Effect of poor manageability and low nutritional value of cyanobacteria on Daphnia magna life history performance

Cyanobacteria may reduce Daphnia fitness through their toxicity, low nutritional value or poor manageability. In this study, the relative importance of the low nutritional value and poor manageability of cyanobacteria on Daphnia life history were determined. To separate these two aspects of cyanobacteria impact, short or long filaments of Cylindrospermopsis raciborskii were given as food to five clones of Daphnia magna either alone or together with the green alga Scenedesmus obliquus. Feeding with short cyanobacterial filaments caused reductions in growth rate (,45%) and fecundity (,60%) in some clones. The deleterious effects caused by cyanobacterial poor nutritional quality could be completely overcome by supplementation with highquality food. Long cyanobacterial filaments caused greater reductions in growth rate (,65%) and fecundity (,80%) in the most affected clones, and the addition of high-quality food only partially diminished the effects of poor filament manageability. In Daphnia fed with long filaments, there was no correlation between body size at maturity and fecundity, and the maximum fecundity was reached by individuals of intermediate size. Decreased body size may, thus, represent an adaptive mechanism against interference with food collection caused by long filaments. The pattern of responses to different food regimes was similar in all Daphnia clones, but they differed in the absolute values of the life history parameters affected. Differences in the fecundity of Daphnia caused by the presence of cyanobacteria may affect clonal composition of cladoceran assemblages, thus, cyanobacteria can shape microevolutionary changes in Daphnia populations.