Environmental shifts revealed by macrofossils and geochemistry in river sediment cores: a high temporal resolution study on the Rhône River (France)

River ecosystems have been highly altered and modified since the 1950s for water control, navigation, hydroelectricity, and agriculture, leading to hydro-sedimentary changes, vegetation shifts, and ecosystem degradation. In this context, secondary channels were particularly affected, as demonstrated by geomorphological and pollution studies (reduced inflow, accumulation of contaminated sediments). It is also clear that rapid changes in vegetation have taken place, but very few studies exist on this issue.

To address this gap, we propose to study and quantify macrofossils from sediment cores extracted from two side channels of the Rhône River, France. The goal is to reconstruct the history of riparian habitats and biodiversity since the 1950s, linking ecological trajectories to pollution trends (already published) and to engineering actions. This approach helps to provide insights into past human impacts at a high temporal resolution (one sample every two years) and to provide key features for future management strategies in alluvial wetlands.

Several sediment cores were extracted from two side channels, located 50 km south of Lyon (France), in an area heavily equipped (dams, dykes and groynes). Radionuclides (¹³⁷Cs and ²¹⁰Pb) and persistent organic pollutant trends were used to date the sediment accumulation. Fifty-two samples were analyzed at 2.5–4 cm resolution, corresponding to a temporal resolution of approximately two years. Plant and animal macrofossils (>400 µm) were recovered, identified, and standardized, with additional characterization of iron slags. Multivariate regression trees were used to highlight temporal succession patterns among taxa.

Macrofossils, especially forest and aquatic taxa, provide a history of habitats and biodiversity since 1950. Three major hydro-ecological phases were identifed and related to local river developments or management. The first phase, before 1977 (date of the dam construction), highlights a diversified riverine forest marked by black alder scales, strawberry seeds and numerous wood fragments. Abundant macrocharcoals and iron slags, correlated with magnetic susceptibility, suggest industrial inputs during this period. Then, the 1980s-1990s represent a second step with the rapid development of terrestrial vegetation with nettles, after dewatering. Finally, around 1999–2000, both sites returned to lentic conditions, with the expansion of wetland and aquatic taxa.

This research demonstrates the value of the macrofossil analysis in order to reconstruct the ecological history of river ecosystems at a high temporal resolution for the last 70 years. The study successfully linked macrofossil data to engineering and management actions, revealing an alternance of riparian dynamics and terrestrialization. These findings provide crucial feedback on the impacts of development, which can be helpful to define reference conditions and monitor long-term ecological changes.