Karen K. Serieyssol

Holocene evolution of two ponds on the Devès Plateau in the French Massif Central

The siliceous sub-fossil content (diatoms, chrysophyte cysts, sponges and phytoliths) of two cores was studied to determine the evolution and environmental changes that have occurred since the formation of two fenland lakes, Lac de Collanges and Lac de Freycenet, on the Devès Plateau, Massif Central, France. Cluster analyses determined eight siliceous zones, whereas a detrended correspondence analysis showed similar changes occurring in both sites, and principal component analysis identified four major shifts occurring over time at both sites, corresponding to the establishment of the fenland, the development of a pond and changes in the terrestrial environment. Four classic pollen zones previously determined for the French Massif Central were recorded in the core. The Boreal and beginning of the Old Atlantic period (9445–9250 cal bp) are marked by the development of the fenland, a decrease in pine trees and loss of grassland. The siliceous component was dominated by chrysophyte cysts indicating a cooler and lower trophic level. The Atlantic period (8365–7852 cal bp) saw maximum expansion of the fenland and the first occurrence and dominance of Aulacoseira perglabra (Østrup) Haworth, indicating higher water levels as a result of climate change. Increasing numbers of sponge spicules and the decrease in A. perglabra showed a shift to dryer conditions. During this phase, greater and more frequent droughts occurred in Collanges compared with Freycenet, most likely due to its smaller drainage basin. With the Subboreal, around 5000 cal bp, a change in conditions occurred: A. lacustris (Grunow) Krammer became the most important centric diatom and phytoliths became an important component. Pine trees increased and total herb and grasses became more important. The increase in total herb and grass pollen, along with the major increase in phytoliths, indicated an increase in human agro-pastoral activity within the area. The Subatlantic saw a decrease in water levels with periods of desiccation. A cyclic pattern of wet and dry phases was documented by a diatom increase in regularly moist and mostly moist subaerial environments. Including counts of phytoliths, sponges and chrysophyte cysts with diatoms allows better interpretation of the changes occurring over time within the area and permitted determination of the arrival of human agro-pastoral activity. The other three groups, namely phytoliths, sponges and cysts, were particularly useful when certain samples contained very few or were totally devoid of diatoms.

Diatom distribution in natural and impacted cut-off meanders of the Allier River, France

Parapotamal cut-off meanders from two different geological catchment regions of the Allier River, 117 km apart (upstream – metamorphic-basalt and downstream – sedimentary), differ in their degrees of infill or depth. In each region, three cut-off meanders were examined; one in each region was impacted by gravel extraction (upstream deepening) and halieutic improvement (downstream deepening), whereas the others were left in their natural state having different silt accumulations either almost cut-off from the river or less silted-up and more open to the river. The sites were monitored for one year for physical and chemical characteristics. Diatom samples collected in summer 2009 were examined along with their associated biocenosis. The two catchments, differing in geology, land-use and water quality, sheltered different diatom communities. Physical and chemical differences were recorded between up- and downstream zones of each cut-off meander due to both the influence of the groundwater (buffering the water temperature among others) and the periodic inflow from the main channel (increased oxygen saturation concentration in downstream end of the cut-off meanders, except for the impacted upstream zone which contained a lot of macrophytes). In the upstream reach (except for one site), the connection with the groundwater from the surrounding catchment, which acts as a hydro-geological reservoir, might explain the highest mineralization and water hardness recorded mainly in the upstream zone of the cut-off meander, and the presence of Pseudostaurosira subsalina (Hustedt) Morales and Thalassiosira weissflogii (Grunow) Fryxell & Hasle. For the downstream reach, in addition to up- and downstream differences, other factors came into play giving rise to different communities, for example, cattle trampling in the cut-off meander leading to the presence of hypereutrophic and polysaprobic taxa or the potential local re-emergence of mineral springs associated with brackish taxa. Moreover, the up- or downstream deepening also induced differences: the extraction of sediment nearer to the river created a system in which water input from the main channel flows in through a larger opening, inducing allochthonous processes. This increased the rivers influence, creating a renewal of earlier morphological and ecological conditions, whereas the gravel extraction that modified the upstream zone increased the influence of groundwater from the surrounding catchment on this area of the site and led to water conditions independent of the main channel. Furthermore, this study has provided an important picture of the environmental variables, mechanisms and processes that drive the distribution of diatoms within the cut-off meanders along the Allier River, which can can be applied in future paleo-environmental studies.