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.

Chironomus Meigen (Diptera, Chironomidae) larvae are usually the largest sediment-burrowing chironomids, and as such often constitute a major part of the freshwater infaunal biomass. However, use of this genus in ecological, environmental and paleoecological studies is hampered by the fact that Chironomus larvae are difficult to identify to species because the larvae of many species are morphologically similar. We used a combination of morphological, cytological and genetic techniques to distinguish Chironomus larvae collected from 31 water bodies located in eastern Canada, producing 17 distinguishable groupings. These groups of larvae were ultimately identified as belonging to 14 known species (C. anthracinus, C. bifurcatus, C. cucini, C. decorus-group sp. 2, C. dilutus, C. entis, C. frommeri, C. harpi, C. maturus, C. nr. atroviridis (sp. 2i), C. ochreatus, C. plumosus, C. staegeri and C. 'tigris') and three other species that remain unidentified (C. sp. NAI-III). No single approach served to delimit and identify larvae of all 17 Chironomus species that we collected. Although we expected that morphological criteria alone would be insufficient, our results suggest that DNA barcoding, using either the mitochondrial cox1 or the nuclear gb2β gene, was also inadequate for separating some Chironomus species. Thus we suggest that multiple approaches will often be needed to correctly identify Chironomus larvae to species.

Summary We set out to determine the feeding behaviours of 15 Chironomus species collected from 16 boreal forest lakes by measuring cadmium (Cd) and stable isotopes of carbon (C), nitrogen (N) and sulphur (S) in larvae. Measurements of S stable isotopes and Cd discriminated between Chironomus species that feed mainly on oxic particles from those that feed mainly on anoxic particles. Our results suggest that C. dilutus, C. entis, C. plumosus and C. staegeri feed mainly on oxic particles (in the water column, in sediment or both), whereas C. anthracinus, C. bifurcatus, C. cucini, C. decorus‐group sp. 2, C. harpi, C. nr. atroviridis (sp. 2i), C. ochreatus, C. spp. NAI‐NAIII and C. ‘tigris’ feed mainly on anoxic sediments. In our study lakes, larval C isotopic signatures were relatively high, which suggests that they feed little on methanotrophic bacteria. Although larval C and N signatures differed between some sympatric Chironomus species, these differences were not consistent among lakes. The absence of a trend among lakes suggests that lake‐specific factors determine larval C and N signatures. Differences in feeding habits and Cd concentrations among sympatric (cohabiting) Chironomus species suggest that pooling them in ecological, palaeolimnological or ecotoxicological studies could obscure trends in nature, thereby limiting their use as monitors of climate change or pollution.

Larvae of the insect Chironomus (Chironomidae: Diptera) have great potential for estimating the bioavailability of sedimentary trace elements because they are common in fine sediments and tolerate high concentrations of these contaminants. Their use as biomonitors is limited by the fact that they are difficult to identify as to species, and the species can differ in their trace element concentrations. To determine whether pooling species would compromise their use as trace element biomonitors, we identified species of Chironomus larvae collected from 22 lakes and measured their concentrations of 9 trace elements. We found that the concentrations of arsenic, barium, cobalt, copper, manganese, and nickel did not generally differ between sympatric Chironomus species, which indicates that they could be pooled for analyses of these trace elements. In contrast, we found that cadmium (Cd), selenium (Se), and zinc (Zn) concentrations differed between species living at the same site according to their feeding behavior, that is, Chironomus species feeding on oxic sediments tended to have higher Cd and Zn concentrations, whereas those feeding on deeper anoxic sediments had higher Se concentrations. Because Se and Zn concentrations in sympatric Chironomus species usually differed by only a factor of 2, separating species based on their feeding behavior might not be as crucial as for Cd if larval Se and Zn concentrations vary greatly from site to site. Environ Toxicol Chem 2019;38:145–159. © 2018 SETAC

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