THE DRAINING OF A LARGE RESERVOIR CONSIDE RED AS AN EXPERIMENTAL BIOMANIPULATION - FIRST RESULTS - FISH CONTROL ON THE STRUCTURE AND BIOCHEMICAL-COMPOSITION OF PLANKTON

The structural evolution of zooplankton and the biochemical compositio n of plankton in Lake Pareloup (Department of Aveyron, France) was stu died for a two year periods : during the progressive draining of this reservoir in 1993, and during the recolonisation phase in 1994. The pl anktonic crustacean are essentially composed of Daphnia longispina, Ce riodaphnia quadrangula, Bosmina longirostris, Cyclops vicinus vicinus, Acanthocyclops robustus and Eudiaptomus vulgaris. The Cladocerans dom inated the population during winter and in early spring 1993. In 1994, their development was maximal during May and they remained proportion ally well represented in summer. The Cyclopoids constituted the main p art of the community in june 1993 as well as at the beginning of the r ecolonisation phase (autumn winter 1993-1994). The maximum average cru stacean density (67 ind./lit) was observed in October 1993 just after the water filling. The potential impact of the zooplankton on the phyt oplanktonic community reveals the preponderant role of small grazers f rom January to June 1993. However, large grazers with an average of 67 % of the potential volume of filtered water represented the prime reg ulators of algal populations from October to June 1994. The average bi omass in spring 1993 was about 59 mu g DW/liter whilst the one recorde d from the same season in 1994 was about 103 mu g DW/liter. A signific ant increase of lipid and protein content of plankton was noted in 199 4. The decrease of the control effected by planktophagous fish, elimin ated during the draining of the reservoir (80 tons) explains the varia tions of structure and biochemical composition of the planktonic popul ation.