Die hard in Lake Bourget! The case of Planktothrix rubescens

Abstract Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was clearly thought that such a proliferation was over. However, against all odds, blooms occurred again in 2016 and 2017 despite apparent very low phosphorus concentrations in surface waters of the lake. Aims of this study were thus to explain the reasons of this come back in order to propose scenarios likely to be helpful to stakeholders who need to know if such proliferations may occur again in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium since a minimum autumn/winter inoculum of P. rubescens was detected the year before. Then, the bloom, that was observed deeper than previous years, was associated to a conjunction of factors already well-known to favour the development of this very competitive species (i.e. mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc…). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, not observed, measured or taken into account here, a plausible scenario could be proposed and may be useful to deciders. One thing remains unclear: where do the cyanobacterium hides when it is not observed during the routine monitoring survey and thus from which place it could initiate its development (nearshore, in the pelagic zone, from the sediment?), unless it is simply not sampled and observed due to methodological bias.