δ13C signatures in chironomids remains as a proxy for past changes in lake functioning : Lake Annecy as a case study
Résumé
The benthic food web, especially chironomids, is increasingly considered in whole lake functioning as a significant contributor of carbon (energy) to the highest trophic level. We studied high resolution temporal evolutions (150 years) of chitinous remains of chironomids δ13C extracted from two sediment cores (30 and 60m) in Lake Annecy. Due to the high number of remains and recent advances in spectrometry, δ13C temporal evolution was conducted for each of the main taxa of the chironomid communities. The lake has been enriched by nutrients and reached a mesotrophic state at ca 1940. A remediation plan implemented in 1967 decreased nutrients in the pelagic zone from the 80s until the present. The aim of the study was to understand temporal changes in lake carbon flow during its recent history. From ca 1850 to ca 1930s δ13C of Micropsectra contracta and Sergentia coracina were constant at around -32‰. From ca 1930 until today, δ13C constantly decreases as low as -38‰. These changes in δ13C chironomids remains occurred simultaneously with changes in chironomids communities studied at the same time. The progressive 13C depletion of M. contracta and S. coracina resulted in an increasing amount of respired carbon assimilated in the trophic foodweb to which they belong. The implication of an increase of the methanogenic-methanotrophic trophic pathway is discussed. The divergence between δ13C of chironomids remains and the biological and chemical characteristics in the pelagic zone monitored since ca 1980s highlights the importance of considering the benthic food web to reconstruct the whole lake ecological functioning.These results reinforce the existing link between the chironomids assemblage characteristics and functional properties in lakes.