Unprecedented changes in mollusc assemblages of the littoral zone in the two largest peri-Alpine French lakes since the last 4500 years

This paper presents changes in mollusc assemblages in the two largest peri-Alpine French lakes, i.e. Lakes Bourget and Annecy, in eastern France during the late Holocene. It is based on new mollusc data obtained in Lake Bourget: i) from three sediment cores spanning the late Holocene period; and ii) from modern samples taken around 1930 and in 2001. These new results are compared with those obtained from the neighbouring Lake Annecy and published in a previous paper. Results show that the modern period is characterised by unprecedented changes in the mollusc assemblages in Lakes Bourget and Annecy over the last 4500 years. The main drivers of the observed changes may be related to: i) the recent arrival of native and exotic species; ii) the considerable regression of lacustrine macrophytes following increased anthropic pressure on these ecosystems from the 1940s onwards; and iii) modifications of the nature and quality of the fine littoral sediments of Lake Bourget, probably linked to the regulation of its water level. In addition, the arrival of new species has led to the creation of no-analogue communities, i.e. which consist of species that are extant today, but in combinations not found until now. As a result, a return to the initial state in terms of community structures is thus difficult to envisage. On the contrary, re-establishing the dominance of charophytes and thus the dominance of gastropods, characteristic of the littoral malacocenoses of the Holocene, is a conceivable objective. Nonetheless, the persistence and extension of the period of hypoxic hypolimnion associated with warmer winters expected in the northern hemisphere could affect the recovery process negatively.