Rivers are important sites for carbon (C) transport and critical components of the global C cycle that is currently not well constrained. However, little is known about C species’ longitudinal and temporal changes in large tropical rivers. The Usumacinta River is Mexico’s main lotic system and the tenth largest in North America. Being a tropical river, it has a strong climatic seasonality. This study aims to evaluate how organic (DOC and POC) and inorganic (DIC and PIC) carbon change spatially and seasonally along the Usumacinta River (medium and lower basin) in rainy (RS-2017) and dry (DS-2018) seasons and to estimate C fluxes into the southern Gulf of Mexico. Concentrations of DOC, POC, DIC, and PIC ranged from 0.88 to 7.11 mg L⁻¹, 0.21 to 3.78 mg L⁻¹, 15.59 to 48.27 mg L⁻¹, and 0.05 to 1.51 mg L⁻¹, respectively. DOC was the dominant organic species (DOC/POC > 1). It was ~doubled in RS and showed a longitudinal increase, probably through exchange with wetlands and floodplains. Particulate carbon showed a positive relationship with the total suspended solids, suggesting that in RS, it derived from surface erosion and runoff in the watershed. DIC is reported for the first time as the highest concentration measured in tropical rivers in America. It was higher in the dry season without a longitudinal trend. The C mass inflow–outflow balance in the RS suggested net retention (DOC and POC sink) in floodplains. In contrast, in the DS, the balance suggested that floodplains supply (C source) autochthonous DOC and POC. The lower Usumacinta River basin is a sink for DIC in both seasons. Finally, the estimated annual C export for the Usumacinta-Grijalva River was 2.88 (2.65 to 3.14) Tg yr⁻¹, of which DIC was the largest transported fraction (85%), followed by DOC (10%), POC (4%), and PIC (<1%). This investigation is the first to present the C loads in a Mexican river.

The present long-term (1993–2013) study was aimed at identifying the effects of global change on the environmental characteristics and the plankton community of the tropical, high-altitude Lake Alchichica, Puebla, Mexico. We found no statistically significant increasing trend in air temperature, but an increase from 2002 to 2013, and interannual variability in the meteorological variables. Accordingly, the water temperature rose from 1993 to 2003 and remained similar until 2013. Nonetheless, on a longer-term (1966-2018), longer than the period considered in the present study, air and water temperatures showed an increasing trend in the range considered indicative of climate change. The lake displayed a characteristic warm monomictic thermal pattern but exhibited interannual variability. The planktonic community composition and structure remained similar. The small chlorophytes (e.g., Monoraphidium minutum) dominated the phytoplankton abundance, while the large diatom Cyclotella alchichicana dominated the biomass. The calanoid copepod Leptodiaptomus garciai dominated both the zooplankton abundance and biomass. However, as the temperature increased the large-size phytoplankton (i.e., C. alchichicana) abundance and biomass increased, but the small-size phytoplankton abundance and biomass, as well as the adult copepod abundance and biomass, decreased. The increase in temperature could have favoured the large-size over the small-size phytoplankton. In addition, the temperature increase could have negatively affected the calanoid copepods in two different ways—direct association with the negative effects of higher temperatures on the physiology of the copepods, or indirect association with the negative effects of higher temperatures on the small-size phytoplankton, which diminish the copepods’ food resource.

Continuous temperature monitoring for two adjacent tropical crater lakes in Mexico at 4200 m amsl shows that the lakes have rhythmic episodes of heating and cooling with a duration of ~ 30 days during the warmest months. The episodes were caused by rise and decline of solar irradiance reaching the lake surface. One lake, El Sol, showed over each heating and cooling episode a stable mixed layer (~ 20 days) and a deeper layer with a weak thermal gradient. Temperatures below the mixed layer warmed progressively by eddy diffusion after the mixed layer formed. Stratification was followed by full mixing of the water column. Within the same crater, an adjacent second lake, La Luna, showed the same cycles of heating and cooling; it stratified daily but not over multiple days. The difference between the lakes (discontinuous polymictic, continuous polymictic) is explained by the lower transparency of El Sol, which led to greater heat uptake near the surface than the more transparent La Luna. Lower transparency of El Sol was caused by modest anthropogenic effects on total suspended solids and nutrient loading, i.e., small deviations from the natural condition of El Sol caused it to differ qualitatively from La Luna. Events observed in these lakes would not have been evident from weekly temperature records.