Biokinetics of cadmium, selenium, and zinc in freshwater alga Scenedesmus obliquus under different phosphorus and nitrogen conditions and metal transfer to Daphnia magna Ri-Qing Yu 1 , Wen-Xiong Wang* Department of Biology, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong Received 31 July 2003; accepted 21 November 2003 ‘‘Capsule’’: Phosphorous enrichment influences metal uptake by algae, but not transfer to a zooplankton grazer. Abstract The uptake of Cd, Se(IV) and Zn by the freshwater alga Scenedesmus obliquus and the subsequent transfer and release budget in Daphnia magna were investigated under different nutrient additions and cell incubation conditions. An increase in ambient phos- phate concentrations from 0.5 mmol l 1 to 50 mmol l 1 significantly increased the intracellular accumulation of Cd (by 18) and Zn (by 5), but decreased the accumulation of Se (by 126) in the alga. The percentage of these metals distributing in the intracellular pool of algae also increased substantially with increasing ambient P concentrations. Nitrate addition from 5.0 to 200 mmol l 1 did not influence the uptake of any of the three metals, although a significant decrease in the intracellular Se distribution was observed. Radiolabeled algae under different nutrient manipulations (semi-continuous culture, starvation, and P-pulse treatments) were used to measure trophic transfer assimilation efficiency (AE) in Daphnia. When the algal cells were grown in a semi-continuous culture, starved for N and P, or were treated with P-pulse, the AEs of Cd and Zn were generally independent of the nutritional conditions, but the Se AE was significantly affected by different P levels. The efflux rate constants, determined during 10 d depuration following 7 days of dietary uptake, decreased significantly for Cd and Zn, but were relatively constant for Se with increasing P concentration. N-addition caused no effect on the metal efflux rate constants. P- or N-additions did not influence the release budget (including molting, neonates, excretion and feces) for all three elements in Daphnia. Our study indicated that phosphate enrichment may substantially increase metal uptake in green alga S. obliquus. Responses of trophic transfer in Daphnia to nutrient enrichment were metal specific. P-enrichment can possibly lead to considerable decrease on Se transfer from algae to zooplankton. # 2004 Elsevier Ltd. All rights reserved. Keywords: Daphnia; Metal uptake; Macronutrients; Eutrophication; Efflux 1. Introduction Metal uptake by phytoplankton and subsequent grazing by zooplankton are critical steps in metal bio- geochemical cycling in both lakes and marine systems. Compared with the considerable progress in under- standing nutrient cycling and trophic interactions in freshwater pelagic systems in recent years (Elser and Urabe, 1999), few studies have been carried out to examine metal cycling in these ecosystems (Twiss et al., 1996; Twiss and Campbell, 1995; Chen et al., 2000). Pelagic stoichiometry of C, N and P in algal cells greatly influences the regeneration and trophic transfer of these macronutrients. It is believed that the stoichiometry of nutrient recycling is a feedback mechanism linking algal nutritional status and grazer dynamics (Elser and Urabe, 1999). Eutrophication with excessive nutrient enrichments has become a worldwide problem impact- ing freshwater ecosystems. An increase in nutrient input leads to phytoplankton blooms and causes consequential 0269-7491/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2003.11.013 Environmental Pollution 129 (2004) 443–456 www.elsevier.com/locate/envpol * Corresponding author. Tel.: +852-2358-7346; fax: +852-2358- 1559. E-mail address: wwang@ust.hk (W.-X. Wang). 1 Present address: Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA.