Depth variation in benthic community response to repeated marine heatwaves on remote Central Indian Ocean reefs

Coral reefs are increasingly impacted by climate-induced warming events.
However, there is limited empirical evidence on the variation in the
response of shallow coral reef communities to thermal stress across depth.
Here we assess depth-dependent changes in coral reef benthic communities
following successive marine heatwaves from 2015 to 2017 across a 5–25 m
depth gradient in the remote Chagos Archipelago, Central Indian Ocean. Our
analyses show an overall decline in hard and soft coral cover and an
increase in crustose coralline algae, sponge, and reef pavement following
successive marine heatwaves on the remote reef system. Our findings
indicate that the changes in benthic communities in response to elevated
seawater temperatures varied across depth. We found greater changes in
benthic group cover at shallow depths (5–15 m) compared to deeper zones
(15–25 m). The loss of hard coral cover was better predicted by initial
thermal stress, whilst the loss of soft coral was associated with repeated
thermal stress following successive warming events. Our study shows that
benthic communities extending to 25 m were impacted by successive marine
heatwaves, supporting concerns about the resilience of shallow coral reef
communities to increasingly severe climate-driven warming events.