Abstract
Particulate organic carbon (POC) derived from inland water plays an important role in the global carbon (C) cycle; however, the POC dynamic in tropical rivers is poorly known. We assessed the POC concentration, flux, and sources in the Usumacinta, the largest tropical river in North America, to determine the controls on POC export to the Gulf of Mexico. We examined the Mexican middle and lower Usumacinta Basin during the 2017 dry (DS) and rainy (RS) seasons. The POC concentration ranged from 0.48 to 4.7 mg L−1 and was higher in the RS, though only in the middle basin, while remaining similar in both seasons in the lower basin. The POC was predominantly allochthonous (54.7 to 99.6%). However, autochthonous POC (phytoplankton) increased in the DS (from 5.1 to 17.7%) in both basins. The POC mass inflow–outflow balance suggested that floodplains supply (C source) autochthonous POC during the DS while retaining (C sink) allochthonous POC in the RS. Ranging between 109.1 (DS) and 926.1 t POC d−1 (RS), the Usumacinta River POC export to the Gulf of Mexico was similar to that of other tropical rivers with a comparable water discharge. The extensive floodplains and the “Pantanos de Centla” wetlands in the lowlands largely influenced the POC dynamics and export to the southern Gulf of Mexico.
Highlights
During the last two decades, several studies mentioned that inland aquatic ecosystems are important agents in the coupling of biogeochemical cycles between continents, the atmosphere, and oceans (e.g., [1,2])
The analysis of the seasonal dynamics of fluvial particulate organic carbon (POC) sources, concentration, and flux in the Usumacinta River basin relies on spatial differences and mechanisms that were determined using hydrogeomorphic features at the basin scale
The middle basin displayed a higher seasonal effect on POC concentration and source due to the potential erosion of terrestrial inputs compared to the lower basin
Summary
During the last two decades, several studies mentioned that inland aquatic ecosystems are important agents in the coupling of biogeochemical cycles between continents, the atmosphere, and oceans (e.g., [1,2]). Rivers have effects on the regional C cycle since they couple landscapes through the lateral export from terrestrial ecosystems [3]. Rivers annually deliver approximately 0.5 Pg of organic carbon (OC) to the world’s oceans, with nearly half arriving in particulate form [4,5,6]. The difference between rockderived and biosphere-derived particulate organic carbon (POC) loads arriving into oceans from terrestrial systems through rivers contributes to the regulation of atmospheric and terrestrial C reservoirs over a geological timescale by sequestration in marine sediments [5,7,8,9]. The fluvial C inventory may be affected by algal in situ production, increasing the POC (autochthonous) and reducing the CO2 concentrations [10]
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