International audienceLac du Bourget, in the Alps, is the largest natural French lake. Following major restoration programmes during the 1970s and early 1980s, involving massive efforts to reduce nutrient loads and pollution in the lake, the water quality has improved over the past two decades. This can be inferred from the increase in the nitrate: phosphate ratio, the intensification of the "clear-water phase" (i.e. the increase in the water column transparency in spring), and the reduction in the total phosphorus and chlorophyll a concentrations. However, blooms of the filamentous, phycoerythrin-rich, non-nitrogen fixing and hepatotoxic cyanobacterium Planktothrix rubescens have occurred since 1996 and have been maintained subsequently, at least during summer and autumn periods. Nutrients (especially phosphorus) are usually thought to be one of the most important factors responsible for cyanobacterial blooms, and so the question is asked if this bloom is a paradoxical outcome of the restoration programs? Using a large set of data taken from surveys of Lac du Bourget, from the literature, and from recent laboratory experiments, and also using field data for the neighboring Lake Geneva, we propose a realistic scenario to account for the population dynamics of the cyanobacterium and the occurrence and maintenance of the bloom in Lac du Bourget. The characteristics of the lake (high water column stability, deepening of the nutrient-depleted layer during the last decade, a long water residence time), local conditions (the nutrient load and charge) and global changes (global warming) all had to be taken into account to explain this bloom. We suggest that the success of P. rubescens in Lac du Bourget is probably due to increased transparency and a longer stratified period following (1) the restriction of other phytoplankton species following reduced phosphorus, which has allowed P. rubescens to make use of organic phosphorus to improve its competitiveness; (2) warmer than average winter/spring periods allowing an earlier water stratification and in fine a competitive advantage to P. rubescens; (3) lower than average surface irradiance, which has also given the low-light preferring P. rubescens an advantage. Finally, this study highlights the importance of long-term data sets in attempting to elucidate the global causes of a major ecological problem (such as this cyanobacterial bloom) and impacts with regard to the function and use of freshwater ecosystems. (c) 2004 Elsevier B.V. All rights reserved

A recently available submersible fluorescent probe was configured and used to survey the vertical distribution of the deep-living toxic and filamentous cyanobacterium Planktothrix (Oscillatoria) rubescens among the autochthonous algal communities in Lake Bourget, France. This in situ measuring spectrofluorometer, which can be used to perform chlorophyll analysis and integrated algal class determination, provides a realistic estimation of the abundance and dynamics of the cyanobacterial population that is known to produce the hepatotoxic heptapeptides microcystin RR and LR. Data provided from in-line measurements using the probe and from P. rubescens cell counts obtained by discrete sampling were closely correlated (r = 0.899, p < 0.01), as were in-line data and spectrophotometric total chlorophyll a measurements (r = 0.775, p < 0.01). A survey conducted from December 1999 to May 2001 revealed that P. rubescens exhibits a deep maximum level (typically between 10 and 15 m) in spring and summer (reaching concentrations of up to 20 µg equivalent chl a l -1 , i.e. 27 000 cells ml -1 ), whereas it spreads from the surface either to the top of the thermocline or to the bottom of the lake, in autumn and winter respectively. We propose that the probe could be used as a powerful tool for assaying the occurrence and dynamics of microalgal blooms, typically toxic cyanobacteria, that call for accurate and rapid monitoring to assess the health of the ecosystem and to alert the authorities about potential risks regarding pumping and use of the lake water for drinking-water production purposes.KEY WORDS: Cyanobacteria · Fluorescence · Bloom · Monitoring · Water supply Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 30: [83][84][85][86][87][88][89] 2002 the organisations that are responsible for these surveys. Hence it is of crucial importance to develop tools to make it possible to carry out rapid and reliable screening for potential contaminants in water. Molecular fingerprinting is often proposed as an alternative method for identifying potentially toxic species, but it involves delay, and still calls for discrete sampling of water and skilled personnel to perform the analysis. Consequently, any system that can cope with the constraints associated with water monitoring, such as immediacy of response, low maintenance costs, specificity, sensitivity, ease of handling, and rapidity of interpretation, is bound to be of interest. The in vivo auto-fluorescence characteristic of pigment-containing microorganisms, such as cyanobacteria and microalgae, has long been envisaged as a potential diagnostic signal (e.g. Yentsch & Yentsch 1979, Yentsch & Phinney 1985, provided that differences in pigment compositions among these phytoplankters lead to differences in fluorescence patterns. A new multi-wavelength submersible probe (FluoroProbe, bbe-Moldaenke) has recently been made available to scientists (Beutler et al. 2002). We used this tool, after simple reconfiguration, to mon...

