The importance of particulate organic matter to invertebrate communities of boreal woodland streams : implications for stream restoration

Allochthonous organic matter originating from streamside vegetation is of central importance in maintaining food webs and diverse life forms in small woodland streams. In this thesis I studied how the seasonal availability of particulate organic matter (POM) is reflected in the seasonality and community composition of stream macroinvertebrates. I also studied the breakdown rates of leaves from three deciduous tree species and their colonization by fungi and macroinvertebrates, as well as the distributional patterns of detritivores and their food resources in the streams of contrasting retentivity. The input of allochthonous detritus in stream Rutajoki, Central Finland, was highly pulsed, peaking in September-October. The standing stocks of benthic organic matter were among the lowest reported for woodland streams. Minimum biomasses and densities of total macroinvertebrates occurred in summer, and maximum in mid-winter. Seasonality was most distinct among shredders, with 40-fold higher biomasses in March than during the summer. The breakdown rates of alder and birch leaves were fast, while willow leaves were processed at a medium rate. Birch leaves contained the highest fungal biomasses while alder supported the highest invertebrate numbers. Invertebrate community composition was similar between the three leaf types, but differed distinctly from the surrounding benthos. The aggregation of detritivores to retentive stream patches was most pronounced in the channelized stream, while in the natural stream they showed only weak association with benthic leaves. Due to simplified habitat structure, channelized streams contained a group of indicator species, whereas shifts between restored and natural streams were more gradual. The poor retention capacity may severely limit the production of detritivorous invertebrates. The presence of various leaf species is one of the key elements in supporting vital populations of detritivores