Abstract
Crustose coralline algae (CCA) play a central role in coral reef framework consolidation and vertical accretion, yet their contribution to reef-scale calcification remains poorly quantified, particularly on reefs dominated by algal turfs. We examined how CCA calcification rates differed across ten reef sites with varying benthic composition around Lizard Island, Great Barrier Reef, Australia. Using tiles from calcification accretion units (CAUs), we quantified CCA carbonate production on light-exposed tile surfaces and related these rates to CCA cover and the Epilithic Algal Ratio (i.e. the spectrum of algal states from CCA to turf dominance), quantified on both tiles and the adjacent reef substrata. CCA calcification rates on tiles varied by up to 6.3-fold across sites, ranging from 2.1 to 13.2 g 100 cm −2 year −1 . Higher CCA cover and reduced turf dominance on the tile surface were strongly related to increased calcification rates. However, tile-based calcification rates were weakly related to CCA cover/turf dominance on adjacent reef substrata, likely because mounted experimental surfaces elevated above the substratum accumulate less sediment, altering CCA versus turf competition. Indeed, on natural reef substrata, reduced CCA dominance was strongly associated with increased turf length, consistent with sediment‑mediated competitive suppression of CCA by turfs. When tile‑based calcification rates were integrated with on-reef benthic cover, site‑level calcification estimates were substantially lower than site-level estimates derived from tiles alone. These findings highlight that experimental tiles may overestimate on-reef carbonate production and that shifts towards long sediment‑laden algal turfs may reduce exposed CCA cover, CCA-based calcification, and therefore reef accretion.