Response of littoral chironomid community and organic matter to late glacial lake level and environmental changes at Lago dell'Accesa (Tuscany, Italy).

International audienceThis study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell'Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes

Organic matter composition and phosphate retention in sediments of a littoral wetland of the Seine estuary (The Vernier Marsh, France).

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Contrasted distributions of triterpene derivatives in the sediments of Lake Caco reflect paleoenvironmental changes during the last 20,000 yrs in NE Brazil

Lipid fractions extracted from sediment layers deposited in a small Brazilian lake during the last 20,000 yrs were investigated by gas chromatography-mass spectrometry. Considerable differences in the distribution and the amount of triterpene derivatives in the aliphatic as well as in the aromatic fractions were observed all along the series. Although no precise identification of these compounds was undertaken, our interpretation of mass spectral data allowed us to discriminate between des-A-triterpenes, mono- and triaromatic derivatives of pentacyclic triterpenes and a series of compounds tentatively identified as diaromatic derivatives of tetracyclic triterpenes. The largest compound diversity was found in the lowest levels of the series dated back to the end of the Last Glacial Maximum when good preservation of terrestrial plant debris was ensured by rapid burial in a semi-arid climate. Then, except for a period corresponding to the Younger Dryas, only des-A-lupane was detected in significant amounts. The high predominance of des-A-lupane is interpreted to result from the development of a belt of Eleocharis sp. (spike-rush) that filtered most of organic inputs from the catchment and was the site of des-A-lupane production and exportation towards the lake centre. During the Younger Dryas, a strong influx of des-A-lupane is attributed to the destruction of the spike-rush belt consecutive to the lowering of the lake level under drier conditions. The temporary destruction of this barrier allowed other triterpene derivatives to reach the lake. The distinct dynamics of des-A and aromatic triterpene derivatives under variable medium conditions led us to hypothesis that these two families of compounds derive from distinct pools that contributed differently to the sediment depending on environmental and climatic conditions. Des-A-lupane was most probably produced in sub-aquatic conditions within the belt of spike-rush. Conversely, aromatic derivatives of triterpenes could have resulted from the degradation of their biological precursors within reducing micro-environments in the catchment