Organic carbon (OC) associated with fluvial bed sediment plays an important role in biotic and abiotic processes operating within drainage basins. Increasingly, there is a need to characterize storage and spatial distributions of OC in aquatic sediments, particularly under-sampled areas like tropical streams. The objectives of this study were to examine in detail the variation of OC concentration with bed sediment grain size, to characterize the influence of grain size variation on relative OC mass storage, and to compare weighted OC values to those in other aquatic sediments worldwide.
The study area selected was a third-order dendritic drainage basin developed in a basaltic complex. Bed sediments along a 6 km section of Manoa Stream were systematically sampled every 50 m for a total of 113 sample site locations. Sediments were partitioned into six size fractions (2Á0 mm) and OC was determined by dry combustion. Data indicate that the OC concentration increases with decreasing grain size, with the greatest values in the 0Á063 mm (silt clay) fraction, approximately 4Á6 times greater than the very coarse sand fraction (1Á00±2Á00 mm). Robust smoothing techniques illustrated a general decrease in OC concentration downstream for the size fractions `0Á25 mm. Bed sediments were dominated by size fractions coarser than 0Á5 mm (80 per cent of the total distribution) and only about 2 per cent in the fractions less than 0Á13 mm. Combining information on OC concentration per size fraction and the mass contribution of each fraction to the whole sample, it was observed that fractions coarser than 0Á5 mm had eight to 12 times the storage of OC per kilogram of bed sediments than the fractions finer than 0Á13 mm. Weighted OC values for Manoa Stream were on average 6Á7 g-OC kg À1 , and these were similar to those reported in the literature for a variety of sediments in aquatic environments, both freshwater and marine. These data provide important information on the relative mass storage of OC in bed sediments and their longitudinal patterns in a tropical fluvial environment.