Abstract
Retention of benthic organic matter (BOM) on the sediment surface of the Njoro River, Kenya was investigated between February 1998 and June 1999 at two sites, differing in channel morphology and riparian vegetation density and composition i.e. openβcanopy and closedβcanopy sites. Monthly accumulation of benthic organic matter (BOM) was highly variable, ranging from 97.3 to 6691.8 and from 206.6 to 1936.3 g ash free dry weight m^β2^ (AFDW) with a mean Β± CL of 1133.3 Β± 794.5 and 834.9 Β± 275.8 g AFDW m^β2^ at the closed and open canopy sites, respectively. Pools retained higher amounts of particulate organic matter than riffles. Surprisingly, differences in retention of BOM at the two sites were not significant. Temporal distribution of benthic organic matter showed no distinct relationship with throughfall and bank runoff input patterns of particulate organic matter. Wood debris comprised 47% and 36% while leaves comprised 34% and 31% of the total BOM at the closed and open canopy sites, respectively; the rest was fruits and fine benthic organic matter (FBOM > 0.1 mm but < 1 mm). Out of 19 plant species recorded at both sites, only 8 species reflected their contribution in BOM with Syzygium cordatum contributing over 80% of the identifiable leaf litter. It is presumed that the variations in spatial and temporal distribution of BOM at the two sites is due to differences in discharge regimes, retention structures and riparian vegetation characteristics.