Trace metal assimilation and release budget in Daphnia magna

Abstract

The assimilation efficiency (AE), efflux rate, and release budget of Cd, Cr(III), Se(IV), and Zn by a freshwater zooplankton Daphnia magna were measured under different food concentrations. The AEs of trace elements by Daphnia on two algal diets (Chlamydomonas reinhardtii and Scenedesmus obliquus) were 30–77% for Cd, 8–44% for Cr, 24–58% for Se, and 7–66% for Zn at food concentrations ranging from 0.136 to 7.50 mg carbon L−1. Metal AEs increased significantly with decreasing food concentrations, with a maximum increase of 5.5 and 4.0X for Zn and Cr, respectively. AEs were generally on the order of Cd > Se > Zn > Cr. Efflux rate constants, determined during 7 d depuration after 8 d of exposure to metals in the dissolved phase or dietary phase, were 0.012–0.216 d−1, with the highest efflux for Zn, followed by Cr > Se > Cd. The relative contribution of different routes of metal loss to the overall metal loss was also quantitatively assessed during the 7‐d depuration period. Metals differed substantially in their routes of release from Daphnia. In general, metal excretion into the dissolved phase was the most important route for metal loss. Molting represented nearly 50–70% and 20–70% of daily metal efflux for Cd and Zn, respectively, following aqueous exposure within the first 4 d but was <20 and <30%, respectively, following food exposure. Release by offspring production contributed substantially to Se efflux by the animals. Up to 44–67% and 16–47% of Se was lost from the animals through reproductive allocation on a daily basis following uptake from the aqueous and dietary phases, respectively. The major routes of Cr efflux were by excretion and feces egestion. Our study suggested that trace metal assimilation and regeneration in Daphnia may play an important role in the biogeochemical fates of metals in lake systems.