Environmental patchiness and processing of maple leaf litter in a backwater of a mountain stream: riffle area vs. debris dams

Abstract

Processing rates of Acer pseudoplatanus leaves were investigated in three types of patches - debris dams, buried and exposed, and a riffle area - of a backwater of the alpine stream Oberer Seebach. The results show a gradient of processing rates from slower, for the leaves buried in the debris dams, to the highest, in the riffle area, with significant differences between these two extremes. Shear stress near leaf bags was significantly higher in the riffle area than in debris dams; however, this probably does not reflect the internal conditions of the bags. It is argued that the accumulation of great amounts of plant debris against the bags in the riffle area, and the relatively hard texture and the uniform perimeter of the leaf discs used in the experiment, might minimize mass losses due to mechanical abrasion by the water current. Nitrogen and phosphorus dynamics of the leaf material suggest that microbial processing did not differ between patches. Likewise, density and biomass of shredders did not help to explain differences in processing rates between patches. Nevertheless, significant taxonomical differences in the shredder assemblages between patches were observed. Differential population dynamics, feeding styles and metabolic capacities of shredders and the different environments in which to obtain food between the studied stream patches may be of great importance in the differences between processing rates.