Alkalinity-enhanced artificial substrates modulate local pH and increase survivorship of early-stage coral recruits

Efforts to restore coral reefs using sexually derived coral recruits are often hindered by low survivorship and growth, hence scalable interventions to improve these parameters are urgently needed. We developed settlement substrates that modify the chemical and hydrodynamic environment to provide localized alkalinity enhancement (AE) within the laminar boundary layer. Cement tiles with four different chemistries and two different surface topographies were tested in a flume to quantify their ability to change local pH under reef-like conditions, and their resulting effect on larval settlement, survivorship, and growth of the endangered Caribbean coral, Orbicella faveolata. Chemistry had minimal effect on initial larval settlement, and textured tiles were preferred over smooth tiles. Substrates that increased pH in the local environment increased post-settlement survivorship, although they did not affect larval growth. Our results indicate that incorporating carbonate additives into cement used for artificial reef structures could effectively enhance the development of coral cover. Alkalinity enhancement through tile-based local pH modulation can improve early coral recruit survivorship, as shown in laboratory-fabricated artificial reef structures made by mixing sodium bicarbonate or sodium carbonate into cement