Recurring bleaching events disrupt the spatial properties of coral reef benthic communities across scales

Marine heatwaves are causing recurring coral bleaching events on tropical
reefs that are driving ecosystem change. Yet little is known about how
bleaching and subsequent coral mortality impacts the spatial properties of
tropical seascapes, such as patterns of organism spatial clustering and
heterogeneity across scales. Changes in these spatial properties can offer
insight into ecosystem recovery potential following disturbance. Here we
repeatedly quantified coral reef benthic spatial properties around the
circumference of an uninhabited tropical island in the central Pacific
over a 9-year period that included a minor and severe marine heatwave.
Benthic communities showed increased biotic homogenisation following both
minor and mass bleaching, becoming more taxonomically similar with less
diverse intra-island community composition. Hard coral cover, which was
highly spatially clustered around the island prior to bleaching, became
less spatially clustered following minor bleaching and was indiscernible
from a random distribution across all scales (100–2000 m) following mass
bleaching. Interestingly, the reduced degree of hard coral cover spatial
clustering was already evident by the onset of mass bleaching and before
any dramatic wholesale loss in island-mean coral cover occurred.
Reductions in hard coral spatial clustering may therefore offer an early
indication of the ecosystem becoming degraded prior to mass coral
mortality. In contrast, the spatial clustering of competitive fleshy
macroalgae remained unchanged through both bleaching events, while
crustose coralline algae and fleshy turf algae became more spatially
clustered at larger scales (200–700 m) following mass bleaching. Overall,
benthic community spatial patterning became less predictable following
bleaching and was no longer reflective of gradients in long-term
environmental drivers that typically structure these remote reefs. Our
findings provide novel insights into how climate-driven marine heatwaves
can impact the spatial properties of coral reef communities over multiple
scales.