Population dynamics of three gammarid species (Crustacea, Amphipoda) in a French chalk stream. Part II. Standing crop

The standing crop of Gammarus pulex pulex (Linnaeus, 1758), G. fossarum Koch in Panzer, 1836 and Echinogammarus berilloni (Catta, 1878) has been studied in a small French chalk stream, the Slack. A brief description of all amphipod species encountered in this river is given, with a key to different life stages. Peaks in abundance could not be explained by population explosions. In spring regions and headwaters population density was highest. The gammarids showed a rather constant zonation in the Slack: G. p. pulex in the unstable spring region; G. fossarum in the upper and middle reaches, confined to the less polluted parts of the latter; and E. berilloni in the middle and lower region of the Slack, where fluctuations in water temperatures increased. Life histories of all three species revealed great inter- and intraspecific differences. Data on size, growth rate, sexual activity, population structure, sex ratio, fecundity and parasitation yielded ample illustrations of these dissimilarities. Laboratory experiments on egg incubation time and reproduction success demonstrated that temperature affects the three species differently. Both seasonal variation and between-year differences in life processes were investigated. The impact of temperature was manifestly important in this aspect too.