Export of particulate organic carbon by the mountainous tropical rivers of Western Ghats, India: Variations and controls

Abstract. The Oubangui is a major tributary of the Congo River, draining an area of ~500 000 km2 mainly consisting of wooded savannahs. Here, we report results of a one year long, 2-weekly sampling campaign in Bangui (Central African Republic) since March 2010 for a suite of physico-chemical and biogeochemical characteristics, including total suspended matter (TSM), bulk concentration and stable isotope composition of particulate organic carbon (POC and δ13CPOC), particulate nitrogen (PN and δ15NPN), dissolved organic carbon (DOC and δ13CDOC), dissolved inorganic carbon (DIC and δ13CDIC), dissolved greenhouse gases (CO2, CH4 and N2O), and dissolved lignin composition. δ13C signatures of both POC and DOC showed strong seasonal variations (−30.6 to −25.8‰, and −31.8 to −27.1‰, respectively), but their different timing indicates that the origins of POC and DOC may vary strongly over the hydrograph and are largely uncoupled, differing up to 6‰ in δ13C signatures. Dissolved lignin characteristics (carbon-normalised yields, cinnamyl:vanillyl phenol ratios, and vanillic acid to vanillin ratios) showed marked differences between high and low discharge conditions, consistent with major seasonal variations in the sources of dissolved organic matter. We observed a strong seasonality in pCO2, ranging between 470 ± 203 ppm for Q < 1000 m3 s−1 (n=10) to a maximum of 3750 ppm during the first stage of the rising discharge. The low POC/PN ratios, high %POC and low and variable δ13CPOC signatures during low flow conditions suggest that the majority of the POC pool during this period consists of in situ produced phytoplankton, consistent with concurrent pCO2 (partial pressure of CO2) values only slightly above and, occasionally, below atmospheric equilibrium. Water-atmosphere CO2 fluxes estimated using two independent approaches averaged 105 and 204 g C m−2 yr−1, i.e. more than an order of magnitude lower than current estimates for large tropical rivers globally. Although tropical rivers are often assumed to show much higher CO2 effluxes compared to temperate systems, we show that in situ production may be high enough to dominate the particulate organic carbon pool, and lower pCO2 values to near equilibrium values during low discharge conditions. The total annual flux of TSM, POC, PN, DOC and DIC are 2.33 Tg yr−1, 0.14 Tg C yr−1, 0.014 Tg N yr−1, 0.70 Tg C yr−1, and 0.49 Tg C yr−1, respectively. While our TSM and POC fluxes are similar to previous estimates for the Oubangui, DOC fluxes were ~30% higher and bicarbonate fluxes were ~35% lower than previous reports. DIC represented 58% of the total annual C flux, and under the assumptions that carbonate weathering represents 25% of the DIC flux and that CO2 from respiration drives chemical weathering, this flux is equivalent to ~50% of terrestrial-derived riverine C transport.

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Particulate Organic Carbon in the Tropical Usumacinta River, Southeast Mexico: Concentration, Flux, and Sources

Water ◽

10.3390/w13111561 ◽

2021 ◽

Vol 13 (11) ◽

pp. 1561

Author(s):

Daniel Cuevas-Lara ◽

Javier Alcocer ◽

Daniela Cortés-Guzmán ◽

Ismael F. Soria-Reinoso ◽

Felipe García-Oliva ◽

...

Keyword(s):

Organic Carbon ◽

Gulf Of Mexico ◽

Particulate Organic Carbon ◽

Water Discharge ◽

Tropical Rivers ◽

Tropical River ◽

C Cycle ◽

Poc Export ◽

Southern Gulf Of Mexico ◽

Global Carbon

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.

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Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin)

Biogeosciences Discussions ◽

10.5194/bgd-9-63-2012 ◽

2012 ◽

Vol 9 (1) ◽

pp. 63-108 ◽

Cited By ~ 1

Author(s):

S. Bouillon ◽

A. Yambélé ◽

R. G. M. Spencer ◽

D. P. Gillikin ◽

P. J. Hernes ◽

...

Keyword(s):

