Relative importance of the trophic and direct pathways on PCB contamination in the rotifer species Brachionus calyciflorus (Pallas)

Abstract

To determine the contribution of food ingestion (trophic pathway) to PCB contamination of Zooplankton in the river Meuse (Belgium), we used ¹⁴C-labelled algae (Dictyosphaerium ehrenbergianum) to measure ingestion and assimilation rates in the rotifer species Brachionus calyciflorus. When the concentration of algae in the culture medium varied from 20 10³ to 200 10³ algal cells ml^-1 (0.12 to 1.18 mg C 1^-1), the Brachionus calyciflorus ingestion rate varied from 0.25 ± 0.12 to 1.52 ± 0.43 ng C ind^-1 h^-1 at 15 °C and from 0.74 ± 0.17 to 5.93 ± 0.61 ng C ind^-1 h^-1 at 20 °C. The assimilation efficiency (ratio of the assimilation rate to the ingestion rate) measured in a culture medium containing 200 10³ algal cells ml^-1 was 55.7 ± 5.8%. Since the PCB concentration measured in the phytoplankton of the river Meuse is about 3 µg PCBs g^-1 D.W., the estimated PCB contamination of Zooplankton ascribable to the trophic pathway ranges from 0.22 ± 0.17 to 1.31 ± 0.77 µg PCBs g^-1 D.W. at 15 °C and from 0.64 ± 0.34 to 5.10 ± 2.10 µg PCBs g^-1 D.W. at 20 °C. The lower figure based on measurements effected at 20 °C is comparable to the actual level measured in Zooplankton samples collected in the river Meuse (0.69 ± 0.20 µg PCBs g^-1 D.W.). The applicability of the formula used in our estimate was checked in a 48-hour in vitro experiment in which the rotifers were fed contaminated algae. The PCB accumulation measured in the rotifers was found to coincide with the calculated PCB contamination. Additional experiments were carried out to determine the contribution of the direct pathway to PCB contamination of Zooplankton living in the river Meuse (0.02 µg PCBs 1^-1 of water; average dissolved organic matter: 3 mg C1^-1). The PCB concentration in Zooplankton resulting from direct uptake of PCBs from the water was estimated at 0.19 ± 0.05 µg PCBs g^-1 D.W. These results show that in Zooplankton living in polluted ecosystems, PCBs are likely to accumulate via the trophic pathway to concentrations up to 30 times higher than by direct contamination. Furthermore, our estimates of PCB contamination via the trophic pathway coincide quite well with actual concentrations measured in situ.