Ecohydrology of a macroinvertebrate metacommunity in a regulated floodplain
Floodplain ecosystems are some of the most biodiverse ecosystems on earth. This level of biodiversity is sustained by a great diversity of habitats and ecological niches, resulting from the interaction between the natural flow and sediment regimes. However, the ecological integrity of floodplains is rapidly deteriorating worldwide because of human activities. Flow regulation for hydropower production is one of the main drivers of ecological degradation in freshwaters and in floodplains in particular. Dams, impoundments and diversions result in a simplification of the natural flow regime, which becomes less variable, this in turn drives a simplification of the riverscape and a decrease in its biodiversity and ecological functioning. The general aim of this thesis is to investigate how communities of aquatic macroinvertebrates are shaped by hydrology in an alluvial floodplain in order to refine our understanding of hydrology – biodiversity linkages in complex ecosystems. To achieve this aim, I conducted an extensive field survey and an innovative field experiment on the Maggia River floodplain, in southern Switzerland. First, I investigated how different flooding regimes and recent flooding history lead to different community structures amongst floodplain habitats. I found that frequent flooding favored rheophilic species over more lentic groups, and limited strong competitive interactions to occur by maintaining low periphyton standing crops. Then, I developed a novel experimental setup, where 24 ponds were excavated out of two gravel bars in the floodplain, and monitored macroinvertebrate metacommunity assembly over 45 days. I showed that the interaction between the local environment and the degree of connectivity to the regional species pool can lead to different assembly trajectories through time. The findings of the experiment were then broadly compared to the temporal changes in metacommunity organization observed in the natural habitats. There, I found that local environmental conditions and the degree of landscape connectivity greatly affected metacommunity structure. Most importantly, I showed that metacommunity organization changed in time during post-disturbance metacommunity assembly, and that the temporal shifts between the main structuring processes differed with the dispersal ability of species. Finally, I used stable isotope analyses to calculate the food chain length within macroinvertebrate communities, and assessed how it responded to gradients of habitat size, productivity and disturbance regime. There, I showed that flooding reduced food chain length, because predators were feeding at lower trophic levels in frequently disturbed habitats. Overall, this study shows that maintaining flow variability and spatial heterogeneity in flood disturbance is key to sustaining floodplain aquatic biodiversity. My research also contributes to the field of metacommunity ecology by showing that metacommunity organization is highly variable in time and especially during assembly after disturbance.