The influence of hydrological seasonality and land use and cover on the relationship between dissolved organic carbon and carbon dioxide along a karst‐draining large tropical river

Abstract

The partial pressure of carbon dioxide (pCO 2 ) in rivers is influenced by carbonate weathering, soil CO 2 , photosynthesis, and the mineralization of dissolved organic carbon (DOC). In tropical regions, elevated temperatures intensify these processes. However, their seasonal dynamics remain poorly understood in large karst‐draining rivers. This study investigated the relationship between pCO 2 and DOC concentrations at both point and river reach scales in the Usumacinta River, the largest tropical watershed in North America. We assessed spatial and seasonal variations in DOC, pCO 2 , and CO 2 air–water flux (FCO 2 ) in relation to land use/land cover and fluvial physicochemical variables during the rainy and dry seasons. We also analyzed longitudinal changes in DOC and pCO 2 (ΔDOC and ΔpCO 2 ) along river reaches. Our results show that pCO 2 was consistently oversaturated, positively correlated with DOC and cropland extent, and negatively correlated with dissolved oxygen. During the rainy season, DOC and pCO 2 were positively related, as were ΔDOC and ΔpCO 2 , suggesting enhanced CO 2 production via heterotrophic metabolism fueled by terrestrial DOC inputs. In contrast, during the dry season, ΔDOC and ΔpCO 2 were negatively related, indicating DOC mineralization by river metabolism exceeds lateral DOC inputs. FCO 2 increased downstream during the rainy season, particularly in lowland reaches influenced by wetlands and agriculture. These findings highlight the role of aquatic metabolism and its interactions with hydrology and land use in regulating CO 2 dynamics in tropical karst rivers. Our study underscores the importance of integrating spatial and seasonal perspectives to better understand carbon cycling and greenhouse gas emissions in tropical fluvial systems.

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