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Abstract. Total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured over a two year period, 1983-1985, in the Orinoco River at Ciudad Bolfvar, 450 km from its mouth. Additionally, samples of the main stem at other localities and main tributaries were included in the present study.The suspended sediment concentrations showed a peculiar pattern of temporal variations: two maxima (May-June and November) and two minima (March-April and August-September).The POC and PN concentrations fluctuated with the water discharge, and higher values were observed during the rising limb than during the falling limb of the hydrogmph. As dry weight percent of solids, the values ranged behveen 0.84 and 9.06% for organic carbon and between 0.08 and 1.45% for total nitrogen. The carbon-nitrogen ratio in the sediment averaged 9.1. The major contribution of organic carbon and nitrogen in particulate form to the Orinoco main stem comes from the left margin tributaries, which have their headwater in the mountainous regions of the Andes.

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