Rapid loss, and slow recovery, of coral reef structural complexity resulting from a prolonged marine heatwave

Globally, climate change-driven marine heatwaves are causing increased coral bleaching and mortality events with cascading effects, including reef habitat complexity. Given that habitat complexity positively influences biodiversity and organismal abundance, such losses have negative ramifications for these ecosystems. Past studies have documented habitat complexity changes following heatwaves, yet the timing and persistence of habitat structural complexity losses arising from these events, and subsequent recovery rates, remain largely unknown. Here, we used Structure-from-Motion (SfM) photogrammetry to survey permanent shallow forereef quadrats (4 m × 4 m; n = 30) in the central equatorial Pacific Ocean at the epicentre of the third global coral bleaching event, the 2015-2016 El Niño, prior to, during and after the event. Reef three-dimensional habitat complexity metrics calculated from the SfM images were then used to examine the immediate and long-term impacts of the mass coral mortality. Coral reef structural habitat complexity, quantified as surface complexity (1.18-fold decline), vector ruggedness measure (VRM; 1.52-fold decline) and fractal dimension (1.03-fold decline), declined significantly during the heatwave. However, contrary to expectation, no subsequent loss (i.e. due to erosion of dead coral colonies) was detected. We found that exposure to underlying local anthropogenic disturbance had already reduced reef complexity before the heatwave, such that the least disturbed sites showed the largest habitat complexity losses from it. All three complexity metrics increased significantly between 2017 and 2019, but not to pre-heatwave levels. Furthermore, through fine-scale morphological classification of the substrate, we show that all coral structure (regardless of whether dead or alive) contributed to habitat complexity, and that live submassive coral had the strongest correlation to the complexity metrics despite it seldom being mentioned in the literature as a morphology providing structural complexity. Morphologies contributing to habitat complexity also shifted over time, as more thermally sensitive corals (e.g. branching, foliose and tabulate) experienced much greater losses than thermally tolerant ones (e.g. submassive and massive). Our results illustrate how rapidly habitat complexity can be lost during climate change induced mass mortality events and imply complexity will be comprised of distinct coral morphologies on future coral reefs and that recovery will take many years.