We investigated the dynamics and diversity of heterotrophic bacteria, autotrophic and heterotrophic flagellates, and ciliates from March to July 2002 in the surface waters (0-50 m) of Lake Bourget. The heterotrophic bacteria consisted mainly of "small" cocci, but filaments (>2 microm), commonly considered to be grazing-resistant forms under increased nanoflagellate grazing, were also detected. These elongated cells mainly belonged to the Cytophaga-Flavobacterium (CF) cluster, and were most abundant during spring and early summer, when mixotrophic or heterotrophic flagellates were the main bacterial predators. The CF group strongly dominated fluorescent in situ hybridization-detected cells from March to June, whereas clear changes were observed in early summer when Beta-proteobacteria and Alpha-proteobacteria increased concomitantly with maximal protist grazing pressures. The analysis of protist community structure revealed that the flagellates consisted mainly of cryptomonad forms. The dynamics of Cryptomonas sp. and Dinobryon sp. suggested the potential importance of mixotrophs as consumers of bacteria. This point was verified by an experimental approach based on fluorescent microbeads to assess the potential grazing impact of all protist taxa in the epilimnion. From the results, three distinct periods in the functioning of the epilimnetic microbial loop were identified. In early spring, mixotrophic and heterotrophic flagellates constituted the main bacterivores, and were regulated by the availability of their resources mainly during April (phase 1). Once the "clear water phase" was established, the predation pressure of metazooplankton represented a strong top-down force on all microbial compartments. During this period only mixotrophic flagellates occasionally exerted a significant bacterivory pressure (phase 2). Finally, the early summer was characterized by the highest protozoan grazing impact and by a rapid shift in the carbon pathway transfer, with a fast change-over of the main predators contribution, i.e., mixotrophic, heterotrophic flagellates and ciliates in bacterial mortality. The high abundance of ciliates during this period was consistent with the high densities of resources (heterotrophic nanoflagellates, algae, bacteria) in deep layers containing the most chlorophyll. Bacteria, as ciliates, responded clearly to increasing phytoplankton abundance, and although bacterial grazing impact could vary largely, bacterial abundance seemed to be primarily bottom-up regulated (phase 3).

Summary 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.

Large, sub-alpine, stratified lakes are directly within the scope of the European Water Framework Directive (WFD) and need adapted monitoring systems. Moreover, anthropogenic eutrophication was frequently the main cause of their water quality degradation in the 20th century. This paper is primarily aimed at demonstrating how in situ sensor-equipped buoys could be the base of monitoring designs to support the WFD objectives. The core of this paper, mainly methodological, focuses on single-depth, high frequency (4 per hour) fluorescence measurements. It shows that the internal wave pattern provides additional information to the singledepth time series to assess phytoplankton dynamics in a stratified water column displaying strong, vertical biomass heterogeneity. The paper deals with the following three aspects: (1) definition of an indicator to determine whether or not the sensor actually

Like many temperate European lakes, Lake Bourget (France) has suffered from eutrophication during the second half of the last century. Due to a remarkable policy restoration program, the lake has been recovering for the past 25 years after a massive decrease in total phosphorus (TP) loading. TP concentrations have decreased from about 100-120 to 20-25 µg/L. Additional efforts are, however, still required to obtain a perennially sustainable good ecological status and model parameterisation of fluxes can assist in predicting future outcomes, especially in the context of global warming. In this paper, a dynamic model (MeroLakeMab) was developed and tested with the purpose to reconstructthe loading history of Lake Bourget and to predict TP concentrations during scenarios of increased temperature, decreased water runoff and decreased P loading. Simulations suggested that the historical TP loading decrease may have been as extensive as 90%. Decreases in water discharge to Lake Bourget at magnitudes forecasted by the Intergovernmental Panel on Climate Change would not affect TP concentrations notably, but marked concentration changes could, however, occur if decreases in runoff would have a strong impact on the TP loading. Increasing temperature effects on yearly mean TP concentrations in the water column would be very small compared to effects from changes in the TP loading.Predictions such as these could be instrumental for future successful lake management.List of Abbreviations. Additional explanatory material is provided in the article text.