This paper presents the recent history of a large prealpine lake (Lake
Bourget) using chironomids, diatoms and organic matter analysis, and
deals with the ability of paleolimnological approach to define an
ecological reference state for the lake in the sense of the European
Framework Directive. The study at low resolution of subfossil
chironomids in a 4-m-long core shows the remarkable stability over the
last 2.5 kyrs of the profundal community dominated by a
Micropsectra-association until the beginning of the twentieth century,
when oxyphilous taxa disappeared. Focusing on this key recent period, a
high resolution and multiproxy study of two short cores reveals a
progressive evolution of the lake's ecological state. Until AD 1880,
Lake Bourget showed low organic matter content in the deep sediments
(TOC less than 1%) and a well-oxygenated hypolimnion that allowed the
development of a profundal oxyphilous chironomid fauna
(Micropsectra-association). Diatom communities were characteristic of
oligotrophic conditions. Around AD 1880, a slight increase in the TOC
was the first sign of changes in lake conditions. This was followed by a
first limited decline in oligotrophic diatom taxa and the disappearance
of two oxyphilous chironomid taxa at the beginning of the twentieth
century. The 1940s were a major turning point in recent lake history.
Diatom assemblages and accumulation of well preserved planktonic organic
matter in the sediment provide evidence of strong eutrophication. The
absence of profundal chironomid communities reveals permanent
hypolimnetic anoxia. From AD 1995 to 2006, the diatom assemblages
suggest a reduction in nutrients, and a return to mesotrophic
conditions, a result of improved wastewater management. However, no
change in hypolimnion benthic conditions has been shown by either the
organic matter or the subfossil chironomid profundal community. Our
results emphasize the relevance of the paleolimnological approach for
the assessment of reference conditions for modern lakes. Before AD 1900,
the profundal Micropsectra-association and the Cyclotella dominated
diatom community can be considered as the Lake Bourget reference
community, which reflects the reference ecological state of the lake.