The macroinvertebrate fauna of augmented river gravels: community composition and function

Abstract

Restoration of riverine ecosystems is a key aspect of river management, driven by concerns over water sustainability and declining biodiversity caused by anthropogenic and environmental perturbations. Anthropogenic perturbations include agricultural sedimentation and nutrient input, deforestation, and river regulation including damming, which disturbs longitudinal, lateral and vertical connectivity. Restoring perturbed riverine systems is costly, and many such restorations are ineffective, with restored systems continuing to degrade. Gravel augmentation, the adding of in-stream gravels, focusses mainly on improving habitat for salmonids, and is one method frequently used to mitigate the impacts of damming. The impact of these augmentations on riverine macroinvertebrates and ecosystem functioning in catchments with altered riparian inputs of leaf litter is, however, still unclear, given the lack of work to date investigating leaf litter combined with macroinvertebrate assemblage structure. This thesis investigates, for the first time, macroinvertebrate assemblage composition, functional diversity and leaf litter breakdown at macro and meso-scales across two low-order, sediment-starved streams on Dartmoor, UK, subject to gravel augmentation. Investigations used macroinvertebrate survey techniques, leaf litter breakdown experiments and univariate and multivariate analyses to explore potential shifts in ecosystem functioning and impacts to the food web related to augmentation. In total, 5641 individuals from 44 taxa were recorded across both catchments. Evidence was found that augmentation led to small and large-scale differences in assemblage structure and functioning, which has the potential to influence both higher and lower trophic levels. Leaf litter breakdown was found to be higher in augmented reaches, although whether associated differences in macroinvertebrate assemblage, diversity, functionality and leaf pack mass loss were due solely to augmentation or a combination of augmentation and other biological and environmental factors remains unclear and merits further study.