Somatic and population growth responses of Ceriodaphnia dubia and Daphnia pulex (Cladocera) to changes in food (Chlorella vulgaris) level and temperature.

We quantified the combined effects of three food levels (0.01 X106, 0.1 X106 and 1.0X106 cells ml-1) and three temperature levels (15, 20 and 25oC) on the somatic and the clonal population growth of two cladocerans Ceriodaphnia dubia and Daphnia pulex for 24 to 42 days. The somatic growth data showed that both the cladoceran species began to increase their body size within 3 days and remained stable beyond a week. Food density had a significant effect on the body size of C. dubia while for D. pulex, food level and temperature as well as their interaction had significant effect on the body size. The population growth of both the cladoceran species was affected by temperature. At 15oC, C. dubia showed little growth at the highest food density, while at low food level, there was some increase in the population abundances, especially after a long lag phase of about 2 weeks. At 15oC and under the lowest tested food level, the population of D. pulex did not increase beyond the inoculation density. Optimal population growth of D. pulex was observed at intermediate temperature and at 1.0X106 cells ml-1 of food level while for C. dubia 25oC and the highest food level yielded highest growth. At low food level and low temperature, the rate of population increase (r) of C. dubia was much lower than that of D. pulex at comparable conditions. On the hand, at higher food levels and temperature regimes, the r of D. pulex was much higher than that of C.dubia. Regardless of the food levels and temperature, the r of C. dubia varied from 0.03 to 0.07 while that of D. pulex varied from 0.01 to 0.08 day-1.

Life table demography of Ceriodaphnia dubia (Cladocera) exposed to copper at different levels and periods.

We studied the demographic responses of Ceriodaphnia dubia exposed to copper at 3 different levels (0.1, 0.2 and 0.4 mg l(-1)) and at 4 exposure periods (3, 6, 12 and 24 hr) in addition to controls (without the heavy metal). The tested levels of Cu concentration and the chosen periods of exposure affected some demographic variables (average lifespan, gross and net reproductive rates and generation time). Depending on the heavy metal concentration and the period of exposure, the average lifespan of C. dubia varied from 18 to 24 day Net reproductive rates (NRR) ranged from 50 to 80 offspring per female per lifespan, while the rate of population increase (r) varied from 0.30 to 0.34 per day Though gross reproductive rates (especially at low and intermediate copper levels and shorter duration of exposure) showed significantly higher values than controls, both NRR and r were either unaffected or decreased. It is therefore necessary to regulate the release of untreated industrial effluents containing copper into freshwater ecosystems. Our study thus highlights the importance of including, not only toxicant concentrations but also exposure time, in ecotoxicological evaluations.