Danielle Benest

Seasonal coupling between phyto- and bacterioplankton in a sand pit lake (Créteil lake, France)

Phyto- and bacterioplankton biomass and activity were simultaneously measured during the course of one year in the shallow Créteil Lake (France).Phytoplankton was dominated, during the whole year, by small-sized organisms (10 to 25 µm). Bacteria were in a majority small coccoids (<0.3 µm). Phyto -and bacterioplankton abundances averaged respectively 3.3 × 106 cells l−1 and 6 × 109 cells l−1.The phasing of the activity and biomass periods suggest a close coupling between phyto- and bacterioplankton. There were two distinct periods of high activity and biomass. Maximal values were observed in summer but an early increase occurred also in winter. Low or undetectable phytoplankton excretion rates, when heterotrophic activity was maximum, indicated a bacterial uptake of up to 100% of the released algal products during the incubation period. Heterotrophic uptake measurements with both glucose and amino acids revealed a seasonal change of the substrates in the lake, glucose uptake being associated more with the maximum activity of the algae, while the amino acids uptake was relatively higher during their decline.The maximal photosynthetic rate averaged 21.5 mgC m−3 h−1 and mean Vmax values were 0.056 and 0.050 mgC m−3 h−1 respectively for glucose and amino acids uptake.

Hatching of Brachionus Rubens O. F. Müller Resting Eggs (Rotifers)

After a dormant period at low temperature (5°C) and darkness, hatching of Brachionus rubens resting eggs is induced by an increase of temperature (10–22°C) in presence of light.

Planktonic bacterial biomass and seasonal pattern of the heterotrophic uptake and respiration of glucose and amino acids in the shallow sandpit lake of Créteil (Paris Suburb, France)

Biomass and activity of planktonic bacteria were investigated during a one year study in a shallow sandpit lake. The shallowness of the lake helped keep the water column homogeneous regarding bacterioplankton. Small free-living bacteria (0.03 µm3 cell−1) dominated the populations throughout the period studied. Bacterial abundances varied from 1 to 11 × 106 cells ml−1. Kinetic parameters (Vmax, K + S and T) were determined with 14C labelled compounds (glucose and amino acids mixture). Vmax values were high and averaged 0.056 and 0.050 µgCl−1 h−1 for glucose and amino acids respectively. Maximal Vmax values were observed in summer at the highest temperatures, but also in early spring. T values were much greater in winter. K + S values were significantly higher for amino acids (3 µg Cl−1) than for glucose (1 µg Cl−1). A low percentage of mineralization (about 25% for both tracers) could be the expression of the high growth efficiency expected when bacteria are growing at the expense of low molecular weight compounds as phytoplankton exudates.

Caractères taxinomiques et spécificité dans le genreAsplanchna (Rotifère)

A comparative analysis of eight characters in two clones ofA. brightwelli and a clone ofA. intermedia, fed withParamecium orBrachionus, is made to determine the intraclonal, interclonal and interspecific variations in this complex of species.The trophi morphology appears the most species specific criterium whereas the other features (size of males, diameter and ornamentation of resting eggs, nuclear number in syncytial glands) show variability, either genetic or related to environmental conditions. The variation observed in the ornamentation of resting eggs and in the nuclear number of vitellogen was distributed along a continuous gradient.

Direct and indirect effects of biomanipulations on periphyton stoichiometry in shallow lakes

Periphyton is a complex mixture of algae, heterotrophic organisms (bacteria, fungi, protozoa), and dead organic matter, attached on diverse substrates in aquatic ecosystems (WETZEL 1983). In littoral regions of shallow lakes, periphyton can represent high biomasses, and several studies have shown that both benthic and pelagic food webs may strongly rely on this resource (e.g., VANDER ZANDEN & VADEBONCOEUR 2002). In the context of eutrophication of freshwater ecosystems, many lakes have switched from a clear water state, with dominance of aquatic macrophytes and high piscivorous/benthivorous-planktivorous ratios, to a turbid water state, with high biomass o f phytoplankton and dominance of benthivorous/planktivorous fishes. Biomanipulations (e. g., manipulations offood web structures by modifying fish communities) can induce a shift from stable turbid state to clear water (PERRow et al. 1997). To maintain this clear water phase, a good understanding of the processes that stabilize systems in this state is essential. In particular, the ro le of periphyton, which competes with phytoplankton and macrophytes for inorganic nutrients, light and C027 must be taken in consideration (JONES et al. 2002). Few works have studied the effect o f food-web structure o n periphyton development. By modifying grazer communities and characteristics oflakes, such as light an d nutrient availability, biomanipulations may greatly affect periphyton in many ways. In particular, following ecological stoichiometry principles, induced modifications of periphyton elemental composition (C:N:P ratios) could greatly affect some important ecological processes, such as grazer-macrophyte-periphyton interactions (CRoss et al. 2005). We analyzed differences in biomass and stoichiometry of the periphytic compartrnent in mesocosms simulating food web structure of nonbiomanipulated and biomanipulated lakes.

Influence of light attenuation on primary production in a shallow sand-pit lake

(1991). Influence of light attenuation on primary production in a shallow sand-pit lake. SIL Proceedings, 1922-2010: Vol. 24, No. 3, pp. 1536-1542.