PHYTOPLANKTON, CILIATE AND ROTIFER DEVELO PMENT AT 2 STATIONS IN THE MIDDLE LOIRE RIVER (FRANCE), DURING A PERIOD OF LOW WATER-FLOW

Planktonic ciliates are numerically an important component of both nan o-and micro-plankton assemblages in standing waters, but little attent ion has been paid to their importance in rivers. To know their role in the middle Loire where microalgae and rotifers can be numerous, the p otamoplankton was studied fortnightly from June to October 1996, a per iod of low water flow. In 1996, excavation works has increased the sus pended matter downstream, while chlorine was used upstream. The increa se in dissolved oxygen, suspended matter, BOD5 and chlorophyll a, asso ciated with the decrease in nutrients downstream, are typical of eutro phic rivers. Algae, with Chlorophytes and Bacillariophytes, were the p rincipal components. Their density increased from the upper site to th e lower, with respective maxima of 50.10(6) cells.l(-1) and 70.10(6) c ells.l(-1). Following algal growth, a large ciliate community develope d, with Oligotriches and Peritriches, with densities up to 6200 cells. l(-1) and 14 340 cells.l(-1) respectively. Simultaneously the rotifers increased, with Brachionids and Trichocercids up to 3610 ind.l(-1) an d 1780 ind.l(-1) from upstream to downstream. In contrast to previous studies, their density decreased downstream, a consequence of excavati on works. As in standing waters, the enrichment in organic matter is r esponsible for the increase in algae and bacteria consumers. The ratio of total heterotrophic to autotrophic biomass (WA ratio) decreased fr om 0.34 upstream to 0.17 downstream. Rotifers, whose downstream decrea se, may be attributed to excavation works and was accentuated by inver tebrate predation, were one of the causes of the increase of green alg ae at the lower site. The rotifer carbon biomass was globally higher t han that of the ciliates; but from the end of summer, ciliates played an important role in the functioning of this lowland river.