Internal Waves; Make and Break in Remote Tropical Atoll Ecosystems

Tropical atoll habitats are often key conservation targets due to being inhabited by several vulnerable species such as reef manta rays and tropical corals. 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 these nearshore dynamics control 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 two cases from a tropical Indian Ocean atoll where both manta rays and deep corals are impacted by local high frequency dynamics:

Firstly, whilst surface coral bleaching is potentially alleviated by internal wave-driven flushing with cold water, deeper corals are impacted by changes in thermocline depth. The Indian Ocean Dipole (IOD) is experiencing increasingly extreme fluctuations that directly impacts on the depth of the thermocline throughout the western IO, in turn modulating the depth range over which internal waves impact the benthic community. In our results 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 the internal waves, rather than alleviating bleaching, 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.

Secondly, at the same site reef manta rays (a highly mobile species) have demonstrated site fidelity at sub-atoll scales where the variation in physical environment is driven by tidally forced surface-to-bottom temperature gradients. Here we explore the dynamics responsible along a lipped gully, which is known for elevated occurrence of manta ray sightings, highlighting the inherent dynamical complexity in these environments and how an understanding of the physical processes enhances the value of biological observations.