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

Abstract

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 lm) dominated by the sand-sized fraction (63-1000 lm). 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 CH 4 transport through Ranunculus stems and an increase in water column CH 4 concentration, while the sediment CH 4 concentration increased 100-fold between February and April. A marked seasonal enrichment in the d 15 N of N 2 dissolved in the pore water correlated with CH 4 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 · 10 )6 Tg CH 4 year )1 ), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.

Methanogenic System of a Small Lowland Stream Sitka, Czech Republic

“…Since a relatively high production of methane has been measured in river sediments (e.g. Schindler & Krabbenhoft 1998, Hlaváčová et al 2005, Sanders et al 2007, Wilcock & Sorrell 2008, Sanz et al 2011, we proposed that river sediments may act as a considerable source of this greenhouse gas which is important in global warming (Hlaváčová et al 2006). …”

Potential carbon fixation via methane oxidation in well‐oxygenated river bed gravels

“…After equilibration, gas samples (50 mL) were withdrawn from the headspace and injected into a gas-chromatograph fitted with a flame ionizing detector (Agilent Technologies). Headspace concentrations of CH 4 were calculated from peak areas calibrated against known standards (Scientific and Technical Gases) and the total amount of CH 4 in a gas-tight vial (headspace plus water) calculated using solubility coefficients (Sanders et al 2007). Chlorophyll analysis-At the end of a set of experiments, all of the gravel (average wet weight 194 g) was collected from each chamber, transferred to a 500-mL bottle (PTFE), weighed, covered with 200 mL of acetone (90% v/v with ultra high purity water) and left to extract for 24 h in a refrigerator.…”

“…However, the groundwater which dominates the flow in many lowland rivers, and particularly of rivers flowing through permeable chalk catchments in the United Kingdom, is supersaturated with methane (Darling and Gooddy 2006). In addition, Sanders et al (2007) showed that this groundwater source is seasonally supplemented by the local production of methane in patches of organic sediments deposited in the lee of macrophyte (Ranunculus penicillatus) stands. We further showed that the case-building larvae of two caddis flies (Trichoptera: Agapetus fuscipes and Silo nigricornis) were consistently 13 C-depleted (mean d 13 C: 241.2% and 240.4%, respectively) relative to the photosynthetic basal resources throughout the year in one such English chalk river (the River Lambourn), and that this depletion was probably attributable to the grazing of a biofilm that supports methane oxidation, thereby providing the caddis larvae with up to 30% of their carbon (Trimmer et al 2009a).…”

Heterogeneous hyporheic zone dechlorination of a TCE groundwater plume discharging to an urban river reach

“…Sand lenses built up beneath trailing macrophytes due to reduced river flow rates and or suspended sediment filtering (cf. Sanders et al, 2007). Centimetre or so thicknesses of dark, presumably reduced, sediment occurred at the lens contact with the river.…”