Tracing methane-derived carbon from microbes to trout in upland streams

Around half of global methane (CH4) emissions are from freshwater ecosystems. While freshwaters frequently have high levels of dissolved CH4, not all is released to the atmosphere. A large proportion is sequestered through methanotrophic bacteria creating a valuable carbon resource potentially accessible to the food web. Methane-derived carbon (MDC) has previously been studied in wetlands and various types of stratifying lakes, where low oxygen concentrations allow for conditions favoured for methanogenesis, but few studies have focused on running waters. This study focused on exploring evidence for the importance of MDC in the food webs of upland streams in Scotland. It aimed to do this through tracing MDC from source to predatory species high in the food web, by quantifying methanogen and methanotroph communities, examining the in-stream niches where they are present, identifying the invertebrates that graze on them using carbon stable isotopes (δ13C) and exploring the drivers causing spatial variations in MDC occurrence. Low δ13C values in brown trout (Salmo trutta) indicated transfer of MDC to higher trophic levels in streams, a phenomenon noted only once before, and the first observation in upland streams. The extent of MDC utilisation, identified through variation in carbon stable isotope values, showed strong spatial variation, with lower δ13C values, linked to catchment land use characteristics, particularly the extent of low soil drainage and pastures. In addition to showing spatial variability, carbon stable isotope values of in-stream basal resources also exhibited marked variation, with stream bryophytes and their associated biofilms having consistently low δ13C values, with some samples reaching below -50‰. Invertebrates in the grazer/scraper functional feeding group were the only invertebrates identifiable as utilising MDC, with four common and abundant taxa identified: Baetidae (Baetis sp.), Goeridae, Heptageniidae and Planorbidae (Ancylus fluviatilis). The use of molecular approaches (sequencing and quantitative PCR (qPCR)) alongside carbon stable isotopes established correlations between low δ13C values and methanogen and methanotroph archaeal bacterial abundances. Methanogenic and methanotrophic community composition and abundance also differed between basal resources. Type I methanotrophs (methanotrophs that use the RuMp pathway) appeared to have the strongest link to MDC contributions, potentially due to their higher affinity for CH4. Upland streams look to have “MDC” hotspots, driven by terrestrial catchment characteristics, favourable for methanotrophs and methanogens. Within these hotspots MDC looks to be an important part of the diet of many abundant invertebrate species, with the freshwater bryophyte Fontinalis antipyretica appearing to be a significant conduit to the invertebrate food web. These findings highlight stream CH4 oxidation as an overlooked resource for aquatic food webs and a potential mitigator of greenhouse gas emissions. Managing streams to support methanotrophic niches, such as bryophytes, will likely bring the dual benefit of diversifying instream carbon sources and slow CH4 emissions through short term sequestration into grazer/scraper invertebrates and fish species. Rising stream temperatures brought about by climate change brings uncertainty around effects on MDC occurrence in streams. With MDC occurrence dependent on CH4 availability, catchment characteristics, stream O2 concentrations, methanotrophic community composition, the presence of bacterial and/or phototroph symbionts, and the presence of MDC-utilising invertebrates; changes to any one of these factors may alter MDC importance. Thus future research should aim to understand how this balance of factors is likely to change within a warming climate.