287 J. N. Am. Benthol. Soc., 2004, 23(2):287–296  2004 by The North American Benthological Society Stable isotope analysis provides fresh insights into dietary separation between Chironomus anthracinus and C. plumosus ANDREW KELLY 1 AND R. I. JONES 2 Department of Biological Sciences, Lancaster University, Lancaster LA1 4YQ, UK J. GREY 3 Max Planck Institute for Limnology, Postfach 165, Plo ¨n 24302, Germany Abstract. We used stable isotope analysis to investigate dietary differences between 2 sympatric species of tubicolous chironomid larvae, Chironomus plumosus and C. anthracinus, from the profundal sediments of 6 eutrophic lakes in the UK and Germany. We found striking variation between lakes in both  13 C(-29.8 to -56.3‰) and  15 N(-7.8 to 14.7‰) in the 2 chironomid species. Moreover, C. plumosus was consistently depleted in both 13 C and 15 N relative to C. anthracinus. Our data support previous reports of interspecific dietary variations between the 2 species, which suggest niche sepa- ration partly on the basis of diet. However, reported differences in the feeding modes of the 2 species could not explain the extreme 13 C- and 15 N-depletion observed in chironomid larvae from several of the lakes. We suggest that the low  13 C signatures result from the ingestion of methanotrophic bacteria and subsequent incorporation of biogenic methane-derived C. Further, the chironomid larvae enhance methanotrophic activity via bioturbation of the surrounding sediment. Significant isotope differences between the 2 species may result from their variable tube morphologies or physiology. Key words: stable isotopes, chironomid larvae, interspecific variation, methane, bacteria, lakes. Chironomid larvae often dominate the ben- thic fauna of lakes, reaching densities of several thousand individuals/m 2 (Coffman 1995). Ben- thic communities rely on sedimentation pro- cesses for their food supply (Brinkhurst 1974), so chironomids play a key role in the recycling of material lost from the water column. Chiron- omid larvae provide a substantial contribution to the diet of benthic predators such as large macroinvertebrates (Hill 1988) or fish (Fox 1989). Pelagic fish also prey upon chironomid pupae during migration to the surface to emerge as imagos (Winfield et al. 2002), and the adults themselves may provide a significant source of energy for terrestrial invertebrate (Col- lier et al. 2002, Sanzone et al. 2003) or vertebrate predators (Davies 1977) inhabiting riparian zones. Thus, chironomids can enhance both benthic–pelagic and aquatic–terrestrial energy flux. Recognition of the energetic importance of 1 Present address: Scottish Universities Environmen- tal Research Centre, East Kilbride G75 0QF, UK. E-mail: andrew.kelly@suerc.gla.ac.uk 2 Present address: Department of Biological and En- vironmental Science, PL 35, 40014 Jyva ¨skyla ¨ Univer- sity, Jyva ¨skyla ¨, Finland. E-mail: r.jones@cc.jyu.fi 3 To whom correspondence should be addressed. E-mail: grey@mpil-ploen.mpg.de chironomid species in freshwater ecosystems has led to many studies of their dietary selec- tion, feeding behavior, and feeding efficiency (see review by Berg 1995). The tubicolous species, Chironomus plumosus and C. anthracinus, often co-exist in the profun- dal sediments of productive European lakes. The 2 species can occupy similar physical and chemical conditions, so it is probable that their niche differentiation is strongly linked to dietary differences. Chironomus plumosus is generally considered to be a filter feeder of suspended or sedimenting material near the sediment–water interface, whereas C. anthracinus is considered an obligate deposit feeder. However, numerous factors can influence the mode of feeding, in- cluding larval size or life stage (Frank 1982), sediment characteristics (Johnson 1984), tube morphology (McLachlan 1977), food availability or quality (Johnson 1985, 1987), and predation or competition (van de Bund and Groenendijk 1994). Chironomus plumosus can also exhibit con- siderable flexibility of feeding mode, switching between filter feeding and deposit feeding un- der different ambient conditions (Hodkinson and Williams 1980). The diets of C. plumosus and C. anthracinus have been well studied, particularly in mesotro-