Methane-Derived Carbon in the Benthic Food Web in Stream Impoundments

<div><p>Methane gas (CH₄) has been identified as an important alternative source of carbon and energy in some freshwater food webs. CH₄ is oxidized by methane oxidizing bacteria (MOB), and subsequently utilized by chironomid larvae, which may exhibit low δ¹³C values. This has been shown for chironomid larvae collected from lakes, streams and backwater pools. However, the relationship between CH₄ concentrations and δ¹³C values of chironomid larvae for in-stream impoundments is unknown. CH₄ concentrations were measured in eleven in-stream impoundments located in the Queich River catchment area, South-western Germany. Furthermore, the δ¹³C values of two subfamilies of chironomid larvae (i.e. Chironomini and Tanypodinae) were determined and correlated with CH₄ concentrations. Chironomini larvae had lower mean δ¹³C values (−29.2 to −25.5 ‰), than Tanypodinae larvae (−26.9 to −25.3 ‰). No significant relationships were established between CH₄ concentrations and δ¹³C values of chironomids (p>0.05). Mean δ¹³C values of chironomid larvae (mean: −26.8‰, range: −29.2‰ to −25.3‰) were similar to those of sedimentary organic matter (SOM) (mean: −28.4‰, range: −29.3‰ to −27.1‰) and tree leaf litter (mean: −29.8 ‰, range: −30.5‰ to −29.1‰). We suggest that CH₄ concentration has limited influence on the benthic food web in stream impoundments.</p></div

Locations of the study impoundments in the Queich River catchment area.

<p>Locations of the study impoundments in the Queich River catchment area.</p

Relationships between pore-water CH₄ concentrations and δ¹³C values.

<p>TNP 4^th, CMI 3^rd, CMI 4^th and SOM stands for Tanypodinae fourth generation, Chironomini third and fourth generation, and sedimentary organic matter, respectively.</p

Average CH₄ concentrations (µmol L⁻¹) and bulk sediment characteristics (TOC, C:N ratio, porosity and δ¹³C) of samples from the studied impoundments (in parentheses are standard errors, ±SE (when applicable).

<p>Average CH₄ concentrations (µmol L⁻¹) and bulk sediment characteristics (TOC, C:N ratio, porosity and δ¹³C) of samples from the studied impoundments (in parentheses are standard errors, ±SE (when applicable).</p

Correlations (<i>rho</i>-values) between δ¹³C values, C:N ratio of bulk sediment, water residence time (min) and CH₄ concentrations (^*<i>p</i><0.05).

<p>δ¹³C_SOM, δ¹³C of bulk sediment; CMI 4^th, fourth instar Chironomini; CMI 3^rd, third instar Chironomini; TNP 4^th, fourth instar Tanypodinae.</p><p>Correlations (<i>rho</i>-values) between δ¹³C values, C:N ratio of bulk sediment, water residence time (min) and CH₄ concentrations (^*<i>p</i><0.05).</p