Explaining metal concentrations in sympatric Chironomus species

Abstract

We compared metal concentrations in larvae of two Chironomus species (Chironomus staegeri and Chironomus tigris) living in the same lake and at the same depth and time. Concentrations of the nonessential metal cadmium (Cd) differed greatly (>8x) between the two species, whereas those of two essential metals differed either much less (zinc [Zn], 2x) or not at all (copper [Cu]). These trends were constant in all seasons. On the one hand, differences in Cd and Zn concentrations between the species were not explained by differences in either their size or their life cycle. Likewise, differential exposure to dissolved metals did not explain larval Cd and Zn concentrations because vertical gradients in dissolved metals did not correlate with depths of larval feeding. On the other hand, the species differed in the type of sediment that they consumed, and measurements of sulfur stable isotopes in larvae confirmed that whereas C. staegeri consumes mostly surface oxic sediment, C. tigris eats mainly deeper anoxic sediment. Because total metal concentrations in gut contents were not correlated with those in larvae, it is likely that metal bioavailability differs between the two sediment types. Overall, our results show that because metal concentrations can differ widely between sympatric congeners, extrapolation from one Chironomus species to another may not be justifiable. Furthermore, because larvae exposed to Zn in the laboratory did not accumulate this metal as they would in the field, we suggest that care is warranted when extrapolating from results obtained in laboratory tests to animals living in the field. © 2008, by the American Society of Limnology and Oceanography, Inc.