Continuous Seasonal River Ebullition Measurements Linked to Sediment Methane Formation.

Laboratory sediment incubations and continuous ebullition monitoring over an annual cycle in the temperate Saar River, Germany confirm that impounded river zones can produce and emit methane at high rates (7 to 30 (g CH4 m(-3) d(-1)) at 25 °C and 270 to 700 (g CH4 m(-2) yr(-1)), respectively). Summer methane ebullition (ME) peaks were a factor of 4 to 10 times the winter minima, and sediment methane formation was dominated by the upper sediment (depths of 0.14 to 0.2 m). The key driver of the seasonal ME dynamics was temperature. An empirical model relating methane formation to temperature and sediment depth, derived from the laboratory incubations, reproduced the measured daily ebullition from winter to midsummer, although late summer and autumn simulated ME exceeded the observed ME. A possible explanation for this was substrate limitation. We recommend measurements of methanogenically available carbon sources to identify substrate limitation and help characterize variation in methane formation with depth and from site to site.

Methane-derived carbon in the benthic food web in stream impoundments.

Methane gas (CH4) has been identified as an important alternative source of carbon and energy in some freshwater food webs. CH4 is oxidized by methane oxidizing bacteria (MOB), and subsequently utilized by chironomid larvae, which may exhibit low δ(13)C values. This has been shown for chironomid larvae collected from lakes, streams and backwater pools. However, the relationship between CH4 concentrations and δ(13)C values of chironomid larvae for in-stream impoundments is unknown. CH4 concentrations were measured in eleven in-stream impoundments located in the Queich River catchment area, South-western Germany. Furthermore, the δ(13)C values of two subfamilies of chironomid larvae (i.e. Chironomini and Tanypodinae) were determined and correlated with CH4 concentrations. Chironomini larvae had lower mean δ(13)C values (-29.2 to -25.5 ‰), than Tanypodinae larvae (-26.9 to -25.3 ‰). No significant relationships were established between CH4 concentrations and δ(13)C values of chironomids (p>0.05). Mean δ(13)C values of chironomid larvae (mean: -26.8‰, range: -29.2‰ to -25.3‰) were similar to those of sedimentary organic matter (SOM) (mean: -28.4‰, range: -29.3‰ to -27.1‰) and tree leaf litter (mean: -29.8 ‰, range: -30.5‰ to -29.1‰). We suggest that CH4 concentration has limited influence on the benthic food web in stream impoundments.

Sediment trapping by dams creates methane emission hot spots.

Inland waters transport and transform substantial amounts of carbon and account for ∼18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (∼0.23 mmol CH4 m(-2) d(-1) vs ∼19.7 mmol CH4 m(-2) d(-1), respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%.