Cyanobacterial bloom termination: the disappearance of Planktothrix rubescens from Lake Bourget (France) after restoration

Like many large freshwater ecosystems in Europe, Lake Bourget suffered from eutrophication during the second part of the 20th century and since the 1980s has been partially restored by reductions in nutrient loadings. Here, we analyse a data set comprised of field measurements of physicochemical and biological variables in Lake Bourget covering the period from 2004 to 2011 and complement this data set with laboratory experiments, to gain an understanding of the changes in phytoplankton community structure during recent years and drivers of these changes. Between 1995 and 2008, Lake Bourget was characterised by the proliferation of the red-coloured filamentous and toxic cyanobacterium Planktothrix rubescens, comprising 34.1-52.6% of the total phytoplanktonic biomass between 2004 and 2008. In 2009, although the contribution of P.rubescens to the total biomass was still considerable (25.3%), it was significantly lower (P<0.05) compared with previous years. The cyanobacterium disappeared completely during the autumn to winter transition of 2009/2010 and has not been recorded since this time. Concomitantly, total phytoplanktonic biomass declined sharply and a new phytoplanktonic community occurred consisting predominantly of mixotrophic genera, such as Dinobryon spp., Rhodomonas, Cryptomonas and a variety of different diatoms such as Stephanodiscus, Cyclotella and Fragilaria. Our findings suggest declines in phosphorus concentration as a key variable in bloom termination, although a number of other factors could also be important, such as temperature-dependent water column mixing, light availability, zooplankton grazing and seasonal cyanobacterial inoculums.