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Abiotic disturbances that directly affect ecosystem functioning may also affect the distribution and composition of functional traits within a community. Shifts in trait composition may further enhance or even limit those effects caused by the abiotic stressors. In this work we asked whether an agricultural landuse gradient would affect both invertebrate detritivore functional diversity and leaf litter decomposition in streams. We further asked how the landuse effect on functional traits would relate to ecosystem functioning, and if their relationship would change across seasons. Using Structural Equation Modelling, which allows the partitioning of both direct and indirect relationships, we show that in the autumn land use had a positive effect on functioning, but this relationship was counteracted by a negative indirect effect on leaf decomposition. Landuse positively affected the presence of detritivore traits that were negatively related to functioning and also promoted trait dominance, which was negatively related to functioning. These results contrast with direct linear regressions between disturbance and functioning, which did not yield any relationship between the two variables. In spring, landuse had no effect on functioning, which is explained by the reduced impact of agricultural disturbances in our boreal streams. Our results emphasise the key role played by trait identity and diversity in mediating the effects of human disturbance on ecosystem functioning. Furthermore, our findings highlight the value of distinguishing the direct effects of human disturbances on ecosystem processes from those mediated through changes in the structure of trophic webs.

The stoichiometric imbalance between consumers and resources can affect both resource processing rates and consumer growth, and thus constitutes a potentially important driver of ecosystem functioning. We hypothesized that imbalances in nitrogen (N), phosphorus (P), and carbon (C) concentrations between detritus and detritivores would have contrasting effects on two related ecosystem processes, with stronger imbalances triggering compensatory feeding while simultaneously constraining detritivore growth. In a stream field experiment, we found that growth of detritivores was constrained by stoichiometric imbalances mostly driven by N limitation, but there was no evidence for compensatory feeding. However, when offered diets of mixed litter with varying N:P and C:N, detritivores preferred the litter species showing the closest match to their own N:P and C:N, which drove accelerated processing of the preferred species in mixture. Our results highlight the role of stoichiometric imbalances between consumers and resources in regulating ecosystem processes.

Stream restoration is a multi-million dollar business that aims at rehabilitating systems impacted by hydrogeomorphological modifications, such as channelization, and ameliorating physical or ecological degradation caused by catchment-scale impacts, such as agriculture or urbanization. Despite extensive programs aimed at restoring habitat complexity in channelized streams, there is little evidence for a recovery of biological diversity, and functional responses have been little assessed. Notably large-scale habitat restorations have recently been undertaken in a river catchment in northern Sweden, including rehabilitation of large habitat structures (massive boulders, large woody debris) originally removed to facilitate timber floating. Based on a hydrogeomorphological measure of habitat complexity, we characterised variability in habitat complexity across 20 stream reaches in the catchment, including reference, channelised and restored sites. We assessed whether increased habitat complexity following restoration affected retention of organic matter (FPOM), the functional diversity and organisation of the detritivore feeding guild, and two ecosystem processes: algal productivity and litter decomposition. Deposition of FPOM increased along the complexity gradient, as did leaf litter decomposition mediated by invertebrates. The increase in invertebrate-mediated decomposition was associated with shifts in the functional composition of detritivore assemblages, with feeding traits associated with more efficient decomposition more prominent in the restored reaches. There was no change in algal productivity at local scales, but increases in shallow, well- lit habitats favourable for algal growth indicate a possible increase in algal productivity at the stream reach scale. Increases in habitat complexity enhanced functioning within the detritital foodweb at local scales, without any changes in the biodiversity of detritivores. Our findings indicate that aspects of functiona