Significant carbonate production on a temperate reef system in southwestern Australia

Calcifying organisms support key geo-ecological functions in shallow tropical and temperate reefs worldwide, including creating habitat structure, producing sediments, and supporting reef accretion. These functions depend on the carbonate budget: the balance between calcium carbonate production and erosion. While carbonate budgets are well characterized in tropical coral reefs, the carbonate budgets of temperate rocky reefs, and their variability across spatiotemporal scales, remain much less well understood. Here, we quantify the carbonate budget of a seaweed-dominated rocky reef ecosystem within the world's largest cool-water carbonate depositional system. We first measured the seasonal and annual calcification rates of key calcifying groups (mobile invertebrates, corals, and coralline algae) across depths of 5, 10 and 18 m at two sites. Using a census-based approach, we then estimated how reef-scale gross carbonate production and erosion varied over depth. Crustose coralline algae exhibited calcification rates similar to those recorded in tropical studies, but their calcification was 2-7 times lower than that of corals. At the reef scale, gross carbonate production ranged from 9 to 2369 g CaCO₃ m^-2 yr^-1, and was dominated by crustose and articulated coralline algae at shallow depths (68-96 % of the total production, 5-10 m depth), with corals becoming the principal contributors (58-62 %) at greater depths. Mobile invertebrates (gastropods and sea urchins) were minor contributors to carbonate production overall (1-3 %). Sea urchins entirely drove bioerosion however, which was relatively low (8-114 g CaCO₃ m^-2 yr^-1) and increased with depth. Although the average net carbonate production in the studied temperate reefs (216-671 g CaCO₃ m^-2 yr^-1) is relatively low compared to that of healthy coral reefs in the region (1400-3880 g CaCO₃ m^-2 yr^-1), the vast expanses of seaweed-dominated rocky reefs in the study area and globally suggest that these ecosystems may play an underappreciated role in cool-water and global carbonate production. Indeed, preliminary estimates suggest that carbonate production across Australia's temperate Great Southern Reef could be comparable to tropical systems renowned for their high carbonate production such as the Great Barrier Reef.