ORIGIN AND DEVELOPMENT OF RIVER ZOOPLANKTON - EXAMPLE OF THE MARNE

Zooplankton composition and growth in the river Marne (France) were st udied on a space scale of 300 km in July 1991. There were three distin ct areas along the river: the immediate reservoir outlet (pK 652), the natural river called middle Marne (from pK 652 to pK 799 downstream) and the channeled river (from pK 799 to pK 975 downstream). A typical lake community, characterized both by an abundance of microcrustaceans and a high zooplankton concentration was found immediately downstream of the reservoir Marne (Der-Chantecocq Lake). Here, large microcrusta ceans (copepods, daphnids), and large rotifers (Keratella cochlearis r obusta and Polyarthra dolichoptera-vulgaris) rapidly disappeared, and small rotifer species (<120 mu m) dominated the plankton. Their popula tions (specially Keratella c. cochlearis) proliferated in the middle M arne as far as 100 km downstream (up to 288 ind. 1(-1)) but were consi derably reduced (20 to 35 ind. 1(-1)) where the river is channeled, al gal resources decline and turbidity increases. The dominance of small organisms such as rotifers, in river plankton is assumed to be the res ult of fish predation on large zooplankton as well as of a short gener ation time which allows their in situ reproduction, in spite of a shor t residence time of the water.