Research Article

Die hard in Lake Bourget! The case of Planktothrix rubescens reborn

¹ CISALB, 42 rue du Pré Demaison 73000, Chambéry cx, France
² Université Savoie Mont Blanc, INRAE, UMR CARRTEL, 75 bis avenue de Corzent 74200, Thonon les Bains cx, France
³ Grand Chambéry, 106 allée des Blachères 73026, Chambéry cx, France

^* Corresponding author: stephan.jacquet@inrae.fr

Received: 10 June 2021
Accepted: 24 August 2021

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 thought that its proliferation was over. However, blooms occurred again in 2016 and 2017 despite apparent low phosphorus concentrations in surface waters of the lake. We have attempted to explain the reasons for this come back in order to develop scenarios helpful to stakeholders who are concerned such proliferations may occur 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 provided a minimum autumn/winter inoculum of P. rubescens was detected the year before. The subsequent bloom was observed deeper than previous years and associated with a conjunction of factors known to favour the development of this 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, a plausible scenario is proposed. One thing remains unclear: where does this cyanobacterium “hide” when it is not observed during the routine monitoring surveys and from which place it could initiate its development (nearshore, the pelagic zone, or from the sediment?).