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.

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Abstract:We investigated the accumulation and biogeochemical cycling of organic matter beneath Ranunculus plants in a lowland river. Organic carbon accumulated beneath the plants at a mean rate of 20 mmol C m 2 h 1 . Annual gross primary production for both Ranunculus and its biofilm, and the microphytobenthos, could account for 26% of the carbon accumulated. The remainder was attributable to organic carbon in both suspended particulate matter (77%) and that associated with sands saltating along the bottom (33%). Maximum carbon oxidation occurred in spring and early summer and declined thereafter. The efflux of CO 2 was greater than the carbon equivalents due to reduction of O 2 , NO 3 and SO 4 2 measured at the surface, which suggested a significant contribution to carbon oxidation from the subsurface and some oxidation via alternative electron acceptors. The peak in carbon oxidation could not be accounted for by either rising temperature or primary production but tracked the quality of recently deposited allochthonous organic matter. The ratio of carbon oxidation to total organic carbon accumulation suggested that 19% of the organic matter deposited was remineralised on an annual basis, although this reached 58% in June. We calculate that a total of 3Ð6 mol N m 2 y 1 was mineralised in the sediment, of which 11% could be accounted for by the measured efflux of NH 4 C . The remainder could be accounted for by the N demand from primary production (67% macrophytes/biofilm; 36% phytobenthos).

The environmental accumulation of plastics worldwide is a consequence of the durability of the material. Alternative polymers, marketed as biodegradable, present a potential solution to mitigate their ecological damage. However, understanding of biodegradability has been hindered by a lack of reproducible testing methods. We developed a novel method to evaluate the biodegradability of plastic samples based on the monitoring of bacterial respiration in aqueous media via the quantification of CO2 produced, where the only carbon source available is from the polymer. Rhodococcus rhodochrous and Alcanivorax borkumensis were used as model organisms for soil and marine systems, respectively. Our results demonstrate that this approach is reproducible and can be used with a variety of plastics, allowing comparison of the relative biodegradability of the different materials. In the case of low-density polyethylene, the study demonstrated a clear correlation between the molecular weight of the sample and CO2 released, taken as a measure of biodegradability.

There is a need to understand the hydro‐ecological significance of surface‐subsurface interactions, including denitrification, in sediments in permeable catchments. Measurement of denitrification in such sediments is complicated by the NO3− reduction zone being relatively deep in the sediment (a few to tens of centimeters), there being 2 significant sources of NO3− (the overlying water and the underlying groundwater) and, in some environments, the impact of macrophytes such as Ranunculus. These factors negate the collection of sediment cores for measuring denitrification by traditional techniques. Instead, we have developed a minipore water probe system that can be used to identify NO3− reduction zones and measure in situ rates of denitrification based on spiking sediment pore waters with 15NO3− followed by a short incubation time (15 min) and subsequent quantification of 29N2 and 30N2. Given the short incubation time, we suggest that the technique can be applied to pore waters with significant advective flow, and present results for fine sand sediments from the River Frome (Dorset, England). The limitations of the technique, including a low depth resolution (2 cm) and intrusion of pore water from outside the sample target depth, are calculated and discussed. Accepting the disturbance caused by the initial insertion of the probes, which is transient, this approach enables denitrification to be quantified under close to in situ conditions without the use of inhibitors and without destruction or isolation of the sediment.