Scale dependency in the hydromorphological control of a stream ecosystem functioning

Abstract

International audiencePhysical 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

Relations of interannual differences in stream litter breakdown with discharge: bioassessment implications

“…, Colas et al. ). These factors might account for much of the unexplained variance of litter breakdown rates in the perennial streams of this study.…”

“…Similarly, in low‐order streams in France, human‐induced changes in flow quantity, and the intensity and frequency of extreme flow events (and geomorphological parameters) were shown to affect microbial decomposition ( k f ) more strongly than water chemistry did (Colas et al. ). Hydrologic changes could give rise to the variability of λ m through numerous potential pathways (unmeasured in this study), for instance, via altered microbial diversity and assemblage structure on litter (Dang et al.…”

“…The effects of site‐level hydrologic fluctuations might be modified by unmeasured reach‐ and patch‐scale attributes (e.g., benthic litter quality and quantity), and/or in‐channel physical features (e.g., channel morphology, availability of hydraulic refugia), thereby inducing varied responses of hydraulic conditions (Turner and Stewardson ) and hence breakdown rates (Colas et al. ).…”

“…While weather-driven differences in hydrologic conditions could account for some spatial variability of litter breakdown rates across study regions, their effects on breakdown rates across years within sites were inconsistent. The effects of site-level hydrologic fluctuations might be modified by unmeasured reach-and patch-scale attributes (e.g., benthic litter quality and quantity), and/or in-channel physical features (e.g., channel morphology, availability of hydraulic refugia), thereby inducing varied responses of hydraulic conditions (Turner and Stewardson 2014) and hence breakdown rates (Colas et al 2017). Notes: Variables selected for use in the variance partitioning analysis, and the directional effect of each variable (+, À) on litter breakdown rates are given (see Table 2 for notations of variables).…”

“…In our study, microbial decomposition and dissolution was influenced more by the degree of flow variability than by water chemistry. Similarly, in low-order streams in France, human-induced changes in flow quantity, and the intensity and frequency of extreme flow events (and geomorphological parameters) were shown to affect microbial decomposition (k f ) more strongly than water chemistry did (Colas et al 2017). Hydrologic changes could give rise to the variability of k m through numerous potential pathways (unmeasured in this study), for instance, via altered microbial diversity and assemblage structure on litter (Dang et al 2005, Judd et al 2006, Zeglin 2015, and primary producers whose priming effects through the release of labile carbon exudates could stimulate bacterial activity (Danger et al 2013).…”