BENTHIC ORGANIC-MATTER DYNAMICS IN 3 STREAMS - RIPARIAN VEGETATION ORBED MORPHOLOGY CONTROL

Benthic organic matter (BOM) estimated by Ash-Free Dry Weight (AFDW) w as analyzed in three streams of the French granitic Massif Central mou ntains. The study sites differed mainly by riparian and catchment vege tation and by their morphology. The nature and area of each retentive structure were determined in each reach. Unexpectedly we did not obser ve greater organic matter accumulation during the leaf fall period or in the most shaded stream. Most autumnal input of organic material was flushed during the high discharge period (October to December). The d ifferences in annual accumulated benthic organic matter (BOM) between streams and the temporal fluctuation in BOM accumulation were explaine d by differences in nature of retention structures and riparian vegeta tion. Thus, riparian vegetation played an important role in the dynami cs of stream organic matter, both by its direct input and as a retenti on structure. The efficiency of retention structures (boulders, roots, branches) greatly depended on water level and discharge.