Explaining metal concentrations in sympatric Chironomus species

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 (.83) between the two species, whereas those of two essential metals differed either much less (zinc [Zn], 23) 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. Measurements of metals in animals can be an important component of ecological risk assessments because they provide the link between metal exposure and metal toxicity (Chapman and Wang 2000). Metal concentrations in animals are also used to estimate the contamination level of whole ecosystems (Phillips and Rainbow 1993; Hare et al. in press). Animals used in this way should be identified to the lowest possible taxonomic level, so that behavioral and physiological differences among taxa will not confound trends in contaminant concentrations among sites or over time (Rainbow 2002; Skubała and Kafel 2004; Buchwalter and Luoma 2005). However, identifying freshwater invertebrates to the species level is often problematic, and thus necessity frequently dictates that species be grouped for contaminant analyses. A case in point is the insect Chironomus (Diptera, Chironomidae, Chironominae, Chironomini); this genus has a worldwide distribution, and larvae are tolerant to and accumulate contaminants in lakes and rivers (Armitage et al. 1995), which makes them good candidates for use as biomonitors in these ecosystems. However, the identification of Chironomus species is difficult without recourse to examining the giant salivary chromosomes of larvae (e.g., Butler et al. 1995), knowledge of which is confined to a small number of specialists. Since congeners can differ markedly in how they take up, store, and lose contaminants (Aoki et al. 1989; Lobel et al. 1990; Rainbow 2002), information is needed to determine if it is justifiable to pool Chironomus species for contaminant analyses. Such studies would also shed light on whether data from toxicity tests using various species of Chironomus are likely to be comparable (Jeyasingham and Ling 2000; Watts and Pascoe 2000; Péry et al. 2005) and if the results of such tests can be extrapolated to congeners in the field. Lastly, comparative studies would be useful for explaining any differences in contaminant concentrations between closely related species. We set out to determine if the larvae of two Chironomus species (Chironomus staegeri and Chironomus tigris) that share the same habitat differ in their concentrations of several trace metals of environmental importance: cadmi1 Present address: Environment Canada, Meteorological Service of Canada, 1141 route de l’Église, Quebec City, Québec G1V 4H5. 2 Corresponding author ([email protected]). Acknowledgments Funding was provided by the Metals in the Human Environment Research Network and the Natural Sciences and Engineering Research Council of Canada. We thank Mac Butler, Louis Croisetière, René Rodrigue, and André Tessier for their comments and assistance.