Bruno Leporcq

Strong effects of amoebae grazing on the biomass and genetic structure of a Microcystis bloom (Cyanobacteria)

Despite its importance for bloom toxicity, the factors determining the population structure of cyanobacterial blooms are poorly understood. Here, we report the results of a two-year field survey of the population dynamics of Microcystis blooms in a small hypertrophic urban pond. Microscopic enumeration of Microcystis and its predators and parasites was combined with pigment and microcystin analysis and denaturing gradient gel electrophoresis of the ITS rDNA region to assess population dynamics and structure. Two main Microcystis morpho- and ITS types were revealed, corresponding to M. aeruginosa and M. viridis. In both years, high population densities of naked amoebae grazing on Microcystis coincided with rapid decreases in Microcystis biomass. In one year, there was a shift from heavily infested M. aeruginosa to the less-infested M. viridis, allowing the bloom to rapidly recover. The preference of amoebae for M. aeruginosa was confirmed by grazing experiments, in which several amoeba strains were capable of grazing down a strain of M. aeruginosa, but not of M. viridis. Zooplankton and chytrid parasites appeared to be of minor importance for these strong and fast reductions in Microcystis biomass. These findings demonstrate a strong impact of small protozoan grazers on the biomass and genetic structure of Microcystis blooms.

Bacterial Community Composition in Three Freshwater Reservoirs of Different Alkalinity and Trophic Status

In order to investigate the factors controlling the bacterial community composition (BCC) in reservoirs, we sampled three freshwater reservoirs with contrasted physical and chemical characteristics and trophic status. The BCC was analysed by 16S rRNA gene amplicon 454 pyrosequencing. In parallel, a complete dataset of environmental parameters and phytoplankton community composition was also collected. BCC in the analysed reservoirs resembled that of epilimnetic waters of natural freshwater lakes with presence of Actinobacteria, Alpha- and Betaproteobacteria, Cytophaga–Flavobacteria–Bacteroidetes (CFB) and Verrucomicrobia groups. Our results evidenced that the retrieved BCC in the analysed reservoirs was strongly influenced by pH, alkalinity and organic carbon content, whereas comparatively little change was observed among layers in stratified conditions.

Phytoplankton ecology of Lake Kivu (eastern Africa): biomass, production and elemental ratios

Lake Kivu, located between Rwanda and the Kivu Province (Democratic Republic of Congo), is the smallest of the East African Great Lakes. It is a deep (max. 489 m), meromictic lake, with an oxic mixolimnion up to 60 m, and a deep monimolimnion rich in dissolved gases, particularly methane (TIETZE et al. 1980, SCHMID et al. 2005). It is also the sole natural lake in which Limnothrissa miodon Boulenger, an endemic sardine from Lake Tanganyika, was introduced to fill an empty niche (COLLART 1954, SIMBERLOFF 1995). Indeed, prior to the introduction, no planktivorous fish was present in the pelagic waters of Lake Kivu. Recent studies have confirmed the oligotrophic status of Lake Kivu: with an annual average chlorophyll a in the mixed layer of 2.2 mg m–3 and low nutrient levels in the euphotic zone, it combines a relatively shallow euphotic layer (~18 m), usually smaller than its mixed layer (20–60 m), with a weak thermal gradient in the mixolimnion (ISUMBISHO et al. 2006, SARMENTO et al. 2006). The most common phytoplankton species are the pennate diatoms Nitzschia bacata Hust. and Fragilaria danica (Kutz.) Lange-Bert., and the cyanobacteria Planktolyngbya limnetica (Lemm.) Komarkova-Legnerova and Cronberg, and Synechococcus sp. (SARMENTO et al. 2006, 2007, 2008). A survey in Lake Kivu covering the vertical and temporal variations of phytoplankton was carried out during 2003–2005, with a large number of plankton primary production measurements and assessment of phytoplankton nutritional status by seston C:N:P ratios.

Identifying the factors determining blooms of cyanobacteria in a set of shallow lakes

Abstract There is a strong interest in developing a capacity to predict the occurrence of cyanobacteria blooms in lakes and to identify the measures to be taken to reduce water quality problems associated with the occurrence of potentially harmful taxa. Here we conducted a weekly to bi-weekly monitoring program on five shallow eutrophic lakes during two years, with the aim of gathering data on total cyanobacterial abundance, as estimated from marker pigments determined by HPLC analysis of phytoplankton extracts. We also determined bloom composition and measured weather and limnological variables. The most frequently identified taxa were Aphanizomenon flos-aquae , Microcystis aeruginosa , Planktothrix agardhii and Anabaena spp. We used the data base composed of a total of 306 observations and an adaptive regression trees method, the boosted regression tree (BRT), to develop predictive models of bloom occurrence and composition, based on environmental conditions. Data processing with BRT enabled the design of satisfactory prediction models of cyanobacterial abundance and of the occurrence of the main taxa. Phosphorus (total and soluble reactive phosphate), dissolved inorganic nitrogen, epilimnion temperature, photoperiod and euphotic depth were among the best predictive variables, contributing for at least 10% in the models, and their relative contribution varied in accordance with the ecological traits of the taxa considered. Meteorological factors (wind, rainfall, surface irradiance) had a significant role in species selection. Such results may contribute to designing measures for bloom management in shallow lakes.

Strong effects of amoebae grazing on the biomass and genetic structure of a Microcystis bloom (Cyanobacteria): Amoebae grazing effects on a Microcystis bloom

Despite its importance for bloom toxicity, the factors determining the population structure of cyanobacterial blooms are poorly understood. Here, we report the results of a two-year field survey of the population dynamics of Microcystis blooms in a small hypertrophic urban pond. Microscopic enumeration of Microcystis and its predators and parasites was combined with pigment and microcystin analysis and denaturing gradient gel electrophoresis of the ITS rDNA region to assess population dynamics and structure. Two main Microcystis morpho- and ITS types were revealed, corresponding to M. aeruginosa and M. viridis. In both years, high population densities of naked amoebae grazing on Microcystis coincided with rapid decreases in Microcystis biomass. In one year, there was a shift from heavily infested M. aeruginosa to the less-infested M. viridis, allowing the bloom to rapidly recover. The preference of amoebae for M. aeruginosa was confirmed by grazing experiments, in which several amoeba strains were capable of grazing down a strain of M. aeruginosa, but not of M. viridis. Zooplankton and chytrid parasites appeared to be of minor importance for these strong and fast reductions in Microcystis biomass. These findings demonstrate a strong impact of small protozoan grazers on the biomass and genetic structure of Microcystis blooms.