Comparaison des peuplements chironomidiens du lac de l'Abbaye obtenus par différentes méthodes d'échantillonnage. Intérêts de la récolte des exuvies nymphales

Une étude des espèces chironomidiennes du lac de l'Abbaye a été effectuée au cours de l'année 1993. Le répertoire spécifique a été établi à partir de l'étude des peuplements imaginaux, nymphals et larvaires échantillonnés selon 5 méthodes: récolte des imagos au filet entomologique, récolte des exuvies nymphales, mise en élevage de stades pré-imaginaux, prélèvements de larves dans les sédiments et mise en place de substrats artificiels. Le peuplement chironomidien obtenu est constitué de 69 espèces. Une comparaison de la composition des peuplements obtenus par les différents modes d'échantillonnage permet de mettre en évidence les particularités de chaque méthode. La récolte des exuvies nymphales semble être la méthode la plus appropriée pour l'établissement d'un répertoire spécifique. Deux espèces dominantes du lac Chironomus anthracinus et Psilotanypus rufovittatus témoignent du caractère polyhumique et désoxygéné du lac. Cependant, la présence simultanée en forte proportion de Tanytarsus niger, Cladotanytarsus iucundus et C. atridorsum témoignent de la faible production pélagique et de la température froide du lac susceptible de minimiser les effets de la désoxygénation sur la communauté chironomidienne.A study of chironomid species was undertaken in Lake Abbaye during the year 1993. Lake Abbaye is located 910 m a.s.l., in a karstic area in the east of France near Switzerland (Figure 1). It covers 80 ha with a maximum depth of 19 m and a mean depth of 7.2 m. Profundal sediment has a high level of organic matter (30.2% dry weight) of mainly terrestrial origin. The accumulation of the organic matter resulted in hypoxic conditions at depth during the summer stratification. The lake is rather cold (maximum surface temperature: 23°C), holodimictic, and is ice-covered from late November to early April. The list of chironomid species obtained resulted from the use of 5 different sampling methods: (i) adults caught with an entomological net; (ii) pupal exuviae sampling at the sites where they were accumulated; (iii) rearing of larvae; (iv) sediment sampling and (v) sampling of larvae using artificial substrates. The sampling site locations are indicated in Figure 1. The present paper gives results and discussions about i) the taxonomic richness and composition of the total chironomid community, ii) the composition of the different communities collected and the characteristics of the five sampling methods, iii) a characterization of Lake Abbaye according to the dominating species.1. Total chironomid communityThe chironomid community of Lake Abbaye consisted of 69 species (Table 1). Compared to other European lakes, this community showed a rather high richness. In some great lakes, however, the number of chironomid species amounted to 140 or 168 species (Lake Innaren, Lake Constance). These particularly high richnesses probably resulted from the great number of substrata according to the great surface of the littoral zone of these lakes. The chironomid community of Lake Abbaye is specific to a mountain area as shown by the high proportion of species collected (82% of the total richness) found by SERRA-TOSIO and LAVILLE (1991) at altitudes exceeding 500 m. According to REISS and FITTKAU (1971), the presence of two particular species (Tanytarsus niger and T. miriforceps) reflects the glacial origin of the lake.2. Composition of the different chironomid communitiesSampling the pupal exuviae allowed us to collect more than 90% of the species with less than 5000 individuals whereas by catching adults this percentage did not exceed 79% with more than 28000 individuals collected and identified. By rearing larvae both the number of species (46 species, 66% of the total richness) and the number of emerging adults (1386 individuals) were low. The least efficient method appeared to be the sediment sampling, which allowed the identification of only 19 taxa (genera and species) compared to the 31 taxa collected by artificial substrates. This study clearly showed that pupal exuviae sampling was the most appropriate method to establish a list of species yielding the greatest ratio between species richness and the effectives.A comparison of the major contributed species among the different communities led to the conclusion that the sampling method strongly affects community composition. Whereas in adult population (Figure 2) the dominating species (relative density=10%) were Tanytarsus niger, Ablabesmyia monilis, A. longistyla and Polypedilum nubeculosum, the community of pupal exuviae (Figure 2) was dominated by Cladotanytarsus iucundus and that obtained by larvae rearing (Figure 2) showed two other (genera and species) dominating species (Psilotanypus rufovittatus and Einfeldia dissidens).The study of the chironomid community by catching adults presented two major difficulties. The first resulted from the sampling of adults that did not emerge from the lake itself. The second problem involved a better sampling of the bigger and/or the more accessible swarms. This method led to an overestimate of the species richness of the lake and modified the species proportions in the community. For example, the great proportion of two species, Ablabesmyia monilis and A. longistyla, in the adult community was observed in none of the other communities (Figure 2).In both larval populations (Figure 3), the genera Chironomus, Tanytarsus, Cladotanytarsus and Procladius dominated. The use of artificial substrates induced, however, a lot of modifications in some genera contributions. For example, the genus Einfeldia was collected at higher relative density in artificial substrates than in the sediment samples. Moreover, a lot of taxa (Glyptotendipes, Microtendipes, Paratendipes, Polypedilum, Cricotopus, Parakiefferiella) were exclusively collected, in larval communities, with the artificial substrates. Despite the well-known differences between the benthic communities collected in-situ and those sampled by artificial substrates (ROSENBERG et RESH, 1982), we think such a technique allows a correct study of the taxonomic richness. The use of identical substrates in different lakes should allow one to define a potential community also related solely to the water quality.3. Characterization of the Lake AbbayeThe study of the taxonomic composition of five different chironomid communities stresses the difficulty in characterizing the lake according to the dominating species. However, comparison between larval community (Figure 3) and imagos or exuviae populations (Figure 2) led to the conclusion that Lake Abbaye could be classified in the category of lakes with Tanytarsus (specially T. niger) and Cladotanytarsus (C. iucundus and C. atridorsum). The particularity of Lake Abbaye is that two groups of species dominated the community. The first, constituted by the aforementioned three species, is dominating in oligotrophic lakes (BRUNDIN, 1949). The second group, represented by Chironomus anthracinus and Psilotanypus rufovittatus, reflected the polyhumic and hypoxic characteristics of the lake. These results indicate that Lake Abbaye has low phytoplanktonic biomass but high organic matter content in its sediment, which originates from terrestrial or littoral inputs (VERNEAUX et al., 1987). The quite low temperature of the water probably minimizes both the degradation of the organic matter and the impact of hypoxic conditions on the chironomid community.4. ConclusionThe present paper clearly demonstrates the importance of the sampling method used to study chironomid communities. Sampling the pupal exuviae in the accumulation zones is the most appropriate method to study chironomids. In order to contribute to a better understanding of the relations between the chironomid species and the environmental conditions, our further research will concentrate on a study of the bathymetric distribution of these species. For this purpose, pupal exuviae will be sampled at different isobaths simultaneously with chemical and physical analyses of the water and sediments. The drift phenomenon of exuviae that could affect the bathymetric pattern of species seemed to be avoided when a criterion of minimum occurrence was taken into account (VERNEAUX, 1996)