Organic Carbon ◽

Particulate Organic Carbon ◽

Chemical Weathering ◽

Dissolved Inorganic Carbon ◽

Low Flow ◽

Tropical Rivers ◽

Particulate Nitrogen ◽

Congo River ◽

Sampling Campaign

Abstract. The Oubangui is a major tributary of the Congo River, draining an area of ~ 500 000 km2 mainly consisting of wooded savannahs. Here, we report results of a one year long 2-weekly sampling campaign in Bangui (Central African Republic) since March 2010 for a suite of physico-chemical and biogeochemical characteristics, including total suspended matter (TSM), bulk concentration and stable isotope composition of particulate organic carbon (POC and δ13CPOC), particulate nitrogen (PN and δ15NPN), dissolved organic carbon (DOC and δ13CDOC), dissolved inorganic carbon (DIC and δ13CDIC), dissolved greenhouse gases (CO2, CH4 and N2O), and dissolved lignin composition. We estimated the total annual flux of TSM, POC, PN, DOC and DIC to be 2.33 Tg yr−1, 0.14 Tg C yr−1, 0.014 Tg N yr−1, 0.70 Tg C yr−1, and 0.49 Tg C yr−1, respectively. Most elements showed clear hysteresis over the hydrograph. δ13C signatures of both POC and DOC showed strong seasonal variations (−30.6 to −25.8 ‰, and −31.8 to −27.1 ‰, respectively) but with contrasting patterns. Our data indicate that the origins of POC and DOC may vary strongly over the hydrograph and are largely uncoupled, differing up to 6 ‰ in δ13C signatures. The low POC/PN ratios, high % POC and low and variable δ13CPOC signatures during low flow conditions suggest that during this period, the majority of the POC pool consists of in situ produced phytoplankton, consistent with concurrent pCO2 (partial pressure of CO2) values only slightly above and occasionally, below, atmospheric equilibrium. Dissolved lignin characteristics (carbon-normalised yields, cinnamyl:vanillyl phenol ratios, and vanillic acid to vanillin ratios) showed marked differences between high and low discharge conditions. We observed a~strong seasonality in pCO2, ranging between 470 ± 203 ppm for Q<1000 m3 s−1 (n=10) to a maximum of 3750 ppm during the first stage of the rising discharge. Water-atmosphere CO2 fluxes were estimated to average ~ 105 g C m−2 yr−1, i.e. more than an order of magnitude lower than current estimates for large tropical rivers globally. While our TSM and POC fluxes are similar to previous estimates for the Oubangui, DOC fluxes were ~ 30 % higher and bicarbonate fluxes were ~ 35 % lower than previous reports. DIC represented 58 % of the total annual C flux, and under the assumptions that carbonate weathering represents 25 % of the DIC flux and that CO2 from respiration drives chemical weathering, this flux is equivalent to ~ 50 % of terrestrial-derived riverine C transport. Although tropical rivers are often assumed to show much higher CO2 effluxes compared to temperate systems, we show that in situ production may be high enough to dominate the particulate organic carbon pool, and lower pCO2 values to near equilibrium values during low discharge conditions.

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Distribution of organic carbon: possible causes and impacts in the Pangani River Basin ecosystem, Tanzania

Environmental Chemistry ◽

10.1071/en17185 ◽

2018 ◽

Vol 15 (3) ◽

pp. 137 ◽

Cited By ~ 1

Author(s):

Juma R. Selemani ◽

Jing Zhang ◽

Ying Wu ◽

Alfred N. N. Muzuka ◽

Karoli N. Njau ◽

...

Keyword(s):

Indian Ocean ◽

Organic Carbon ◽

Particulate Organic Carbon ◽

River Basin ◽

Dissolved Inorganic Carbon ◽

C3 Plants ◽

Tropical Rivers ◽

Limited Information ◽

Wet Season ◽

The Indian Ocean

Environmental contextUnderstanding the sources of organic carbon and its spatial and seasonal variation is essential for implementing measures to control water pollution. There is, however, only limited information about organic carbon in east African rivers. This study reports the distribution of dissolved and particulate organic carbon in the Pangani River Basin, using isotopes to trace sources of carbon to the basin and its flux to the Indian Ocean. AbstractThere is limited information on organic carbon in African rivers, especially from the eastern side. Here, we report distribution and impacts of total suspended matter (TSM), and dissolved and particulate organic carbon (DOC & POC) in the Pangani River Basin (PRB) ecosystem together with their fluxes to the Indian Ocean. δ13C was also used to trace sources of carbon in the basin. Results showed that the basin is supplied with carbon from allochthonous sources dominated by C3 plants, with higher levels of TSM and DOC in the wet season than in the dry season. Several factors, including altitude, temperature, rainfall, lithology and anthropogenic activities, have a significant influence on the seasonal and spatial distribution of organic carbon in the basin. High discharge in the wet season mobilised terrestrial organic carbon to elevate concentrations of DOC, POC and TSM. Mean concentrations of DOC, dissolved inorganic carbon (DIC), POC and TSM in PRB were in ranges comparable to that in other tropical rivers but their fluxes were lower than in most tropical rivers around the world. Diverting water from the river for irrigation and hydroelectric power production was one of the factors that reduced the flux of carbon. Observed hypoxic conditions in the reservoir indicates that the quality of water for human and aquatic ecosystem health is possibly threatened by a high level of organic carbon; furthermore, the trends of increasing population, deforestation, temperature and rainfall will likely increase the concentration of organic carbon in the future. Better management of waste, afforestation and reforestation are recommended to restore degraded natural forest, so as to reduce uptake of organic carbon from the terrestrial environment.

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Contribution of transparent exopolymeric particles (TEP) to estuarine particulate organic carbon pool

Marine Ecology Progress Series ◽

10.3354/meps11294 ◽

2015 ◽

Vol 529 ◽

pp. 17-34 ◽

Cited By ~ 12

Author(s):

S Annane ◽

L St-Amand ◽

M Starr ◽

E Pelletier ◽

GA Ferreyra

Keyword(s):

Organic Carbon ◽

Particulate Organic Carbon ◽

Carbon Pool ◽

Organic Carbon Pool ◽

Transparent Exopolymeric Particles

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The Carbon:²³⁴Thorium ratios of sinking particles in the California Current Ecosystem 2: Examination of a thorium sorption, desorption, and particle transport model

10.26434/chemrxiv.7803698 ◽

2019 ◽

Author(s):

Michael Stukel ◽

Thomas Kelly

Keyword(s):

Organic Carbon ◽

Water Column ◽

Particulate Organic Carbon ◽

Transport Model ◽

California Current ◽

In Situ Measurements ◽

Power Law Distribution ◽

Sinking Particles ◽

Organic Carbon Concentration

Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon:thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U-234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

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