L'évolution comparée des biocénoses rotatoriennes du Léman et du Lac de Constance montre un parallélisme remarquable lors des changements continus de l'état trophique, tant avant qu'après le niveau maximal d'eutrophisation atteint par ces deux lacs. La comparaison avec un certain nombre d'autres milieux soumis également à un changement de leur niveau trophique met en évidence une variation très nette des effectifs des espèces méso-eutrophes et eutrophes en fonction de la progression ou de la régression de l'eutrophisation; ces changements sont beaucoup moins marqués en ce qui concerne l'association des espèces oligo-mésotrophes.
Evolution of rotifer biocenosis during changes of the trophic state in Lake Geneva and comparison with Lake Constance
Eutrophication of a lake must be expected to cause both abiotic and biotic responses. The changes in the trophic state of a lake certainly occur at all trophic levels, but to different extents depending on the character of the ecosystem; the subsequent evolution of the rotifer biocenosis is one of the most pronounced features occurring as an indirect consequence of eutrophication.
In the last three decades, the rotifer community in Lake Geneva has presented important changes in structure, as a result of continuous changes in the trophism of the lake. This study covered a period of time sufficient to show possible true modifications of the community structure.
Lake Geneva has undergone anthropogenic eutrophication since the 1950's. The lake reached its highest nutrient level during 1979-1980, which has decreased though since 1981 with the improvement of phosphorus removal in sewage treatment plants. A similar trend was observed in Lake Constance, with more important quantitative changes connected with previous results dating from the oligotrophic stage of this lake.
Table 1 gives the variations in abundance of rotifer species observed during the eutrophication increase and during the decrease of the eutrophication level in some lakes of different trophic status.
Figure 1 indicates the quantitative changes of the main rotifer species in Lake Geneva and Lake Constance. Compared to the maximal trophic level (indicated by arrow), data from Lake Geneva for 1959-1987 are similar to those obtained in Lake Constance between 1963 and 1978, if the previous data obtained for this lake during its oligotrophic stage are excluded.
Figure 2 presents the general trend observed for the different trophic indicator groups during the increase or decrease of eutrophication. Throughout the eutrophication process, these groups exhibited a general increase for most of the different species, with the ratio "number of increasing species/number of decreasing species" growing from the oligo-mesotrophic group to the eutrophic one. During the decrease of eutrophication, the strength of meso-eutrophic and eutrophic indicator groups diminished more than that of the oligomesotrophic group.
Many species have appeared during the eutrophication increase, mainly belonging to meso-eutrophic and eutrophic indicator groups, together with the settlement of some oligo-mesotrophicspecies (Ascomorpha saltans, Synchaeta oblonga, Notholca caudata). The proportion of meso-eutrophic and eutrophic species increased in the two lakes, but there is a trend in populations of the oligo-mesotrophic species to decrease, leading to a precocious disappearance of Ploesoma hudsoni and later of Ascomorpha ovalis, Gastropus stylifer, Ploesoma truncatum in Lake Constance, and of Synchaeta tremula and Notholca foliacea in both lakes.
Decreasing eutrophication reduces the number of some mesoeutrophic (Trichocerca longiseta and T. pusilla in Lake Geneva, T. rousseleti in Lake Constance) and eutrophic species (Anuraeopsis fissa and Trichocera cylindrica in Lake Geneva, Pompholyx complanta in Lake Constance). However, the water quality has mot improved enough to allow a further increase in oligo-mesotrophic species, except for Conochilus unicornis in both lakes, and for Ascomorpha ecaudis, Kellicottia longispina and Synchaeta oblonga in Lake Geneva.
Since 1981, the oligo-mesotrophic indicator group is numerically dominant in Lake Geneva (BALVAY and LAURENT, 1989c), preceding the euryecious one (Asplanchna priodonta + Keratella cochlearis), white the strength of the meso-eutrophic group decreases and the eutrophic one remains almost steady.
- trophic state,
- indicator species,
- Lake Geneva,
- Lake Constance