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

The importance of sedimenting organic matter, relative to oxygen and temperature, in structuring lake profundal macroinvertebrate assemblages

We quantified the role of a main food resource, sedimenting organic matter (SOM), relative to oxygen (DO) and temperature (TEMP) in structuring profundal macroinvertebrate assemblages in boreal lakes. SOM from 26 basins of 11 Finnish lakes was analysed for quantity (sedimentation rates), quality (C:N:P stoichiometry) and origin (carbon stable isotopes, d13C). Hypolimnetic oxygen and temperature were measured from each site during summer stratification. Partial canonical correspondence analysis (CCA) and partial regression analyses were used to quantify contributions of SOM, DO and TEMP to community composition and three macroinvertebrate metrics. The results suggested a major contribution of SOM in regulating the community composition and total biomass. Oxygen best explained the Shannon diversity, whereas TEMP had largest contribution to the variation of Benthic Quality Index. Community composition was most strongly related to d13C of SOM. Based on additional d13C and stoichiometric analyses of chironomid taxa, marked differences were apparent in their utilization of SOM and body stoichiometry; taxa characteristic of oligotrophic conditions exhibited higher C:N ratios and lower C:P and N:P ratios compared to the species typical of eutrophic lakes. The results highlight the role of SOM in regulating benthic communities and the distributions of individual species, particularly in oligotrophic systems

Classification biologique des lacs jurassiens à l'aide d'une nouvelle méthode d'analyse des peuplements benthiques I. Variété et densité de la faune

