The summer carbon budget of fucoid forests and encrusting coralline barrens on a NW Mediterranean rocky reef

BACKGROUND AND AIMS: Shifts from macroalgal forests to encrusting coralline barrens are widespread on temperate rocky reefs, yet differences in their trophic state (i.e., autotrophic versus heterotrophic) remain unclear. We conducted a field experiment to compare variations in gross community productivity (GCP), community respiration (CR), net community productivity (NCP) and calcification rates between forests formed by Ericaria brachycarpa and encrusting coralline barrens, and assessed how these processes regulate total dissolved inorganic carbon (DIC) fluxes. METHODS: In summer 2022, we conducted light and dark incubations on a shallow rocky reef at Giannutri Island (NW Mediterranean) to measure DIC and total alkalinity changes. We performed incubations in E. brachycarpa forests, encrusting coralline barrens, and manipulated forest plots where canopies were removed to specifically assess the contribution of understorey assemblages to DIC fluxes. We calculated hourly GCP, CR, NCP and calcification rates, as well as diel DIC fluxes due to photosynthesis/respiration and calcification/dissolution processes. To enhance the interpretation of carbon fluxes, we quantified the biomass of major producer and consumer groups in each plot. KEY RESULTS: Hourly GCP rates were higher in the forest than in the barren, indicating a greater amount of CO2 fixed through photosynthesis. There was also a trend for forests to host a larger biomass of consumers that drove CR rates, as shown by the canopy removal treatment. NCP was greater in forests than barrens. Over a daily cycle, total DIC fluxes, mainly driven by photosynthesis and respiration, were positive and did not differ among habitats. CONCLUSIONS: Our results indicate that, in summer, both macroalgal forests and encrusting coralline barrens are heterotrophic.