Abstract
AbstractMethane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production. We assessed methanotrophy and photosynthetic production under differing conditions of light and methane concentration. We measured methane oxidation and photosynthesis in gravel sediments from adjacent shaded and unshaded stretches of 15 chalk rivers in southern England, and also in 30 artificial channels in which we manipulated light and methane experimentally. The capacity for methane oxidation was 78% higher in the shade than unshaded areas, indicating a denser, or more active, methanotrophic assemblage on shaded riverbeds, and the difference was most pronounced when methane concentration was high. Across the 15 rivers, methanotrophic production ranged from 16 to 650 nmol C cm−2 d−1 and net photosynthetic production from 256 to 35,750 nmol C cm−2 d−1. The relative importance of methanotrophy to their total production (i.e., photosynthetic and methanotrophic) increased with methane concentration and ranged from 0.1–2.4% and 0.2–13% in unshaded and shaded areas, respectively. Over an annual cycle in one river, the response of the methanotrophs in the shade to a high summer methane concentration was ∼ five times greater than in the open; in winter, there was no effect of shading on methane oxidation. The response of methanotrophy to shading and methane concentration in the artificial channels resembled that found in the rivers. Methanotrophy makes a non‐negligible (here up to ∼ 13%) contribution to particulate carbon production in these streams, is disproportionately greater in the shade, and constitutes a distinct carbon pathway available for their food webs.
Highlights
Methane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production
Microbial methane oxidation has been well studied in lakes and wetland sediments, where the methane concentration can be high (Hershey et al 2015; Oswald et al 2015; Segarra et al 2015) and methanotrophic production contributes considerable particulate carbon to the food web to the extent that it can be detected in the biomass of macroinvertebrates and fish, and even exported to the terrestrial ecosystem via emerging adult insects (Grey and Deines 2005; Ravinet et al 2010; Jones and Grey 2011; Grey 2016)
Rmo is the rate of methane oxidation, V is the volume in cm3 taken up by 1 g of gravel (0.95), CCE is the carbon conversion efficiency which we have shown previously to be 0.5 (50%) for eight typical chalk streams, d is the depth over which we have integrated the methane oxidation [15 cm is the conservative estimate of riverbed depth over which methane oxidation occurs at a similar rate to that at the surface (Shelley et al 2014)]; and h is the number of hours over which methanotrophy was assumed to occur (24)
Summary
Methane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production. Methanotrophy makes a non-negligible (here up to 13%) contribution to particulate carbon production in these streams, is disproportionately greater in the shade, and constitutes a distinct carbon pathway available for their food webs. Microbial methane oxidation has been well studied in lakes and wetland sediments, where the methane concentration can be high (lM–mM range) (Hershey et al 2015; Oswald et al 2015; Segarra et al 2015) and methanotrophic production contributes considerable particulate carbon to the food web to the extent that it can be detected in the biomass of macroinvertebrates and fish, and even exported to the terrestrial ecosystem via emerging adult insects (Grey and Deines 2005; Ravinet et al 2010; Jones and Grey 2011; Grey 2016)
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