Emission of methane from chalk streams has potential implications for agricultural practices

• Published in: Freshwater biology Vol. 52; no. 6; pp. 1176 - 1186
• Main Authors: SANDERS, I.A, HEPPELL, C.M, COTTON, J.A, WHARTON, G, HILDREW, A.G, FLOWERS, E.J, TRIMMER, M
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.06.2007; Blackwell Publishing Ltd
• Subjects: ecosystem function; erosion; Freshwater; land use; methane; Ranunculus; Ranunculus penicillatus; sediment; sediments; soil erosion
• ISSN: 0046-5070; 1365-2427
• DOI: 10.1111/j.1365-2427.2007.01745.x

1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function. 2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 μm) dominated by the sand-sized fraction (63-1000 μm). 3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH₄ transport through Ranunculus stems and an increase in water column CH₄ concentration, while the sediment CH₄ concentration increased 100-fold between February and April. A marked seasonal enrichment in the δ¹⁵N of N₂ dissolved in the pore water correlated with CH₄ flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin. 4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems. 5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 x 10⁻⁶ Tg CH₄ year⁻¹), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.