Influence of food density on Daphnia pulex life history during a multigenerational exposure to external gamma radiation.

The present study examined the combined effects of food availability and chronic gamma radiation, a multiple stress remaining scarcely explored. Several experiments aimed: to measure changes in life history in a well-studied Daphnia pulex clone (TCO) on a range of four food levels (from low to saturating), in order to select two contrasting food levels (namely limiting and optimal) in our experimental setup; to investigate life history responses to chronic external 137Cs gamma radiation (from 5.0 µGy h-1 to 28.6 mGy h-1) for three parthenogenetic generations, at the two selected food levels; to quantify potential radiation effects on algal growth (without daphnids) in the two phytoplankton species used as food for D. pulex. Our results showed that food availability strongly dominated D. pulex responses in every experiment. First, survival, growth and neonate production significantly differed among food levels in unexposed daphnids. Second, lethal and sublethal radiation effects appeared largely weaker than anticipated from literature knowledge in Daphnia spp., especially in the higher range of dose rates (≥ 4.85 mGy h-1) after several generations. Third, a significantly greater algal growth was detected with increasing dose rate in Raphidocelis subcapitata (an algae with a greater nutritional quality), reaching + 7% at 0.551 mGy h-1, + 22% at 4.85 mGy h-1, + 36% at 26.8 mGy h-1 (after 24 h). At the same time, Chlamydomonas reinhardtii (an algae with a poorer nutritional quality) showed no significant change in algal growth with increasing dose rate. Without measurements of algal densities during daphnid irradiation, one could hypothesize that a radio-induced increase in food quantity and quality, as observed in R. subcapitata, might compensate for energy costs associated with chronic gamma radiation and account for the lack of significant life history effects in daphnids.