diponegoro University, Semarang,Indonesia, 2.Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan, 3.Center of Atmospheric and Oceanic Studies, Department of Geophysics, Graduate School of Science,Tohoku University, Sendai, Japan, 4.Department of Oceanography, Faculty of Fisheries and Marine Science,Diponegoro University, Semarang, Indonesia, 5.Advanced Institute for Marine Ecosystem Change (WPI-AIMEC), Tohoku University, Sendai, Japan, 6.Marine Science Institute, University of the Philippines Diliman,Quezon, Philippines, 7.Disaster Management Program, Faculty of Science and Health Technology,Navamindradhiraj University, Bangkok, Thailand, 8.Atmospheric Science Research Group, Faculty of EarthScience and Technology, Institut Teknologi Bandung, Bandung, Indonesia, 9.Department of Atmospheric andOceanic Science, University of Maryland, College Park, MD, USA)
Keywords:
Marine heatwaves,Coral reef,Remote sensing,Sentinel-2,Nusa Dua
Historically, the region from Southern Java to Bali has experienced the most intense and long-duration marine heatwaves (MHW) within the Indonesian Seas. Nusa Dua, located in Southern Bali, is part of the Coral Triangle region, a region of diverse coral reef ecosystems that previously had a history of bleaching. The impact of MHW on short-term coral reef changes is well studied, but long-term analysis in the region is highly scarce, especially with limited observational data. This study aims to investigate MHW's contribution to the decrease in healthy coral cover by combining historical remote sensing methods with field observations.
MHW was defined as prolonged discrete anomalous warm water exceeding the 90th percentile of the climatology baseline. Detection was done using sea surface temperature (SST) data collected over 40 years from the Operational Sea Surface Temperature and Ice Analysis (OSTIA). Historical mapping of healthy coral reef coverages was done using Sentinel-2 imagery and classified using a random forest algorithm after a series of in-air and water corrections, achieving an overall accuracy of 76% and a Kappa coefficient of 0.52. Field observation was conducted for ground truth validation using the Line Intercept Transect (LIT) along with the retrieval of water quality parameters (pH, DO, TSS) to investigate potential local anthropogenic stressors.
The result indicates three intense and long-duration MHW were recorded from 2015 to 2022. MHW that occurred during the west monsoon period shows a significant decrease in healthy coral coverage, with mean absolute intensity exceeding the coral thermal threshold of 29°C. Linear regression between SST and coral coverage during the west monsoon period shows R2 of 0.516, significantly higher than the west monsoon in non-MHW periods (R2 = 0.006), thus showing a good relation between MHW and healthy coral coverage loss that occurred during this season. The most severe decrease happened in 2016, coinciding with strong El-Niño, resulting in a total loss of 39.7% from the initial coverage, with massive Porites sp. reported to be one of the most impacted.
In contrast, the east monsoon periods serve as the recovery period for the coral since the SST is lower (24–25°C). This cool SST is driven by easterly winds from the southern coast of Java, which generate offshore Ekman transport and induce upwelling that lifts a cold water mass from the deeper column to the surface, where the coral is located. MHW with low intensity may not disrupt the recovery process, but high intensity may, especially when the mean absolute intensity value exceeds the coral thermal threshold, as such in the case of 2016, where recovery was minimal (10.94%). This supports the finding that the impact of MHW on coral reefs should be measured by its absolute values rather than its intensity. Field observational data further support this conclusion.
MHW was defined as prolonged discrete anomalous warm water exceeding the 90th percentile of the climatology baseline. Detection was done using sea surface temperature (SST) data collected over 40 years from the Operational Sea Surface Temperature and Ice Analysis (OSTIA). Historical mapping of healthy coral reef coverages was done using Sentinel-2 imagery and classified using a random forest algorithm after a series of in-air and water corrections, achieving an overall accuracy of 76% and a Kappa coefficient of 0.52. Field observation was conducted for ground truth validation using the Line Intercept Transect (LIT) along with the retrieval of water quality parameters (pH, DO, TSS) to investigate potential local anthropogenic stressors.
The result indicates three intense and long-duration MHW were recorded from 2015 to 2022. MHW that occurred during the west monsoon period shows a significant decrease in healthy coral coverage, with mean absolute intensity exceeding the coral thermal threshold of 29°C. Linear regression between SST and coral coverage during the west monsoon period shows R2 of 0.516, significantly higher than the west monsoon in non-MHW periods (R2 = 0.006), thus showing a good relation between MHW and healthy coral coverage loss that occurred during this season. The most severe decrease happened in 2016, coinciding with strong El-Niño, resulting in a total loss of 39.7% from the initial coverage, with massive Porites sp. reported to be one of the most impacted.
In contrast, the east monsoon periods serve as the recovery period for the coral since the SST is lower (24–25°C). This cool SST is driven by easterly winds from the southern coast of Java, which generate offshore Ekman transport and induce upwelling that lifts a cold water mass from the deeper column to the surface, where the coral is located. MHW with low intensity may not disrupt the recovery process, but high intensity may, especially when the mean absolute intensity value exceeds the coral thermal threshold, as such in the case of 2016, where recovery was minimal (10.94%). This supports the finding that the impact of MHW on coral reefs should be measured by its absolute values rather than its intensity. Field observational data further support this conclusion.