Comparaison des peuplements chironomidiens du lac de l'Abbaye obtenus par différentes méthodes d'échantillonnage. Intérêts de la récolte des exuvies nymphales Comparison of the chironomid communities of Lake Abbaye (Jura, France) collected using five different sampling methods. Avantages of the pupal exuviae sampling

Résumé

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.

Mots-clés:

• Lacs,
• Chironomidae,
• exuvies nymphales

Abstract

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 community

The 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 communities

Sampling 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 Abbaye

The 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. Conclusion

The 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).

Keywords:

• Lakes,
• Chironomidae,
• pupal exuviae