Knowledge of functional aspects of communities in intermittent streams currently is lacking, and the role of terrestrial detritus as a resource in open-canopy streams is uncertain. Our main objective was to characterize and quantify the macroinvertebrate shredder assemblages in 3 intermittent open-canopy streams on the island of Öland in the Baltic Sea, Sweden, by estimating secondary production. Estimates of annual shredder secondary production ranged from 0.005 to 13.6 g ash-free dry mass (AFDM) m⁻² y⁻¹ among sites. Shredder production and amount of organic matter were positively related, but shredder production was more strongly influenced by the duration of the summer dry period. Production decreased with increase in the length of the summer dry period, but shredder production was still high at sites with long dry phases and was comparable to estimates of shredder production in permanent forested streams, results suggesting that terrestrial organic matter might be an important energy resource in open-canopy streams. Shredder species richness decreased with increase in length of the summer dry phase, and shredder species composition at sites with a long dry period was dominated by drought-tolerant taxa. Our results suggest that ecosystem functions might shift from being sustained by many taxa to being sustained by fewer taxa as stream ecosystems experience moderate droughts of similar length (2–4 mo).

Shredder feeding is a vital process in making decomposition products available to biota in streams. To investigate which food sources shredders in open-canopy streams exploit, we conducted a feeding preference experiment with the invertebrate detritivores Limnephilus bipunctatus and Nemoura sp., which are commonly found in open-canopy streams on the Swedish island of A-land in the southern Baltic Sea. Leaves of birch, Swedish whitebeam, and shrubby cinquefoil; dead and fresh grass; water moss; and algae were offered to the shredders in multi- and single treatments. We hypothesized that food with high nutritional value would be preferred. Both taxa preferred leaves of shrubby cinquefoil, a bush common in the riparian zone of A-land streams; additionally Nemoura sp. also chose algae. Dead grass, the most abundant food source in the streams during the whole year, was the least consumed food type. The fresh food types had highest nutritional value, measured as carbon to nitrogen content. Therefore, food quality could not alone explain the preference of shrubby cinquefoil. However, among the detritus type offered, shrubby cinquefoil had the highest nutritional value. Shrubby cinquefoil may constitute one important energy source to these open-canopy stream ecosystems and may be essential in maintaining an abundant shredder community in these streams. Thus, the results of this study indicate that detrital resources are indeed important in open-canopy stream systems.

1. Allochthonous detritus is the major source of energy in forested streams, but less is known of the importance of terrestrial subsidies to open-canopy streams. Here, we used stable isotope analysis to assess the importance of allochthonous versus autochthonous energy sources to invertebrate shredders in four open-canopy streams in Sweden. Shredders and potential food sources were analysed at both open sites and those with deciduous trees in the riparian zone.2. Mixing models showed that allochthonous coarse particulate organic matter was the most important energy source to shredders at both the open and wooded sites, suggesting that terrestrial subsidies may be an important process in open-canopy streams, just as they are in forested streams.3. However, shredders at open sites had a larger proportion of biofilm in their diet than at wooded sites, indicating an ability of shredders to adjust their diet with food availability.4. We also used the carbon and nitrogen stable isotope signatures to assess the size of the feeding niche of shredders. Feeding on both allochthonous and autochthonous sources at open sites was reflected in a larger feeding niche than at wooded sites for one of the three species analysed. There was substantial overlap of the feeding niche among shredder species, indicating a high functional redundancy within this guild.

Predictions of effects of global climate change include decreased runoff for many parts of the world, which will result in drying of streams. Information of the effects of drought on aquatic ecosystems is limited and little is known of the effects on ecosystem functions. Our main objective was to measure the direct effects of drought on leaf litter breakdown by invertebrate shredders in a controlled laboratory experiment. We hypothesized a decreased breakdown at high drought level. Single-species and multi-species treatments with three shredder species (Asellus aquaticus, Limnephilus bipunctatus, and L. flavicornis) were set up in an experiment with three drought level treatments, control, medium, and high drought (6 cm water level, 1 cm water level, and water level below sediment surface, respectively). Breakdown measured as leaf litter loss was significantly lower in both medium and high drought treatments compared to the control. Previously, decreased breakdown due to drying has been reported, but attributed to low densities of invertebrate shredders. We show that even when shredders are present, drought decreases the breakdown. Drought treatments also induced earlier pupation for the caddisfly L. flavicornis. Shifts in species phenology due to drought, e.g., earlier emergence, may affect species ability to adult survival and reproduction. Shifts in timing of emergence may also affect terrestrial food webs, where emerging aquatic insects may constitute an important food subsidy. Our knowledge of the complex effects of droughts in aquatic systems is limited with an urgent need of extended knowledge of the ecological effects of droughts on freshwater ecosystem functioning.