Rivers and streams are often supersaturated with methane, but what does this mean for the life residing in them?
To build a more complete picture of methane’s importance as a carbon source in freshwater streams this PhD project investigates methane derived carbon (MDC) at three different spatial scales. At the largest scale, how does a stream’s catchment influence methane’s sequestration into stream systems? Narrowing down the scale further, we look at pathways of methane through stream food webs. Which species within the web are the vectors to which methane enters the food web, and how dependent are they on methane as a carbon source? Lastly, we look at the microbiological level, what stream niches dictate the abundance of methane producing and methane oxidising organisms?
To address all these questions streams were studied across the Upper Tweed and Gala tributaries of the River Tweed, Scotland. Stream catchment influences were analysed through based on carbon stable isotope analysis of brown trout (S. trutta) and spatial data about catchment characteristics. There were significant trends in relation to MDC influence on trout stable carbon isotope values and catchment characteristics, most notably with land use and the abundance of low drainage areas. For stream communities, mixing-models based on both carbon and nitrogen stable isotope data identified the presence of key functional feeding groups/species (grazers) as important vectors of MDC into the food web. To understand the microbial communities responsible for methane production/oxidation amplicon sequencing and qPCR are used to compare methanogenic archaea and methane oxidising bacteria abundance with the overall bacterial community at different instream niches. This was further supported with carbon stable isotope data from these niches to identify MDC in organic matter such as biofilms, sediments, invertebrates and macrophytes.