Biogenic methane contributes to the food web of a large, shallow lake

Abstract

Summary Biogenic methane as an alternative carbon and energy source for freshwater organisms has been receiving increasing attention, but the phenomenon is still poorly understood for shallow lakes. We measured the carbon and nitrogen stable isotope signatures (δ13C, δ15N) for key groups of pelagic and benthic organisms, including crustacean zooplankton, chironomid larvae, young‐of‐the‐year and adult fish, to assess whether biogenic methane contributes to pelagic and benthic food webs in a large, shallow lake, Lake Võrtsjärv, Estonia. In the southern part of the lake, covered by macrophytes, crustacean zooplankton showed strong seasonal variation of δ13C, with the lowest values occurring in the period of oxygen depletion. Chironomid (Chironomus plumosus) larvae showed high isotopic variability within the lake, with strongly 13C‐depleted and 15N‐depleted signatures (down to δ13C −64.0‰ and δ15N −2.6‰) in the macrophyte‐covered area. Our results indicate that carbon derived from biogenic methane can contribute seasonally to the benthic food web and, to some extent, also to the pelagic food web, where the lake is covered by macrophytes. Moreover, δ13C values for roach (Rutilus rutilus), perch (Perca fluviatilis) and pike (Esox lucius) from the macrophyte‐dominated area were on average 3.5‰ lower than those for the same fish from the plankton‐dominated lake area, suggesting some carbon derived from methane is transferred up through the food web. Although no direct evidence is available, our results, together with previous studies of the lake, suggest that protozoans could be a potentially important link from methane‐oxidising bacteria to animals higher in the web.

The Incredible Lightness of Being Methane-Fuelled: Stable Isotopes Reveal Alternative Energy Pathways in Aquatic Ecosystems and Beyond

“…Such lakes are often shallow and contain considerable stands of macrophytes; while methanogenesis is certainly proceeding in the sediments, much of the CH 4 produced might be routed via the plant stems and via ebullition (routes 1&2 in Figure 1) and hence, sidestep incorporation into the food web [although see reference to Agasild et al (2014), below]. Since the whole water column is well oxygenated, there is no distinct boundary where MOB will accumulate and thus it is unlikely that zooplankton will feed heavily upon MOB (Jones and Grey, 2011).…”

“…Interestingly, the latter lake was sampled at various sites and it was only at one particular site dominated by vegetation that low δ 13 C values were recorded in both zooplankton and chironomids. Agasild et al (2014) postulated that the stands of macrophytes prevented wind mixing from disturbing the sediments, and that dissolved oxygen in the water column was reduced by the restricted circulation of water and gas exchange between the water surface and the atmosphere and by increased oxygen demand from the decomposition of organic matter; all processes which would lead to greater MDC being available to the food web.…”

“…From stratifying lakes, (Harrod and Grey, 2006) and (Ravinet et al, 2010) have found isotopic evidence of MDC contributing (up to ∼12%) to bream (Abramis brama) and to ruffe (Gymnocephalus cernuus), respectively, while in a shallow, well-mixed Pantanal (tropical) wetland lake (Sanseverino et al, 2012) could trace MDC into various fish species. Even from the very shallow lake Võrtsjärv, Agasild et al (2014) reported that at sites amongst the macrophytes where zooplankton and chironomid larvae were most 13 C-deplete, there was a corresponding decrease in δ 13 C for roach (Rutilus rutilus), perch (Perca fluviatilis), and the apex predator, pike (Esox lucius), indicative of trophic transfer of MDC to the very top of the food web. To date, evidence from rivers has not been reported, but given the extremely abundant nature of the primary consumers (particularly cased caddis flies) that appear key to linking MOB into the food web in such systems, the pathway is certainly in place .…”

The more, the merrier: heterotroph richness stimulates methanotrophic activity

“…Depending on their metabolic capabilities, microorganisms can be broadly grouped into autotrophs (primary producer) and heterotrophs (decomposers), with the autotrophs forming the base of food webs. Similarly, in methane-driven ecosystems, the methanotroph can be considered as a primary producer and interacts to form close association with the heterotrophs (Hutchens et al, 2004;Iguchi et al, 2011;van Duinen et al, 2013;Agasild et al, 2014). Possibly, the methanotroph and heterotrophs are mutually codependent.…”

Zoobenthos are minor dietary components of small omnivorous fishes in a shallow eutrophic lake

“…larval chironomids has been reported for only relatively large fish species (Harrod and Grey 2006;Ravinet et al 2010;Agasild et al 2014). Therefore, the tube-building behaviour of larval chironomids should have negative effects on the trophic transfer of methane-derived carbon to higher consumers in littoral habitats where small fishes predominate.…”