Local alkalinity enhancement using artificial substrates increases survivorship of early-stage coral recruits

Efforts to restore coral reefs using sexually derived coral recruits are often hindered by their low survivorship and growth, hence scalable interventions to improve these parameters are urgently needed. Here, we developed novel settlement substrates that modify the local chemical and hydrodynamic environments to provide local alkalinity enhancement (AE) within the laminar boundary layer to aid in coral restoration. Cement tiles with four different chemistries and two different surface topographies were tested in a novel flume system to quantify their ability to change local pH under reef-like conditions and their effect on larval settlement, survivorship, and growth of the endangered Caribbean coral, Orbicella faveolata. Chemistry had a minimal effect on the initial settlement of coral larvae, and textured tiles were preferred over smooth tiles. However, substrates that created a more alkaline local environment increased post-settlement survivorship. The increased survivorship of O. faveolata recruits on AE tiles was not due to increased growth on AE tiles, although trends in growth were dependent on chemistry and topography of tiles. Our results indicate that mixing sodium bicarbonate or sodium carbonate into cement used to fabricate artificial reef structures could be an effective means to enhance the development of coral cover. Significance StatementReef-building coral populations in South Florida and the Caribbean are nearing crisis, with low survivorship rates of new recruits being identified as a critical bottleneck limiting their recovery. Since ocean acidification is known to stress corals and hinder their growth, we hypothesized that enhancing substrate alkalinity could help mitigate ocean acidification effects by boosting coral growth and increasing survivorship during their critical early stages. This hypothesis was experimentally tested by adding sodium bicarbonate or sodium carbonate to settlement substrates and studying coral settlement preference, and coral spat growth and survival. Our results showed significantly higher survivorship on alkalinity enhanced substrates, indicating that this could be a promising restoration intervention.