Diverging carbonate budgets following tropicalisation of temperate reefsBiogeochemical Cycling

Océane Attlan¹, Thomas Wernberg^{1, 2}, Georgina Wood^{1, 3}, Albert Pessarrodona¹

¹The University of Western Australia, Australia - ²Institute of Marine Research, Norway - ³Flinders University, Australia

The redistribution of species globally has emerged as one of the most significant biological responses to anthropogenic climate change. Temperate ecosystems are facing rapid shifts in their community structure and functions, with habitat-forming corals, turfs or warm-affinity seaweeds increasingly dominating areas previously occupied by kelp forests. This transition has a notable impact on the calcifying community, yet there remains a significant gap in understanding the implications for ecosystem function, particularly the production and the bioerosion of calcium carbonate, i.e., carbonate budget, a cross various alternative states. Here, we aim to quantify how shifts in the dominant habitat-forming species alter the net carbonate budget of temperate reefs under various scenarios of tropicalisation. To assess the producers (corals and calcifying algae) and bioeroders (urchins and parrotfishes) of calcium carbonate, we conducted surveys across a tropical to temperate transition zone on the west coast of Australia. We calculated carbonate production at the site level following the Indo-Pacific ReefBudget method, which revealed that carbonate production in turf and coral dominated reefs is driven by plating Acropora spp and exceeded that of temperate kelp forests. In contrast, other alternative states exhibited lower carbonate budgets than kelp forests, due to sparse calcifiers and the presence of parrotfishes. Bioerosion rates were low but shifted from being driven by urchins in temperate kelp forests to parrotfishes in tropicalised reefs. Our results show that, following tropicalisation, the carbonate budgets of temperate reefs are profoundly affected by the habitat-forming species that end up dominating the seafloor, leading to either a loss or a gain in carbonate material. Such changes likely have significant implications for the inorganic carbon cycle and the geo-ecology functions of temperate reefs.