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

L'A39 et les étangs bressans

Collection Gérer l'environnementNational audienc

Assessing Biological Orders of river sites and biological structures of watercourses using ecological traits of aquatic insects

International audienceA new method is proposed to assess Biological Orders (BO) of river sites and to compare biological structures of watercourses using two general ecological traits (typological preferendum and typological amplitude) of 187 species of aquatic insects (Plecoptera, Trichoptera, Ephemeroptera) specified in a previous database (Verneaux et al., 2003). A community is defined as an arrangement of species which have various abundances, typological preferenda (tp) and typological amplitudes (ta). In the field, any species assemblage was considered possible, and a river site community was analyzed using two descriptors: its relative species richness and its Biological Order, calculated using the abundance and the ecological traits ( tp, ta) of the recorded species. The BO of a river site, with a range from 0 to 10, constitutes a mean community point which plots the site on a theoretical upstream-downstream gradient defined by a species continuum ( Verneaux et al., 2003). The grounds and the BO assessment are presented, followed by the method applied to a site of the upper course of the Loue River ( French Jura). Community changes which occurred between 1975 and 2000 are specified. The Loue pattern was then plotted with other watercourses in a typological graph where biological structures of rivers are compared using a biological templet. This method, showing differences to the templet, allows to estimate biological changes referable to different levels of disturbance. Biological Orders and the significance of the biological templet are then discussed. The proposed typological analysis, giving both the Biological Order of a river site and its distance from the biological templet, constitutes a useful tool for running waters diagnosis and biological integrity assessment

Evolution des assemblages de Chironomidae (Insecta : Diptera) pendant le Tardiglaciaire dans le lac du Lautrey (Jura, France) : essai de reconstruction paléoenvironnementale.

International audienc

Les macroinvertébrés dans l’évaluation du fonctionnement trophique lacustre : mesures isotopiques et analyse des communautés

National audienc

Relationships of macrobenthos with dissolved oxygen and organic matter at the sediment-water interface in ten French lakes

International audiencePrevious papers have underlined the major role played by dissolved oxygen and organic matter in macrobenthos distribution in lakes, especially in the bathymetric decrease in faunal richness. The present study deals with relationships between these two factors and a biotic index based on endobenthos taxon richness and density. Biological and environmental data refer to 40 sites plotted at 4 different relative depths in 10 lakes of the French Jura (9 lakes) and Pre-Alps (Lake Annecy). From 735 samples of soft sediment and 640 values of dissolved oxygen, the observed relationships of total organic carbon (TOC % dw) and dissolved oxygen (DO% sat) with a biodiversity gradient are presented in three figures. Despite the complexity in determining any biodiversity, the limiting effect of minimum DO values and maximum TOC values on the biotic potential are underlined. DO proved to be limiting below 90 % saturation and very limiting below 65 %. For TOC, a preferendum appeared around 3 % dw. From samples selected in non-limiting conditions of oxygen or organic carbon contents, each of these two factors was connected with the biodiversity index by a regression. In lake typology a 10% sediment organic matter content indicates the onset of dysbiotic features for macroconsumers. The proposed results might be used as guidelines for lake monitoring and functional studies

The Lake Biotic Index (LBI): an applied method for assessing the biological quality of lakes using macrobenthos; the Lake Chalain (French Jura) as an example

International audienceThe paper proposes a new biological method for assessing the quality of lakes. It is based on the comparison of littoral and profundal macroinvertebrate communities sampled in soft sediments. Two depths were sampled: one at the littoral zone (Zl=2 m) to define a littoral index (Bl) related to the biogenic potential of the lake (quantitative data), the other at the deep zone (Zf=0.66 Zmax) to define a taxonomic deficit index (Df) connected with the quality of the water-sediment interface in deep zone (qualitative data). These two indexes were combined in a Lake Biotic Index (LBI), with a range from 0 to 20, which gives an evaluation of the biogenic capacity of a lake to the development of macroconsumer communities. Using the Bl and Df values any lake can be plotted on a typological graph which is helpful to characterize lakes both quantitatively (oligo- to polybiotic) and qualitatively (eu- to dysbiotic). Lake Chalain (French Jura) is presented as an example of the method application with its macrofauna and the index calculation. Then, this lake was included in the typological graph with nine other French lakes studied using the same method. Each lake, characterized by its Bl, Df and LBI values, can be compared to the others in a typological scheme. The proposed indexes are discussed and bases for interpretation of the results are presented. The proposed classification of lakes gives an evaluation of their biogenic capacity concerning macroconsumers and can be usefully compared with other classifications, especially with trophic and sediment types, in functional studies as well as in lake diagnosis, in the context of the Water Framework Directive (Directive 2000/60/CE)

Les macroinvertébrés dans l’évaluation du fonctionnement trophique lacustre : mesures isotopiques et analyse des communautés

National audienc

Quantitative assessment of the reliability of chironomid remains in paleoecology: effects of count density and sample size

International audienceRandom distributions for a wide range (1–100,000) of chironomid head capsules (HC) were simulated on a 1-m2 surface. The number of HC found in circular surfaces equivalent to standard core diameters (90 and 63 mm) was estimated 1000 times, over the range of tested densities. For each number of HC found in the samples, the range of simulated densities was estimated using a threshold probability (p > 0.95). This enabled us to develop equations to infer HC density from sample counts. Because of the threshold probability for comparable sample counts, the equations yield higher estimated densities under a random distribution than for a regular distribution. The probability of sampling at least one HC was >0.95 for densities of 900 HC m−2 for the 90-mm core and 1400 HC m−2 for the 63-mm core. For a specific sample count, the range of actual densities was ~10 times higher for the 63-mm core than the 90-mm core. Comparison with field larval densities revealed that most densities were too low to be suitable for annually resolved reconstruction of a quantitative signal, using current corer sizes, although a large number of populations can support sub-decadal analyses. Nonetheless, some lakes exhibit population sizes large enough to reconstruct robust quantitative estimates of past chironomid abundances. This work provides guidance to reconstruct species dynamics and fine-scale time series analyses in paleoecology