Les auteurs proposent une nouvelle méthode de classification biologique des lacs fondée sur une analyse comparative de la macrofaune des sédiments. Cette recherche inclut la définition préalable d'un protocole d'échantillonnage permettant la comparaison de systèmes lacustres de profondeur maximale très différente. Ce protocole original est caractérisé par : — des prélèvements de sédiments meubles effectués à une profondeur en zone littorale (Z0 : 1-2 m) et à trois profondeurs, relatives à la profondeur maximale des systèmes étudiés, en zone limnétique (Z1 = 0,25 Zmax, Z2 = 0,50 Zmax, Z3 = 0,75 Zmax) ; — un nombre d'échantillons par profondeur prospectée (ni) proportionnel à la longueur (L en km) de chaque isobathe (n0 = 1,8√10 L et n1, n2, n3 = 1,4√10 L). 325 stations ont été réparties, suivant un protocole standard, dans 9 lacs du Jura et 104 taxons, dont 98 genres ont été recensés. Une analyse factorielle des correspondances, appliquée aux stations et aux genres, montre que les stations se distribuent le long de l'axe F1, dans un ordre de variété faunistique décroissante et de profondeur croissante, exprimant l'évolution de la variété et de la densité en fonction des modifications des conditions écologiques avec la profondeur. Une première différenciation des lacs a été obtenue en fonction de la variété totale et de la densité globale des 9 systèmes étudiés. Ces deux critères permettent de distinguer des lacs très biogènes et des lacs peu biogènes (variété totale et densité globale faibles). Cette deuxième catégorie de lacs regroupant toutefois des systèmes dont les caractéristiques écologiques différaient nettement, une seconde classification biologique a été établie par la prise en compte : — d'un critère quantitatif : le couple variété-densité en zone littorale exprimant le potentiel biogénique des systèmes (Bo = vo √do) ; — d'un critère qualitatif : la régression de la variété générique en fonction de la profondeur relative. Une hypothèse de linéarité de la distribution bathymétrique relative de la variété faunistique permet le calcul d'indices de distribution faunistique (f¡ = kiv¡/vo) qui traduisent l'évolution des conditions écologiques avec la profondeur. Ces deux critères (Bo et f) constituent les coordonnées d'un système d'axes comportant 4 niveaux (2 niveaux quantitatifs et 2 niveaux qualitatifs) qui permettent de classer les lacs en fonction de leur aptitude biogénique

Classification biologique des lacs jurassiens à l'aide d'une nouvelle méthode d'analyse des peuplements benthiques II. Nature de la faune

Le présent article constitue la deuxième partie d'un triptyque consacré à la classification biologique (parties I et II) de 9 lacs de la Franche-Comté jurassienne en relation avec les facteurs écologiques (partie III). Cinquante-quatre taxons ont été répertoriés par l'application d'un protocole original d'analyse comparative de la macrofaune des sédiments (Verneaux et al. 1993a). Appliquées à quarante-deux taxons déterminés au genre, des analyses factorielles des correspondances permettent de sélectionner vingt-trois taxons (taxons repères) hiérarchisés par analyse de la distribution bathymétrique de leur densité relative et de leur fréquence. Utilisés pour caractériser les peuplements benthiques à 4 profondeurs (Z0 : 2 m, Z1 : 0,25 Zmax, Z2 : 0,50 Zmax, Z3 : 0,75 Zmax) ils permettent d'établir une classification des systèmes lacustres qui est alors comparée à une précédente classification obtenue par analyse de la biodiversité en zone littorale (B0 = v0 √d0) et de l'évolution de la variété avec la profondeur relative exprimée par un indice de déficit taxonomique fi = kvi/v0 (Verneaux et al. 1993a). Cette étude de la nature de la faune montre : — qu'en zone littorale, où les conditions mésologiques sont optimales, les systèmes étudiés, non pollués globalement, possèdent tous des taxons repères sensibles et se différencient essentiellement par le critère adéquat quantitatif B0, — que la modification de la nature de la faune avec la profondeur constitue un critère d'appréciation du mode de fonctionnement des systèmes susceptible de compléter utilement l'indice de déficit taxonomique fi. Ce travail souligne la nécessité d'associer, en matière de typologie, une analyse des 3 critères fondamentaux que sont la variété, la densité et la nature de la faune

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

The 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 = 2m) 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 helpfull to characterize lakes both quantitatively (oligo- to polybiotic) and qualitatively (eu- to dysbiotic). Lake Châlain (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)

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

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Lateglacial paleoenvironmental reconstruction using subfossil chironomid assemblages from Lake Lautrey (Jura, France)

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