Temperature effects on bioconcentration of DDE by Daphnia

1. Lake temperatures vary with season, latitude, elevation and as a result of thermal pollution. Ln addition, lake temperatures may increase with global warming. Radiotracer experiments were conducted to determine the effects of temperature on the bioaccumulation of lipophilic organic contaminants by zooplankton. Daphnia pulex were exposed to C-14-labelled DDE, a stable metabolite of the organochlorine pesticide DDT, in particle-free water for 24 h. An increase in temperature from 5 to 25 degrees C resulted in a 314% increase in bioconcentration factor (the ratio of contaminant concentration in the organism to contaminant concentration in the water). 2. To mimic the fluctuating temperatures experienced by zooplankton during diel vertical migration, we conducted experiments in which animals were exposed to 25 degrees C for 12 h in the light, then 15 degrees C for 12 h in the dark. Exposure to this fluctuating temperature regime for 48h resulted in a 27-33% increase in bioconcentration factor relative to a constant 20 degrees C control. 3. Live animals accumulated more than twice the amount of DDE than freshly killed animals, indicating that the activity of the organism is important in bioconcentration. This finding sheds light on the possible mechanisms for the increase in bioconcentration at higher constant temperatures. Daphnia pump more water through their branchial chambers at higher temperatures. Thus, if the thoracic limbs are an important site of contaminant uptake, then animals are exposed to more contaminant molecules at higher temperatures. Other possible mechanisms include changes in the thickness of the diffusive boundary layer and changes in cell membrane permeability.