Hazel leaf breakdown in two low-order streams differing in the functional efficiency of their detritivore assemblages

Using a reciprocal exchange experimental design, the processing rate of haz el leaf packs was measured in two low-order streams in south-west Ireland t hat differed in their macroinvertebrate assemblages and in their susceptibi lity to acid pulses during spates. One stream had a deciduous riparian zone and the other flowed through a conifer plantation. The influence of the de tritivore assemblage relative to the initial microbial conditioning was ass essed. Hazel leaf packs and plastic 'habitat-only' packs were initially con ditioned in each stream and were then either transferred to the other strea m or replaced in the stream of conditioning. The rate of macroinvertebrate colonisation and hazel leaf pack breakdown were subsequently measured in al l four treatments. There was no significant difference between streams in l eaf mass loss during the conditioning period. During the subsequent exposur e period, there was a significant difference between streams in hazel leaf breakdown rate, regardless of the stream in which the leaf pack was conditi oned. Within each stream, detritivores (shredders and deposit feeders) did not differentially colonise packs conditioned in the different streams, ind icating that there was no consequential difference in initial conditioning between streams. However, there were significant differences between the st reams in their colonising detritivore assemblages, both in the abundance of individual taxa and in the relative dominance of different functional feed ing groups. From this study it would appear that the impact of detritivore activity in the leaf packs outweighs any potential differences in microbial processing between the two streams and also exceeds the effect of differen tial conditioning on the attractiveness of the detritus. Factors influencin g the structure of the detritivore assemblages are thus of primary importan ce in the decomposition of leaf litter in these systems.