Temporal Dynamics of the Carbonate System in a Tropical Rhodolith Bed from a Protected Caribbean Bay

Natalia Rincón-Díaz^1*Carlos E. Gómez²Valentina Piñeros Pérez¹Félix Alvarado-Jiménez¹Samuel Nuñez¹Rocio Garcia Urueña¹

• ¹Universidad del Magdalena, Santa Marta, Colombia
• ²Universidad de los Andes, Bogotá D.C, Colombia

Coastal zones are key players in the global carbon cycle, yet the temporal dynamics of their carbonate system, particularly in tropical rhodolith habitats, remain understudied. Here, we investigated the carbonate chemistry variability of Gairaca Bay, a protected tropical bay within Tayrona National Natural Park, Colombian Caribbean. Sampling across three contrasting habitats, a rhodolith bed at depths 1, 7 and 15 m, the bay entrance (Outer bay - 10 m depth), and a shallow sandy-bottom area (Inner bay - 1 and 6 m depths), was conducted during 2023 and 2024. Temperature, salinity, and pHT were measured in situ; total alkalinity (TA) was determined via open-cell titration, and additional carbonate parameters (DIC, pCO₂, HCO₃⁻, CO₃²⁻, and Ωarag) were calculated. Statistical analysis (PERMANOVA) revealed significant seasonal differences in temperature (F = 248.42, p < 0.05), salinity (F = 49.02, p < 0.05), total alkalinity (TA) (F = 7.65, p < 0.001), and dissolved inorganic carbon (DIC) (F = 2.54, p < 0.001), with no significant variation across sites or depths. Upwelling periods were characterized by cooler, saltier waters, (25.9 ± 1.14 °C and 34.48 ± 0.46) elevated TA and DIC, and slightly lower pHT and Ωarag values, whereas non-upwelling periods exhibited warmer temperatures, reduced salinity, (30.0 ± 0.76 °C and 33.36 ± 0.28), and higher pHT and Ωarag. Seasonal delta analysis indicated that the most pronounced variations occurred during the non-upwelling season, amplifying freshwater discharge and intensifying inter-seasonal differences. Among sites, the outer bay exhibited the highest variability in pHT and Ωarag, while the inner bay was the most stable for TA and DIC. In contrast, rhodolith bed bottom showed high variability in TA but remained relatively stable for both pHT and Ωarag, particularly during the non-upwelling season. These patterns support the potential role of rhodolith beds as biogeochemical refugia in acidification-prone tropical environments.