Many large European rivers have undergone multiple pressures that have strongly impaired ecosystem functioning at different spatial and temporal scales. Global warming and other environmental changes have favored the success of invasive species, deeply modifying the structure of aquatic communities in large rivers. Some exogenous species could alter trophic interactions within assemblages by increasing the predation risk for potential prey species (top-down effect) and limiting the dynamics of others via resource availability limitation (bottom-up effect). Furthermore, large transboundary rivers are complex aquatic ecosystems that have often been poorly investigated so that data for assessing long-term ecological trends are missing. In this study, we propose an original approach for investigating long-term combined effects of global warming, trophic resource decrease, predation risk, and water quality variations on the trait-based structure of macroinvertebrate and fish assemblages over 26 yr (1985-2011) and 427-km stretch of the river Meuse (France and Belgium). The study of temporal variations in biological, physiological, and ecological traits of macroinvertebrate and fish allowed identifying community trends and distinguishing impacts of environmental perturbations from those induced by biological alterations. We provide evidence, for this large European river, of an increase in water temperature (close to 1°C) and a decrease in phytoplankton biomass (-85%), as well as independent effects of these changes on both invertebrate and fish communities. The reduction of trophic resources in the water column by invasive molluscs has dramatically affected the density of omnivorous fish in favor of invertebrate feeders, while scrapers became the major feeding guild among invertebrates. Macroinvertebrate and fish communities have shifted from large-sized organisms with low fecundity to prolific, small-sized organisms, with early maturity, as a response to increased predation pressure.

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Studies dealing with quantitative decline of freshwater fish species with long-time series data are rather scarce and primarily limited to migratory species. We present an original work integrating over two decades investigations on roach (Rutilus rutilus L.) stock, one of the dominant cyprinid fish species in the River Meuse (Belgium). This study aimed to quantify changes in the stock of roach after recent indicators (captures from sport fishermen) suggested a sharp decline. Three methods were used: capture-mark-recapture (CMR) estimation, catch per unit of effort by gillnetting (CPUE) and long-term monitoring of fish-passes. Population dynamics of roach was also investigated. CMR method demonstrated a decrease of 91 % of the roach stock compared to earlier data (1993-2003), with current densities close to 300 roach ha-1. This was also confirmed by a decrease of 95 % in CPUEs and a decrease between 93 and 98 % in two fish-passes during the same period. On the other hand, growth and mortality did not indicate radical changes before and after the decline was found. As roach stock decline is generalizable to the whole Belgian's Meuse following estimations in two other sites, further investigations are needed to explain the reasons underlying this decline in order to take conservation measures. First, the uptake of primary production (i.e. phytoplankton) by invasive clams is supposed. And secondly, an increase in predation pressure by the Great Cormorant is effective as this avian predator was found in high densities from 2000 until 2006.

Metabolic rates seemed lower for chub than for roach, especially when they were fed with Chironomidae. A Chironomidae-based diet would be more profitable to chub, and the high associated growth rate could increase the development of the fish larvae. The HL and TEF were in the range of those reported in the literature. These results will be helpful for field-based studies, because they can help to increase the accuracy of models. Copyright © 2017 John Wiley & Sons, Ltd.

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