Distribution of benthic macroinvertebrates and leaf litter in relation to streambed retentivity: implications for headwater stream restoration

The distribution of leaf litter and benthic macroinvertebrates was studied in two boreal streams: a stream with a natural streambed (Merenoja), a channelized stream (Rutajoki), and the latter stream after its restoration. Leaf release experiments were performed to characterise the retentive structures of each stream. Benthic samples were collected from patches that retained artificial leaves (`retention sites') and from randomly located patches (`random sites'). Retention sites contained significantly more benthic leaves than random sites in each stream, but the difference between patch types was most distinct in the channelized stream. Densities of shredders and other detritivores were distinctly higher in retention than in random sites in Rutajoki, both before and after restoration. In Merenoja, shredders were evenly distributed among the patch types. Restoration clearly increased the trapping efficiency of Rutajoki, but only slightly reduced the aggregation of detritivores to retentive stream patches. Our study highlights the importance of detritus aggregations for stream invertebrates, especially in channelized streams.