Scale dependency in the hydromorphological control of a stream ecosystem functioning

Physical habitat degradation is prevalent in river ecosystems. Although still little is known about the ecological consequences of altered hydromorphology, understanding the factors at play can contribute to sustainable environmental management. In this study we aimed to identify the hydromorphological features controlling a key ecosystem function and the spatial scales where such linkages operate. As hydromorphological and chemical pressures often occur in parallel, we examined the relative importance of hydromorphological and chemical factors as determinants of leaf breakdown. Leaf breakdown assays were investigated at 82 sites of rivers throughout the French territory. Leaf breakdown data were then crossed with data on water quality and with a multi-scale hydro- morphological assessment (i.e. upstream catchment, river segment, reach and habitat) when quantitative data were available. Microbial and total leaf breakdown rates exhibited differential responses to both hydromorphological and chemical alterations. Relationships between the chemical quality of the water and leaf breakdown were weak, while hydromorphological integrity explained independently up to 84.2% of leaf breakdown. Hydrological and morphological parameters were the main predictors of microbial leaf breakdown, whereas hydrological parameters had a major effect on total leaf breakdown, particularly at large scales, while morphological parameters were important at smaller scales. Microbial leaf breakdown were best predicted by hydromorphological features defined at the upstream catchment level whereas total leaf breakdown were best predicted by reach and habitat level geomorphic variables. This study demonstrates the use of leaf breakdown in a biomonitoring context and the importance of hydromorphological integrity for the functioning of running water. It provides new insights for envi- ronmental decision-makers to identify the management and restoration actions that have to be un- dertaken including the hydromorphogical features that should be kept in minimal maintenance to support leaf breakdown

Assessment of functional integrity of eutrophic streams using litter breakdown and benthic macroinvertebrates

Currently, interest is growing in evaluating stream functional integrity while river assessment schemes are still exclusively based on structural indicators determined from various aquatic communities. Although some approaches relying on macroinvertebrate functional groups and combinations of traits have been advocated as means to assess ecosystem function, there has been no attempt to test the reliability of these methods with any direct functional indicator even though litter breakdown has been recently proposed as a functional indicator of stream impairment. The purpose of this study was to compare nine benthic macroinvertebrate-based structural metrics with functional metrics based on leaf litter breakdown in coarse and fine mesh bags in nine streams distributed along a eutrophication gradient. In coarse mesh bags, a 10-fold drop in breakdown rate of alder litter indicated a high sensitivity of this functional indicator to the deleterious effects of ammonium and its associated products, ammonia and nitrite. In contrast, microbial breakdown measured in fine mesh bags did not vary substantially along the gradient. Taxonomic and functional structures of macroinvertebrate assemblages were altered in the most eutrophic streams, as shown by drops in IBGN (French biotic index), BMWP (Britain biotic index) and EPTC (Ephemeroptera, Plecoptera, Trichoptera and Coleoptera) richness. Shredder richness and an ecological diversity index calculated from functional groups defined from multiple trait combinations exhibited the highest correlation with litter breakdown rate. Our results indicate that indirect assessment of stream functional integrity by structural indicators is realistic but requires specific metrics not necessarily based on traits or functional groups. Leaf litter breakdown is confirmed as a reliable indicator for direct assessment of streams impacted by eutrophication