Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparianvegetation

1. The seasonal dynamics of the benthic macroinvertebrate assemblage, and t he subset of this assemblage colonising naturally formed detritus accumulat ions, was investigated in two streams in south-west Ireland, one draining a conifer plantation (Streamhill West) and the other with deciduous riparian vegetation (Glenfinish). The streams differed in the quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the conifer str eam being more acid at high discharge. We expected the macroinvertebrate as semblage colonising detritus to differ in the two streams, due to differenc es in the diversity and quantity of detrital inputs. 2. Benthic density and taxon richness did not differ between the two stream s, but the density of shredders was greater in the conifer stream, where th ere was a greater mass of benthic detritus. There was a significant positiv e correlation between shredder density and detritus biomass in both streams over the study period. 3. Detritus packs in the deciduous stream were colonised by a greater numbe r of macroinvertebrates and taxa than in the conifer stream, but packs in b oth streams had a similar abundance of shredders. The relative abundance of taxa colonising detritus packs was almost always significantly different t o that found in the source pool of the benthos. 4. Correspondence analysis illustrated that there were distinct faunal diff erences between the two streams overall and seasonally within each stream. Differences between the streams were related to species tolerances to acid episodes in the conifer stream. Canonical correspondence analysis demonstra ted a distinct seasonal pattern in the detrital composition of the packs an d a corresponding seasonal pattern in the structure of the detritus pack ma croinvertebrate assemblage. 5. Within-stream seasonal variation both in benthic and detritus pack assem blages and in detrital inputs was of similar magnitude to the between-strea m variation. The conifer stream received less and poorer quality detritus t han the deciduous stream, yet it retained more detritus and had more shredd ers in the benthos. This apparent contradiction may be explained by the inf luence of hydrochemistry (during spate events) on the shredder assemblage, by differences in riparian vegetation between the two streams, and possibly by the ability of some taxa to exhibit more generalist feeding habits and thus supplement their diets in the absence of high quality detritus.