Many streams run through naturally-unforested, agricultural, and residential landscapes and thus do not have closed riparian canopies. Little is known of food web dynamics and ecosystem function in such open-canopy streams. Similarly, our knowledge of ecosystem processes in intermittent streams and effects of drought on stream ecosystem function are limited, despite that predictions of climate change effects include increased frequency and duration of droughts in freshwaters. The aim of my thesis was to increase our knowledge of invertebrate shredder assemblages and ecosystem function in open-canopy intermittent streams and to reveal the role of terrestrial organic matter in open-canopy streams. Invertebrate shredders, as primary consumers of detritus, significantly contribute to the ecosystem function decomposition of organic matter. Thereby, shredders are important vectors for transferring detrital energy into stream food webs.

I found high-density and species-rich shredder assemblages in three open-canopy intermittent streams on the island of Öland in the southern Baltic Sea, Sweden. My results showed that terrestrially derived organic matter was the shredder’s main food source and the amount of high-quality CPOM potentially limited shredder production. However, through supplementing their feeding with algae, a high shredder production, similar to production estimates in forested permanent streams, could be maintained in these open-canopy intermittent streams. Instead, my results indicate that the primary physical factor constraining shredder production in intermittent streams is the length of the summer drought period. Furthermore, drying simulated as decreasing water levels in an experiment, decreased invertebrate shredder feeding activity and consequently, the breakdown rate of organic matter. Drying also altered a caddisfly shredders’ life cycle phenology and such evident induction of earlier pupation due to drying has not been shown for aquatic insects earlier.

Food webs and ecosystem function and processes in open-canopy and intermittent streams deserve further attention, especially as these types of streams are very common worldwide. My findings indicate that the energy base in open-canopy streams may be terrestrially derived organic matter, as in forested streams, but constraints imposed by habitat traits and differences in organic matter input size may have significant consequences on stream productivity. My results may be important for increased understanding of potential effects of land use changes (i.e. forestry, agriculture, urbanization) and environmental changes (i.e. climate change) on stream ecosystems.