Abstract
Structural complexity underpins the ecological functioning of coral reefs. However, rising ocean temperatures and associated coral bleaching threaten the structural integrity of these important ecosystems. Despite the increased frequency of coral bleaching events, few studies to date have examined changes in three-dimensional (3D) reef structural complexity following severe bleaching. The influence of local stressors on reef complexity also remains poorly understood. In the wake of the 2015-2016 El Niño-induced mass coral bleaching event, we quantified the effects of severe heat stress on 3D reef structural complexity across a gradient of local human disturbance. Using Structure-from-Motion photogrammetry we created 3D reconstructions of permanent reef plots and observed substantial declines in reef structural complexity, measured as surface rugosity and terrain ruggedness, and a detectable loss of habitat volume one year after the bleaching event. 3D reef complexity also declined with increasing levels of human disturbance, and with decreasing densities of branching and massive corals. These findings improve our understanding of the effects of local and global stressors on the structural foundation of coral reef ecosystems. In the face of accelerating climate change, mitigating local stressors may increase reef structural complexity, thereby heightening reef resilience to future bleaching events.
Highlights
Habitat complexity has long been known to play an important role in structuring natural communities[1]
Kiritimati presents the opportunity to examine the effects of local stressors on coral reef structural complexity: the atoll is characterized by a gradient of local human disturbance with the majority of its population concentrated on the northwest side of the island[39] (Fig. 1), resulting in a diverse spectrum of reef states ranging from highly-degraded sites near the villages to near-pristine ones on the eastern side of the island (Fig. 2)
All digital elevation model (DEM) were rendered with a cell size of 1.0 cm, which is within the range of the Ground sampling distance (GSD) computed for all models
Summary
Habitat complexity has long been known to play an important role in structuring natural communities[1]. Mass coral bleaching can severely reduce reef carbonate budgets[19], shifting reefs to a state of net erosion and limiting their ability to recover lost structure This loss is compounded by increases in the abundance of common bioeroding organisms[20] following mass bleaching, and the proliferation of dead coral substrate, which is more eroded[21]. The few studies that have used SfM to quantify changes in reef complexity following coral bleaching have employed these techniques within only one[31,32] to three[30] reef plots, limiting our ability to understand the effects of bleaching on reef structure at a wider scale. This study capitalized on a severe pulse heat stress event to examine the effects of bleaching-associated mass coral mortality on 3D reef structural complexity and the influence of underlying local human disturbance on these changes. We aimed to (1) quantify the change in 3D reef structural complexity on Kiritimati resulting from the 2015-2016 El Niño, (2) determine the effect of local anthropogenic stressors on levels of structural complexity and the degree of structural change, and (3) examine the relationship between shifts in benthic composition and changes in coral reef structural complexity
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