Diverging carbonate budgets following tropicalisation of temperate reefs

Climate-driven shifts in habitat-forming species abundance and distributions are transforming how ecosystems produce and cycle carbon, nutrients, and energy. Temperate kelp forests are increasingly being replaced by tropical or warm-affinity habitat-forming species like turf seaweeds and corals, i.e., “tropicalised”. These changes can have cascading effects on associated species such as calcifying organisms, which contribute to sediment generation and carbon cycling via the production of calcium carbonate. This study quantified carbonate production and erosion across a temperate kelp forest and three tropicalised reef states following the 2011 marine heat wave: kelp-turf mix, warmer-affinity seaweeds, and coral-turf dominance. Results show gross carbonate production was highest in coral-turf reefs (1.85 ± 0.65 kg m⁻² yr⁻¹), lowest in warmer-affinity seaweed reefs (0.04 ± 0.02 kg m⁻² yr⁻¹), and intermediate in temperate kelp forests (0.60 ± 0.19 kg m⁻² yr⁻¹). These differences were linked to the abundance of corals in the coral-turf state and scarce calcifying algae in the warmer-affinity seaweed state. Bioerosion played a moderate role in the overall budget, but the dominant bioeroders differed between habitats: urchins in temperate reefs contributed 80% less than parrotfishes in tropicalised reefs. Overall, tropicalisation can either increase or decrease carbonate availability, depending on the dominant habitat-formers. These shifts may significantly impact inorganic carbon cycling and the structural and functional integrity of coastal reef ecosystems.