Impact of marine heatwaves and cold spells on coral reef ecosystem in a tropical region: a case study of Lombok Waters, Indonesia

Coral reefs, vital marine ecosystems, are highly vulnerable to thermal stress-induced bleaching. Regional-scale bleaching events are often triggered by elevated sea surface temperature (SST) associated with global warming. While sustained temperature anomalies exceeding ± 1 °C above the long-term maximum monthly mean can induce bleaching, extreme short-term warming and cooling events, known as marine heatwaves (MHW) and marine cold spells (MCS), respectively, pose significant threats. These events, characterized by prolonged periods of anomalous temperature extremes, can have far-reaching ecological and economic consequences. This study investigated the spatial distribution of MHW and MCS in the Lombok Sea and their relationship with coral bleaching. Using daily SST data from 1998 to 2022, we identified MHW and MCS events and analyzed their frequency, intensity, and duration. Additionally, Landsat imagery (Landsat 5, 6, and 7) and field surveys conducted in 2016 were employed to assess changes in coral cover over time. Our results show that the effect of MCS and MHW cause coral bleaching, particularly in years with high frequency, intensity, and duration of these events. Extreme daily intensity (anomaly of SST > 1.5 °C) of MCS in 2000 and MHW in 1998, 2016, and 2022 led to a significant decrease in coral reef cover of approximately 30%. Furthermore, we observed that large-scale climate patterns such as the El Niño-Southern Oscillation (ENSO) can exacerbate MHW (in 1998 and 2016) conditions and contribute to coral decline. These results underscore the urgent need for effective coral reef management strategies to mitigate climate change and ocean warming impacts.