BREAKDOWN AND INVERTEBRATE COLONIZATION OF LEAF-LITTER IN 2 CONTRASTING STREAMS - SIGNIFICANCE OF OLIGOCHAETES IN A LARGE RIVER

The breakdown of willow (Salix alba) and beech (Fagus silvatica) leave s and their colonization by invertebrates were studied in a third-orde r mountain stream (breakdown rate k = 0.0147 and 0.0052.d-1, respectiv ely) and a seventh-order lowland river (k = 0.0094 and 0.0027.d-1). Wi llow leaves broke down three times faster than beech leaves; this diff erence was twice that between streams. Total invertebrate densities an d biomass and the proportion of shredders were higher in the large riv er; therefore, differences in breakdown capacities between streams can not be attributed to differences in invertebrate community structure. Instead, microbial processing and abiotic fragmentation are suggested as factors controlling breakdown rates. Owing to the abundance of naid ids, up to 2000 invertebrates per leaf bag were recorded in the large river. These oligochaetes appeared to use litter accumulations as a mi crohabitat that provided shelter and a rich supply of food in the form of fine particulate organic matter. Tubificid oligochaetes accounted for 55% of total biomass on partly degraded leaves at this site. Their abundance and regular occurrence inside willow leaves suggest that tu bificids enhance leaf fragmentation and possibly mediate the incorpora tion of organic matter in river sediments once the plant tissue is suf ficiently macerated.