The synergistic effects of temperature and food concentration of life history parameters of Daphnia.

Daphnia parvula experiences a wide range of thermal (4°-30°C) and food conditions during its annual population cycle. We used cohort life table experiments to determine the synergistic effects of temperature and food concentration on its life history parameters. A 3×3 factorial design experiments was conducted with animals raised at all combinations of three naturally experienced temperatures (10°, 15° and 25° C) and food levels (0.02, 0.20 and 2.0 mg C/l). D. parvula showed an increase in survivorship with decreasing temperature at all food levels. Fecundity parameters (number of broods/female, brood size and net reproductive rate) increased with increasing food at the two lower temperatures but showed a mid-range food optimum at the highest temperature. Development rates and realized rates of increase (r) showed an increase with both increasing temperature and food such that they were maximum at the highest temperature-food level treatment. The life history parameters, average lifespan, age at first reproduction, brood duration time, brood size and number of young per reproductive female all showed significant interaction between temperature and food as was suggested by trends in R o and r. Temperature had a reduced effect on fecundity, development rates and realized rates of increase at the low food level.Population birth rates of continuously reproducing zooplankton are typically calculated by the egg-ratio method (Edmondson 1960) and are based on thermally controlled development rates which neglect the effects of food limitation. Significant synergistic temperature-food effects on brood duration time and other life history parameters of Daphnia parvula suggest that food limitation and foodtemperature interaction should be considered when calculating field population birth rates. A comparison of realized rates of increase from this study with similar life table data for Daphnia parvula raised on natural food from Lake Oglethorpe indicate that synergistic effects are negligible in this eutrophic system due to abundant resources. However, synergistic effects are probably important in oligotrophic systems where resources are limited.