Impacts of water quality on Acropora coral settlement: The relative importance of substrate quality and light

Coral larval settlement patterns are influenced by a vast array of factors; however, the relative roles of individual factors are rarely tested in isolation, leading to confusion about which are most crucial for settlement. For exam-ple, direct effects of the light environment are often cited as a major factor influencing settlement patterns, yet this has not been demonstrated under environmentally realistic lighting regimes in the absence of confounding factors. Here we apply programmable multispectral lights to create realistic light spectra, while removing corre-lating (but not obvious) factors that are common in laboratory settlement experiments. Using two common spe-cies of Acropora - key framework builders of the Great Barrier Reef - we find little evidence that light intensity or changes in the spectral profile play a substantial role in larval settlement under most environmentally realistic settings but can under more extreme or artificial settings. We alternatively hypothesise and provide evidence that chronic light conditions and recent sediment exposures that impact benthic substrates (e.g., crustose coral-line algae) have a greater impact on settlement success. Under these conditions, there was a decrease of up to 74% settlement success. Management of water quality conditions that impact the quality of benthic-settlement sub-strates therefore should present a priority area of focus for improving coral recruitment. (c) 2021 Elsevier B.V. All rights reserved.