Temporal variation and fluxes of dissolved and particulate organic carbon in the Apure, Caura and Orinoco rivers, Venezuela

• The DOC ranged between 1,5 and 6,8 mgC l−1; 2,07–4,9 mgC l−1 and 1,66–5,35 mgC l−1 in the Apure, Caura and Orinoco rivers.
• The DOC concentrations show maximum values during rising waters due to washout processes of soils.
• The inverse relationship between POC (in % of TSS) and TSS concentrations in the three rivers fits a logarithmic model.
• The mean annual TOC flux of the Orinoco was 4,27 × 106 TonC yr−1. The Apure contributes with 11,5% and the Caura with 7,1%.
• The decrease in the flux of TOC in the Orinoco River could be associated to interannual variability of water discharge.

The concentrations of total suspended sediments (TSS), dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured in water samples taken monthly in the Apure, Caura and Orinoco rivers during a hydrological cycle (between Sept. 2007 and Aug. 2008). The DOC concentration values ranged between 1.5 and 6.8 mgC l−1 in the Apure River; 2.07 and 4.9 mgC l−1 in the Caura River and 1.66 and 5.35 mgC l−1 in the Orinoco River. The mean concentration of DOC was 3.9 mgC l−1 in the Apure River, 3.24 mgC l−1 in the Caura River and 2.92 mgC l−1 in the Orinoco River at Puerto Ordaz. The three rivers showed a similar temporal pattern in the concentrations of DOC, with higher DOC values during the increasing branch of the hydrograph due to wash-out processes of the organic material stored in soils. The mean concentration values of POC were 1.33 mgC l−1; 0.77 mgC l−1 and 0.91 mgC l−1 in the Apure, Caura and Orinoco rivers, respectively. The inverse relationship found between the percentage in weight of the POC and the concentrations of TSS in the three rivers fits a logarithmic model, as it has been previously reported for other worldwide rivers. The POC concentrations in the Orinoco River showed a positive relationship with the TSS, suggesting that the POC in the Orinoco is the result of terrestrially organic matter. Although the fluxes of organic carbon (OC) in the three studied rivers are dependent on the values of water discharge, the fluxes of DOC during the increasing branch of the hydrograph are higher than those found during the decreasing stage, due to the yield of organic material accumulated in soils during the preceding dry season. The mean annual flux of total organic carbon (TOC) of the Orinoco River at Puerto Ordaz was about 4.27 × 106 TonC yr−1. Of this, 3.28 × 106 TonC yr−1 (77%) represents the flux of DOC and about 0.99 TonC yr−1 (23%) represents the flux of POC. The mean annual input of TOC from the Apure River to the Orinoco River was about 4.92 × 105 TonC yr−1 (11.5%), while the contribution of TOC from the Caura River to the Orinoco River was estimated at 3.05 × 105 TonC yr−1 (7.1%).The values of annual transport of TOC calculated for the Apure, Caura and Orinoco rivers were lower than those reported twenty years ago. This could be related to interannual variations of precipitation in the Orinoco Basin, due to runoff variations can have a strong effect on the fluxes of OC from land to rivers.