Seawater-conditioned cement as a viable substrate for coral larval settlement

Cement-based materials are commonly used in artificial reefs, but this application encounters a challenge due to their elevated surface pH, which is not ideal for larval recruitment. To address this concern, the present study employed a simple seawater pre-conditioning process to lower the cement surface pH and examined the time required to reach the normal seawater pH range. Four groups of experimental tiles were prepared by mixing ordinary Portland cement with oyster shell powder at mass ratios of 1:0, 1:10, 1:20 and 1:30. Seawater in immediate contact with these tiles displayed initial pH levels of 10.6, 10.0, 9.6 and 9.0, respectively. Within a week of seawater conditioning in darkness, all tested ratios, including pure cement (1:0), displayed a decrease in their impact on seawater pH, which returned to the natural level of 8.1. The conditioned tiles were subsequentially used to induce settlement of coral larvae of Acropora tumida over a three-day period under a day-night cycle. Across all tested ratios, the tiles exhibited statistically similar rates of coral settlement, with an average of 0.53–0.86 larvae observed on each tile (15 mm × 15 mm). These findings suggest that when given sufficient time of pre-conditioning to adjust the surface pH, cement can be a suitable substrate for coral larval recruitment and for implementation in artificial reefs.