Heterotrophic metabolism in a forest stream sediment:: surface versus subsurface zones

The heterotrophic metabolism of a forest stream sediment was studied during leaf fall, immediately after a flood that completely scoured and homogenized the first 10 to 12 cm of sediment. After this event, differences were found between the surface sediment (0 to 3 cm depth) and the subsurface sediment (7 to 10 cm depth). Higher extracellular enzymatic and respiratory activities were measured in the surface than in the subsurface sediment. The higher heterotrophic activity was related to the higher quantity and quality of the organic matter, which accumulated in the surface sediment (higher chlorophyll and bacterial densities). The heterotrophic metabolism in the surface sediment followed a marked seasonal pattern that correlated with the variations in the environmental parameters (discharge, nutrients). However, no temporal pattern was observed in the subsurface sediment, less affected by the physical changes (e.g. scouring), where no significant change in any activity was recorded throughout the study period. There was a significant decrease in heterotrophic metabolism in the subsurface sediments. This difference can be attributed to the lower availability of algae-derived material in the subsurface sediments.