The effects of Chilomonas on the life history traits of Daphnia longispinaunder semi-natural conditions and the implications for competition in the plankton

Assuming that heterotrophic flagellates (H.F.) can sustain cladoceran life cycles, particularly at periods of low food and high detritus conditions, t he growth, reproduction and life span of Daphnia longispina were studied un der conditions of summer food limitation. They were fed both natural resour ces and natural food enriched with a culture of the colourless Chilomonas ( ovoid cell, 8 x 25 mu m). Four H.F. morphotypes occurred in the natural wat er and the first experiments with Daphnia, showed that the cladoceran would most easily ingest those of 5 mu m-10 mu m, while the addition of Chilomon as severely depressed the H.F. of 2 mu m. The capability of this flagellate to ingest small H.F. was confirmed using fluorescent particles ranging fro m 0.94 to 3.95 mu m. As a consequence, Daphnia could control the abundance of H.F., but also compete with the largest morphotypes. To study the influe nce of the H.F. on the life span of Daphnia, the cladoceran was acclimated for two generations, before the start of the experiments. In semi-natural c onditions, constant temperature, without predators and fed natural water, t he summer daphnids achieved smaller sizes, produced fewer offspring and liv ed for a shorter time than when Chilomonas was added to their diet. Despite the addition of 560 mu g C l(-1), this was not sufficient to enhance the D aphnia reproduction to the level of the spring population fed natural water . These results clearly illustrate that the heterotrophic flagellates contr ibuted significantly to improving the reproduction and survival of the daph nids. They emphasise the importance of the microbial loop as a link to larg er consumers, particularly when unpalatable or nutritionally inadequate alg ae are dominant, which is often so in lakes during summer.