The importance of methane derived carbon as a basal resource in chalk stream food webs: the effect of light availability and methane concentration.
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Methane is oversaturated relative to the atmosphere in many rivers, yet its cycling and fate is poorly understood. While photosynthesis is the dominant source of autotrophic carbon to rivers, chemosynthesis and particularly methane oxidation could provide alternative sources of primary production where the riverbed is shaded or at depth beneath the sediment surface. I highlight geographically widespread methanotrophic carbon fixation within the gravel riverbeds of over 30 chalk rivers and in 15 of these, the potential for methane oxidation (methanotrophy) was also compared to photosynthesis and stable isotope analyses were used to trace methane into the wider food web. Detailed concurrent measurements of photosynthesis and methanotrophy in one large chalk river over a complete annual cycle, showed methanotrophy to be active to at least 15cm into the riverbed and to be strongly substrate limited. The seasonal trend in methanotrophic production reflected that of the riverine methane concentrations, and thus, the highest contribution to autotrophic production was in mid-summer. At the sediment surface, photosynthesis was limited by light for most of the year with heavy shading induced by dense beds of aquatic macrophytes and riparian vegetation. Across 15 rivers in midsummer, methane derived carbon was estimated to contribute 18% of production (methanotrophic plus photosynthetic) in well illuminated riverbeds and 51% in the shaded areas (median values). With warming conditions and associated increasing methanogenesis in fine sediments, methanotrophy is predicted to prevent increased methane emissions from rivers due to the strong kinetic response of methane oxidation. The gross carbon fixation efficiency of methane oxidation was calculated as 50% and was conserved across eight rivers with varying methane oxidation capacities and ambient methane concentrations. Methanotrophic production is widespread, efficient and most important when ambient methane concentration is high and light availability is low
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