River networks have a dendritic structure, where organisms with purely aquatic life histories have limited dispersal connectivity between suitable habitats in separate subcatchments. This leads to characteristic patterns of high beta diversity in headwater reaches, and higher alpha diversity downstream where different subcatchments’ species pools can mix. Rivers also by nature have directional flow, transporting resources such as terrestrial litter input from upstream to downstream. Along the way, these resources are processed by different microbial, fungal, and macroinvertebrate communities. As many freshwater ecosystems are net heterotrophic, they are dependant on these allochthonous subsidies to function. To examine biodiversity in a dendritic river network and its effect on ecosystem functioning through subsidy processing, we studied 61 stream and river reaches in a 740 km2 area of the Thur drainage basin in eastern Switzerland. In each reach, we surveyed the macroinvertebrate community and measured microbial community respiration, as well as breakdown rates of a standardized cotton strip and of leaf litter of the dominant tree Fagus sylvaticus. We also measured environmental conditions such as stream chemistry and temperature, and extracted geographic information about land use, topography, and position in the river network.
Results/Conclusions
Biodiversity patterns in many ways conformed to theoretical expectations for dendritic networks. Macroinvertebrate species richness was low in headwater reaches and peaked in mid-sized reaches and in a network module in the middle of the drainage, and beta diversity was often high across headwaters, indicating that they are reservoirs of unique diversity. However, links between the observed biodiversity patterns and ecosystem functioning were non-intuitive. Surprisingly, decomposition of leaf litter by the microbial community (in fine mesh bags excluding most invertebrates) did not correspond to microbial community respiration, nor did decomposition of leaf litter by the total community (in coarse mesh bags) correspond to macroinvertebrate community diversity or the abundance of shredding taxa. Furthermore, we saw strong effects of anthropogenic stressors modulating patterns of both diversity and ecosystem functioning, interfering with predictions made for pristine riverine networks. Specifically, macroinvertebrate abundance and diversity was much lower in one major subcatchment with high effluent loads from manufacturing and wastewater treatment, and decomposition of the standardized cotton strip was low across this entire subcatchment. Overall, our study allows us to address both the structure and function of riverine communities across a large scale, and identify how anthropogenic stressors may alter these patterns and processes.