Satellite monitoring of coral reef responses to the 2022 volcanic ash deposition of the Hunga Tonga–Hunga Ha‘apai

Abstract Volcanic ash deposition can influence coral reef ecosystems, yet its impacts remain poorly quantified due to the rarity and limited field observations. Here, we combined Sentinel-2 imagery with a shallow-water radiative transfer model to retrieve bottom reflectance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mtext>b</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> from 2019 to 2025 across five islands arranged along a distance gradient from the 2022 Tonga eruption. This physics-based method enables consistent retrievals without reliance on in situ calibration data, which is particularly valuable for data-sparse and remote reef systems. Coral pixels were extracted and classified as sensitive or stable based on their interannual <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mtext>b</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> ranges, and for each class the seasonal harmonic curve was fitted using the median <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mtext>b</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> at each imaging time. Across the near-field islands, sensitive pixels were mainly concentrated in shallow habitats (0–10 m), whereas in far-field regions they shifted toward deeper waters (10–20 m). In shallow reefs, coral responses were controlled by the interaction between volcanic ash fertilization and ongoing climate stress, with near-field reefs showing post-eruption recovery and far-field reefs experiencing only weak and short-lived improvement. In deeper waters, coral responses were dominated by light limitation and background climate forcing, resulting in persistent decline in far-field reefs and only transient post-eruption perturbations in near-field reefs. These findings provide multi-year, spatially coherent evidence that volcanic disturbance interacts with ongoing climate stress in depth- and distance-dependent patterns, highlighting the value of habitat-scale satellite diagnostics for improved monitoring and management of coral reef ecosystems.