Inhibition of coral settlement at multiple spatial scales by a pervasive algal competitor

Larval settlement to the benthos can be influenced by physical and chemical cues. On coral reefs, the macroalga Lobophora is known to negatively impact coral recruitment, though the scales at which it affects coral larvae is unclear. We used aquarium experiments to mechanistically assess the response of larvae from 3 Acropora species to Lobophora at multiple spatial scales, and complemented these experiments with an analysis of the effects of Lobophora on Acropora spp. field recruitment patterns. The smallest scale (0−10 cm) focused on the effects of the distribution of Lobophora across an experimental tile, with settlement declining 60% for 2 of the species when a 15 cm piece of Lobophora was distributed throughout a 100 cm tile, compared to the control. The intermediate scale (5−15 cm) focused on the effects of increasing algal biomass on settlement, with settlement for all species negatively associated with algal biomass. Settlement decreased almost 50% in the highest treatment (6.2 g of Lobophora in the tanks), compared to the control. The mechanism of settlement inhibition was also tested at this scale, with waterborne compounds highlighted as a key settlement inhibitor. Lobophora also impaired overall settlement at the largest scale (0−100 cm), decreasing settlement by 40−50%, regardless of its location relative to the settlement substrate. Finally, Acropora field recruitment patterns also demonstrated a negative effect of Lobophora on coral recruitment in situ. Our results reveal the ability of Lobophora to inhibit coral settlement at multiple spatial scales, which may contribute to large-scale recruitment failure on coral reefs following disturbances.