Roger Pourriot

Origin and development of river zooplankton: example of the Marne

Zooplankton composition and growth in the river Marne (France)were studied on a space scale of 300 km in July 1991. There werethree distinct areas along the river: the immediate reservoir outlet(pK 652), the natural river called middle Marne (from pK 652to pK 799 downstream) and the channeled river (from pK 799 topK 975 downstream). A typical lake community, characterized both byan abundance of microcrustaceans and a high zooplanktonconcentration was found immediately downstream of the reservoir Marne(Der-Chantecocq Lake). Here, large microcrustaceans (copepods,daphnids), and large rotifers (Keratella cochlearisrobusta and Polyarthra dolichoptera-vulgaris) rapidlydisappeared, and small rotifer species (<120 μm) dominated theplankton. Their populations (specially Keratella c. cochlearis)proliferated in the middle Marne as far as 100 km downstream(up to 288 ind. l-1) but were considerably reduced (20 to35 ind. l-1) where the river is channeled, algal resourcesdecline and turbidity increases.The dominance of small organisms such as rotifers, in riverplankton is assumed to be the result of fish predation on largezooplankton as well as of a short generation time which allowstheir in situ reproduction, in spite of a short residencetime of the water.

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.

Workshop on Culture Techniques of Rotifers

Culture techniques of Rotifers can be considered of two types according to whether the number of associated species is known or not (Dougherty et al., 1960). The cases of monoxenic (one associated species), xenic (unknown number of associated species) and clonal cultures will be successively examined in the following.

Relations Entre les Tintinnides et Quelques Variables Biotiques et Abiotiques d'un Lac de Gravière

Summary Seasonal fluctuations of Tintinnopsis lacustris have been studied throughout 8 consecutive years. Their depth distribution has also been investigated during one of these years. Simultaneous collected data on temperature, concentration of dissolved oxygen and chlorophyll a, phytoplankton abundances, and, also during one year, bacterial biomass allowed to examine relationships between the abundance of tintinnids and these variables. Although tintinnids avoid anoxic water at bottom, temperature and dissolved oxygen did not influence their distribution within the water column. In winter, the scarcity of predators and competitors, more than the abiotic factors, often permitted the development of a large population of Tinitinnopsis lacustris . The doubling time of the population decreased indeed when the temperature increased. The abundance of T. lacustris in relation to algal and bacterial biomass tends to show that the tintinnids would only consume a part of the nannophytoplankton (Cryptophyceae species in particular).