Stable isotope signatures of benthic invertebrates in arctic lakes indicate limited coupling to pelagic production

Abstract

We tested the hypotheses that benthic macroinvertebrates, especially chironomid larvae in the Tribe Chironomini, from small oligotrophic arctic lakes are only weakly linked to pelagic food sources, and that they are trophically linked to biogenic methane. All offshore benthic macroinvertebrates sampled from 20 lakes were depleted in 13 C compared with nearshore benthic macroinvertebrates. Nearshore macroinvertebrates had ␦ 13 C consistent with feeding on periphyton and/or terrestrial detritus. However, ␦ 13 C of offshore Chironomini, the dominant macroinvertebrate group, and oligochaetes, was more depleted than that of other offshore macroinvertebrates to a degree that could only be explained by a diet that included carbon derived from biogenic methane. Seston ␦ 13 C showed a small shift toward greater depletion in deeper lakes, but ␦ 13 C of Chironomini, oligochaetes, and predatory chironomids varied as a function of dissolved oxygen concentration (DO) at the sediment-water interface, which was generally lower in shallow lakes that were deep enough to stratify. In lakes with lower DO in bottom waters, the importance of methane-based carbon to Chironomini was greater. Experimental 15 N enrichment of phytoplankton in small-and mid-sized arctic lakes resulted in limited 15 N enrichment of most benthic macroinvertebrates, indicating that their productivity was not closely tied to pelagic production. It is likely that aerobic benthic metabolism of dissolved organic carbon (DOC) also is an important energy source for benthic macroinvertebrates in sediments because the link between benthic macroinvertebrate production and contemporary pelagic primary production was weak regardless of dependence on biogenic methane.

Fossil chironomid δ13C as a proxy for past methanogenic contribution to benthic food webs in lakes?

“…A sample of Chironomus anthracinus-type head capsules with a d 13 C value of -36.9% was found in sediments from a Siberian tundra pond. This is comparable to d 13 C values reported in living Chironomus larvae from Arctic lakes in Alaska, Sweden and Finland (Hershey et al 2006;Jones et al 2008). Chironomus species are the taxa most commonly reported to incorporate methane-derived carbon by feeding on MOB (Grey et al 2004;Jones et al 2008).…”

Biogenic methane in freshwater food webs

“…Almost one-third of the lakes contained chironomid larvae with an estimated 40% or more of their carbon biomass derived from methane, and as much as 70% in some circumstances. Jones et al (2008) C of chironomids and oligochaetes and dissolved oxygen concentration was also reported by Hershey et al (2006) from small arctic lakes in Alaska. Thus, it appears that the low d…”

“…Kiyashko, Narita & Wada, 2001;Grey et al, 2004a;Deines & Grey, 2006;Hershey et al, 2006); this presumably relates to differences in redox conditions and hence in the availability of suitable microhabitats for MOB. However, even in offshore zoobenthos communities, it appears that chironomid larvae often exhibit low d…”

“…1; Kiyashko et al, 2001;Hershey et al, 2006;Syväranta, Hämäläinen & Jones, 2006), and this apparent paradox needs to be explained. Some taxa, such as large bivalve molluscs, are essentially filter feeders that rely predominantly on POM sedimenting from the pelagic and will not access MOB in sediments; indeed, large profundal mussels have been promoted as surrogate measures of phytoplankton d 13 C (Cabana & Rasmussen, 1996).…”

Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen

“…Using multiple approaches (litter exclusion, gut contents, biomarkers, and stable isotopes), a number of authors have reported that some fishes and benthic invertebrates in streams and the littoral zones of rivers and lakes are indeed supported in part by terrestrial organic matter (e.g., refs. [9][10][11][12][13][14][15][16]. Demonstrating a terrestrial influence on pelagic food webs in lakes had been both more difficult and more controversial.…”