Did nearby mangroves protect corals during heat stress and flood-plume disturbance?

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Coral reefs are increasingly exposed to multiple climate-driven disturbances. Concurrent thermal stress and flood-plume exposure may become more common, particularly in nearshore mangrove-lined habitats where terrestrial runoff and marine stressors can coincide. The ecological consequences of these combined stressors across mangrove-coral reef ecotones remain poorly understood.

Researchers examined coral community changes at Pioneer Bay, Orpheus Island, Great Barrier Reef, over three years, from 2023 to 2025. The period included a thermal stress event in 2024, followed by concurrent thermal stress and flood-plume disturbance in 2025. They surveyed six reef-crest sites annually, spanning an intertidal mangrove-coral gradient and adjacent reef habitats beyond the ecotone. They assessed coral colony density, genus richness, bleaching prevalence, benthic and coral genus composition alongside thermal and rainfall data.

There was no evidence of coral community change between July 2023 and July 2024, but rapid, bay-wide reorganization had occurred by April 2025. Across gradient sites, mean colony density fell from 16.78 to 2.97 colonies per m² and mean genus richness fell from 16.58 to 5.17 between 2024 and 2025. Reef sites beyond the gradient declined similarly, but retained comparatively higher live hard coral cover than gradient sites, indicating that mangrove proximity did not buffer coral declines. Across all sites, benthic composition shifted from coral-dominated to turf-algae-dominated substrate.

Colony density declined most strongly in the sensitive genera Acropora, Pocillopora and Montipora, but also declined in normally more stress-resistant taxa Favites, Lobophyllia and Galaxea. Porites remained the most abundant genus in 2025, although its colony density fell by approximately 25% from the previous year. The results indicate that concurrent thermal and flood-plume disturbance can rapidly restructure inshore coral communities, including normally stress-tolerant taxa.