Enhanced tolerance of aquatic organisms to metal toxicity is one of the important issues of environmental monitoring programs. Determination of dominant uptake route(s) of metals may help to better predict the toxic effects posed by metals. This study aimed to investigate the importance of Zn uptake routes on tolerance and energy reserves of Daphnia magna. Neonates of D. magna were exposed to water-borne zinc, dietary zinc (algae Pseudokichneriella subcapitata loaded with Zn) or to combination of both for 4 days. LC50 (48 h) values of Zn were considerably different from different zinc pre-exposures. Four-day pre-exposure of D. magna neonates produced enhanced tolerance to Zn toxicity. The lowest LC50 values were found in controls (48.2 mu M) (no Zn added to their exposure medium and food) and after water-borne Zn pre-exposures (46.2 mu M). The level of tolerance increased when dietary Zn was included in pre-exposures, reaching the highest level of LC50 value (70.8 mu M) in the highest pre-exposure concentration of diet and water-borne combination experiment. The energy reserves of D. magna also varied significantly under different pre-exposure routes of zinc. In all cases, control animals contained lowest levels of protein, sugar and lipid. Likewise, they represented the lowest energy reserves. Protein levels were highest in the highest dietary Zn exposure, and lowest in the water-borne exposures. Highest and lowest sugar levels were measured in the lowest and highest water-borne Zn exposures, respectively. In contrast, lipid levels were higher in the higher Zn exposure of all exposure routes, the combination exposure resulting in highest lipid levels. The highest total energy reserve was measured in animals that lived in the highest Zn exposure of diet and water-borne combination experiment, mainly due to greater lipid reserves in algae reared in Zn containing media. Results suggest that the dietary exposure route should be considered carefully in natural monitoring studies, and be considered in regulatory assessments of zinc and population dynamics of cladocerans. (c) 2005 Elsevier Inc. All rights reserved.