The relative importance of water and food as cadmium sources to Daphnia magna Straus.

Knowledge of the transport pathways of metals into aquatic organisms is paramount in determining the metal's potential mechanism of toxicity. To determine the relative importance of water and food as cadmium (Cd) sources for the cladoceran Daphnia magna grazing on the algae Chlorella vulgaris, we measured cadmium accumulation and toxicity (feeding inhibition and survival) in three genetically different clones of D. magna subsequent to water, food, and water and food exposures. We found that Cd uptake from water and food was independent of source and additive in effect, with D. magna juveniles accumulating twice as much Cd from water than from food (algae). However, the efficiency with which Cd was assimilated by D. magna from its algal diet was much higher (10%) than from water (0.3%). Uptake and toxic responses were inversely related: tolerant clones accumulated more Cd. As a consequence, models based on uptake of metals from the combined routes of water and food may be reliable to predict metal dynamics in the field, but may fail to predict toxic effects since tolerance to metals is not necessarily linked to reduced total uptake of metals.