Context-dependent coral physiological responses to heat stress across a natural turbidity gradient.

Corals in turbid environments may be more resilient to thermal stress than their clear-water counterparts, yet the physiological trade-offs affecting corals subject to multiple stressors are still poorly understood. During the summer of 2023, corals reefs in the Dampier Archipelago (Western Australia) experienced high temperatures and heat stress (>31.5 °C and 14.4 degree heating weeks), reduced pH (>0.1 unit decline), and a ∼30% increase in light attenuation, providing a natural laboratory to assess coral metabolic responses to multiple stressors. We determined photosynthesis, respiration, and calcification rates of a common reef-building coral (Acropora millepora) over two seasons (spring and summer), at four sites spanning a cross-shelf turbidity gradient. Despite higher turbidity and reduced light at the inshore sites, coral photosynthesis-to-respiration ratios (P:R) were comparable to those at the clearer offshore sites. P:R and calcification rates declined at all sites during summer heat stress, with the highest relative decline in calcification (88%) observed at the most inshore site. Contrasting respiration and calcification responses among turbid sites suggest that trade-offs between growth and survival functions under heat stress may depend on the balance between energetic inputs (influenced by light and nutrient availability) and other energetic costs (such as sediment rejection). Combined, these findings highlight the importance of local environmental conditions for modulating coral stress responses, and the context-dependent nature of thermal tolerance on turbid reefs.