Ecosystem impacts due to thermocline depression by the 2019 extreme Indian Ocean Dipole event

Tropical atoll habitats are often key conservation targets due to being inhabited by several vulnerable species such as reef manta rays and tropical coral species. These atolls are subject to both basin scale forcing through the Indian Ocean Dipole (IOD), monsoonal variation, and local processes. The steep slopes surrounding these atolls support highly dynamic, energetic nearshore ecosystems which vary over sub-kilometre spatial scales that are poorly resolved in general circulation models. Improving our understanding of how physical oceanographic processes control these local ecosystems, through both in-situ observations, and fine scale models, is critical for enabling informed policy decisions and efficient use of conservation resources. Here we summarise the impact of the local fine scale processes, which are heavily modulated by the monsoon and Indian Ocean Dipole (IOD), on a tropical atoll ecosystem in the central Indian Ocean (IO).

The IOD is experiencing increasingly extreme fluctuations with direct impacts on the depth of the thermocline throughout the western IO. In our observations from 2019, the IOD deepened the thermocline to an unprecedented depth of 100 m, subjecting mesophotic corals to temperatures typical of surface waters and causing significant bleaching. High resolution numerical modelling shows that internal waves, rather than alleviating bleaching, further exacerbate the heating effects preferentially advecting high temperature surface water to increased depths. The wave influence is, however, highly localised, necessitating designated studies at individual sites to understand the spatial heterogeneity in internal wave impacts.

At smaller sub-atoll scales, the IOD also influences the feeding behaviour of reef manta rays, which are more frequently detected in the presence of tidally forced surface-to-bottom temperature gradients. The site of most manta ray detections in the study area is a lipped gully, situated at 60-70 m depth, and colloquially named 'Manta Alley'. During deeper thermoclines, the cooling events within Manta Alley, with which increased reef manta presence is associated, are precluded from occurring due to the deep thermocline, impacting feeding behaviour.

Our results highlight the inherent dynamical complexity in these environments, with the impacts of basin scale processes cascading down to local scales. Improving our understanding of how these dynamics cross-interact with each other, as well as the local ecosystem, enhances the value of biological observations, presenting the opportunity for better informed and more effective conservation strategy.