Contrasting coral growth and heat stress responses across a natural turbidity gradient

Abstract Coral growth and reef accretion are increasingly impacted by ocean warming and declining water quality, yet the interactive effects of heat stress and turbidity on coral growth remain equivocal. Using close-range photogrammetry, we quantified annual linear extension rates of five common hard coral morphotypes during a moderate heat stress year (2021–2022; 7.3 degree heating weeks; DHW) and an extreme heat stress year (2022–2023; 11.9 DHW) in a nearshore reef system in Western Australia. Environmental gradients across this system were pronounced, with mean turbidity (KD490) more than doubling from offshore to inshore, and maximum water temperatures up to 1.3 °C higher at sites nearer the mainland. Under moderate stress conditions, corymbose Acropora and massive Porites growth increased with distance from the mainland, reflecting cross-shelf gradients in turbidity and water temperatures. Following extreme heat stress, growth declined across all coral morphotypes and a breakdown in cross-shelf growth patterns was observed. Growth declines were best explained by accumulated heat stress (mean and maximum DHW), though relative tolerance of inshore corals may indicate enhanced thermal resistance conferred by turbidity. Fast-growing corymbose and tabular/plating Acropora showed moderate declines in growth between years (31 and 38%, respectively), yet only the latter suffered high whole-colony mortality (70%). Slower-growing, stress-tolerant taxa (massive Porites and unifacial Turbinaria ) exhibited modest declines in growth (14 and 18%, respectively) and lower whole-colony mortality (< 5%). Bifacial Turbinaria displayed the largest decline in growth of all morphotypes (56%), possibly reflecting greater susceptibility to the additive stressors of temperature and turbidity. These findings highlight the complex interplay between turbidity and temperature in shaping coral growth and survival, revealing trade-offs between thermal tolerance and calcification that may influence reef-building capacity and resilience of coral reefs.