Comment on egusphere-2022-467

Coral reefs face increasing pressures in response to unprecedented rates of environmental change at present. The coral reef physical framework is formed through the production of calcium carbonate (CaCO3) and maintained by marine organisms, primarily hermatypic corals, crustose coralline algae (CCA), and other calcifying algae. The Kimberley bioregion, located in the northern part of Western Australia, has largely escaped land-based anthropogenic impacts and this study provides important metabolic data we are lacking on reef-building calcifying corals and macrophytes from an undisturbed set of marine habitats. Specimens of the dominant coral and algal taxa were collected from the reef platform of Browse Island located on the mid-shelf just inside the 200 m isobath off the Kimberley coast. During experimental light/dark incubations, all algae were net autotrophic producing 6–111 mmol O2 m−2 day−1. In contrast, most corals were net consumers of O2 with average net fluxes ranging from −42 to 47 mmol O2 m−2 day−1. The net change in pH was generally negative for corals and calcifying algae (−0.01 to −0.08 h−1). Corals, Halimeda (a calcifying green algae) and Galaxaura (a calcifying red algae) had positive calcification rates in light ranging from 4.2 to 18.4 g CaCO3 m−2 d−1. Resulting net calcification rates were all positive and ranged from 1.9 to 9.9 g CaCO3 m−2 d−1 and were strongly correlated to net O2 production. In intertidal habitats around Browse Island, estimated relative contributions of coral and Halimeda to the reef production of CaCO3 were similar at around 600–840 g m−2 year−1. The low reef platform had very low coral cover of < 3 % which made a smaller contribution to calcification of ~240 g CaCO3 m−2 year−1. Calcification on the subtidal reef slope was predominantly from corals, producing ~1540 g CaCO3 m−2 year−1, twice that of Halimeda. The relative contributions of the main reef builders, in these undisturbed areas, to net community metabolism and CaCO3 production and reef metabolism is important to understand exclusively climate-driven coral bleaching and mortality.