Organic carbon decomposition was measured in rhodolith beds using the Teabag Index (TBI; Keuskamp et al., (2013; https://doi.org/10.1016/j.ecolind.2022.109077). The data includes measurements relating to carbon decomposition including mass loss, organic carbon loss, decomposition rate (k) and stability factor (S). Oxygen (O2), dissolved inorganic carbon (DIC) and calcium carbonate (CaCO3) flux measurements were also obtained to measure sediment respiration and photosynthesis rates (using O2 and DIC fluxes) and calcification and dissolution rates (using CaCO3 fluxes). Measurements were obtained from in vitro incubations of rhodolith bed community samples (contained in Perspex cores). Samples were collected by hand using scuba from Loch Sween (Scotland; 56.031837, -5.601581; Water depth = 7m) and contained sediment (~8cm), macroalgae and calcifying fauna on the top of the sediment. Experiments were run in the summer and winter. Samples were collected on 2021/04/28 for the summer experiments and 2021/01/01 for the winter experiments. Separate mesocosms (referred to as cores) collected for each sampling campaign. There were 4 treatments for this experiment: T CO2: Ambient pCO2 (Bubbled gas composition = 400ppm) and temperature; T+ CO2: Ambient pCO2 (400ppm) and elevated temperature (3°C); T CO2: Elevated pCO2 (750ppm) and ambient temperature; T+ CO2+: Elevated pCO2 (750ppm) and elevated temperature (+3°C). For each core, light and dark incubations were run for 2 hours to calculate light and dark flux measurements. Net (daily) flux was calculated by multiplying light and dark incubations by the respective amount of hours spent in the light or dark. O2 fluxes were calculated by using optic spots, with O2 concentrations taken at the start and end of the incubation. DIC fluxes were determined by measuring the DIC concentration of water samples at the start and end of the incubation. DIC was determined using an Automated Infra-Red Inorganic Carbon Analyser (AIRICA). CaCO3 fluxes were calculated using the alkalinity anomaly technique with total alkalinity measured via titration. Information of flux calculations can be found in Martin et al. (2007). All flux measurements were corrected for considering seawater blanks from both light and dark cores. O2 fluxes were used to calculate organic carbon (OC) production, and CaCO3 fluxes were used to calculate inorganic carbon (IC) production. Carbon Dioxide (CO2) drawdown was calculated as OC - IC, assuming that 1 mol of IC produced = 1 mol of CO2.Flux measurements were taken from 2021/09/07 - 2021/09/21 in the summer, and 2022/01/20-2022/01/31 in the winter. As experiments were run in the lab, conditions were maintained at treatment levels. Following flux measurements, green and rooibos tea was buried 8cm into the sediment. For each treatment, 3 cores had green tea bags and 3 cores had rooibos tea bags buried within the sediment of the core. Two cores were left undisturbed, and 2 cores were disturbed in the same manner as adding the tea but with no tea bags inserted as a procedural control. Similarly to the teabags buried in situ, bags were buried at 8cm depth with dry mass loss, OC wt% loss, S and k calculated at the end of the 92 days. Green and Rooibos tea was buried at Loch Sween so that in vitro experiments could be compared with in situ measurements. The data was collected to determine if the decomposition of labile (green tea) and refractory (rooibos tea) carbon types varied with global warming and ocean acidification and if that would change rhodolith bed communities capacity to act as a source or a sink.