Use of allochthonous and autochthonous carbon sources by Chironomus in arctic lakes

Larval Chironomus commonly dominate the zoobenthos of lakes and serve as an important food resource for fish. Stable isotope studies that trace carbon (C) through lake food webs have shown that Chironomus, as well as other consumers, are often depleted in C-13 relative to pelagic seston and detritus (BUNN & BOON 1993). Two general mechanisms for the isotopic depletion relative to basal resources have been proposed: (1) consumers may be selectively feeding on algae that may have a more negative delta C-13 than bulk seston (HAMIL-TON et al. 1992, LENNON et al. 2006); (2) consumers may be feeding on significant quantities of methane oxidizing bacteria (MOB; KIYASKO et al. 2001. GREY et al. 2004, JONES & GREY 2004, ELLER et al. 2005. HERSHEY et al. 2005, 2006), which also have a more negative delta C-13 than pelagic seston, detritus, or bulk sediments. Allochthonous inputs can support benthic or pelagic components of lake food webs, especially in small lakes (FRANCE 1995, HECKY & HESSLEIN 1995, KRITZBERG et al. 2004). Furthermore, MOB may be an especially important food source in lakes with high dissolved organic carbon (DOC). Hypolimnetic CH4 accumulation is correlated positively with epilimnetic DOC (HOUSER et al. 2003). Pelagic MOB production was estimated to be of similar magnitude to production by other heterotrophic microbes in humic Lake Kjelsasputten (HESSEN & NYGAARD 1992). Within the sediments, tubes produced by larval chironomids have higher CH4 oxidation rates and higher densities of MOB than surrounding sediments (KAJAN & FRENZEL 1999). Previous research in arctic Alaskan lakes suggests that phytoplankton-derived food sources are of low importance and MOB are of greater importance in the nutrition of larval Chironomini (HERSHEY et al. 2006). Here we used intact sediment cores as mesocosms to experimentally evaluate the role of seston and DOC as basal carbon resources supporting Chironomus production.