Effects of climate change and light limitation on coral recruits

Climate change impacts and light attenuation from suspended sediments due to runoff, natural resuspension or dredging, can both impede the replenishment of coral populations. Here we tested the independent and combined impacts of climate change (current temperature and dissolved CO2, and two future climate scenarios) and a one-month-long light attenuation period at 5 different light levels (0.1 to 4 mol photons m−2 d−1) on early Acropora millepora recruits. Additionally, we evaluated whether the effects were age dependent by comparing responses of recruits that were one-month-old (‘early attenuation’) vs two months old (‘late attenuation’). Recruit survival, size and Symbiodiniaceae densities increased slightly under moderate future climate conditions (current temperature +0.44°C, 692 ppm pCO2), but decreased under a more severe climate scenario (+0.94°C, 985 ppm pCO2). Light attenuation significantly decreased recruit survival, size and Symbiodiniaceae densities only for recruits exposed to the late attenuation, suggesting an increasing reliance on photosynthesis as recruits age. Under the more severe climate scenario tested, recruit survival was diminished by both climate change (≤ 18 ± 4 [SE]% in the early attenuation) and light limitation (≤ 32 ± 6% in the late attenuation) compared with controls. However, there was no interaction between future climate scenarios and light attenuation indicating these effects were additive. This study demonstrates the potential effects of light limitation and future climate conditions on coral recruitment success and highlights the need to manage the timing of sediment-generating activities near reefs to optimise light availability for several months post-settlement