Long‐term trends in trait structure of riverine communities facing predation risk increase and trophic resource decline

Abstract

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.

River habitat homogenisation enhances trophic competition and promotes individual specialisation among young of the year fish

“…Benthic macroinvertebrate abundance is similar in the two studied sites (Latli, Descy et al, 2017). The family richness is slightly higher at Waulsort (28) than at Ham-sur-Meuse (25) mainly due to exotic taxa that represent >80% of the sampled organisms.…”

“…In another study, we showed that plankton decline and increased predation risk (e.g. the great cormorant Phalacrocorax carbo and the European wels Silurus glanis ) influenced trophic structure of the fish community of the Meuse River (Latli, Descy et al., ), which could potentially differ according to the diet type and the importance of human perturbation.…”

“…During the last 25 years on the French and Belgian Meuse River, water temperature increased almost 1°C whereas orthophosphate, nitrate, and suspended matter concentrations decreased (Latli, Descy et al, 2017) along with phytoplankton biomass, which fell by 85% ( Figure 1). The zooplankton followed the same trend: during the 1990s the mean abundance of Rotifera was between 300 and 500 individuals (ind)/L through the summer with maxima close to 4,000 ind/L.…”

“…However, if food is abundant, two species could share the same resources without negatively impacting each other (Pianka, ). For example, omnivorous fish suffer less from river regulation and habitat destruction than fish having more specialised diets (Aarts, Van Den Brink, & Nienhuis, ; Latli, Descy, et al., ).…”

“…A major event in the recent history of the Meuse River was the establishment of non-native filter-feeding bivalves, dreissenid and corbiculid mussels, which contributed to reduce the phytoplankton and zooplankton biomass by 85% over a period of 15 years (Pigneur et al, 2014). As in many European and American freshwater ecosystems (Higgins, Vander Zanden, Joppa, & Vadeboncoeur, 2011;Ward & Ricciardi, 2007), the invasive filter-feeders have impacted most biota in the Meuse River, from plankton to piscivorous fish (Latli, Descy et al, 2017). However, the impact on fish communities varied according to the degree of channelisation.…”