Constanza Ricaurte-Villota ^1* Magnolia Murcia-Riaño ¹ Jose Martin Hernandez-Ayon ²

• ¹ José Benito Vives of Andréis Marine and Coastal Research Institute, Santa Marta, Colombia
• ² Institute of Oceanological Investigations, Autonomous University of Baja California, Ensenada, Mexico

We investigated the spatiotemporal variation in the partial pressure of CO₂ (pCO₂), pH, total alkalinity (TA), and dissolved inorganic carbon (DIC) and their main drivers from 2017 to 2022 in Gayraca Bay and Chengue Bay in the northeastern Colombian Caribbean coast. Forcing agents included river runoff, coastal upwelling, and the El Niño-Southern Oscillation (ENSO). The carbonate system variables exhibited significant seasonal variability modulated by annual changes in coastal upwelling, rainfall, and river runoff. Low (high) pH, pCO2, and calcium carbonate saturation state (Ω) values were observed with high (low) DIC and TAin the dry and wet seasons, respectively. During El Niño, negative coastal upwelling anomalies prevailed over the entire year, resulting in negative pCO₂ anomalies and positive Ω, DIC, and TA anomalies. During La Niña, positive rainfall anomalies prevailed, although these alternated with periods of positive upwelling anomalies. Under this condition, during periods of higher rainfall, negative pCO₂, DIC, and TA anomalies and positive Ω anomalies were observed, whereas during periods of higher upwelling, positive pCO₂, DIC, and TA anomalies and negative Ω anomalies were observed. At the beginning of 2022, undersaturated levels of Ωcalc and Ωarag (<1) were observed, which may have hindered coral calcification. This may pose a risk to corals under future climate change scenarios. A Taylor series decomposition analysis showed that TA and DIC are important and consistent drivers of changes in pCO2, pH, and Ωarag from marine to estuarine conditions due to seasonal and interannual changes in rainfall and river runoff, with interannual variability possibly influenced by ENSO events. The observed trends in pH and pCO₂ were primarily driven by a decrease in DIC and TA, attributed to an increased input of river runoff, despite rising atmospheric CO₂ levels. This behavior stands in contrast to the typical ocean acidification trends observed in other regions. This study provides a novel 6-year time series of the carbonate system for the Colombian Caribbean and serves as a baseline to evaluate the effects of global warming in the region.