Environmental Factors Affecting Methane Cycling in Narragansett Bay
Wetlands and estuaries are strong sources of methane to the atmosphere due to high rates of methanogenesis. These environments contain diverse and extensive microbial communities that are responsible for processing organic matter, with tidal flow responsible for exchange between marine, freshwater, and estuarine sources. While most methane is produced in sediment, as methanogenic Archaea generally require anoxic conditions, methane oxidizing bacteria (methanotrophs) regulate emissions in oxygenated waters through the consumption of methane. Therefore, flood tide has the potential to provide wetlands—salt marshes, in particular—with organic matter and methanotrophic communities, creating a unique environment which features the co-occurrence of both methanogenesis and methanotrophy. Ebb tide would then be responsible for the transport of methane and potentially microbial communities to nearby waters. There has been limited research investigating the importance of salt marshes to marine methane cycling. ^ The objective of this thesis is to study the spatial and temporal distribution of methane in Narragansett Bay and investigate the role of salt marshes in Bay cycling under various environmental conditions using stable isotope analysis. The latter was achieved through incubations of porewater and outlet samples collected from Fox Hill Salt Marsh in December (7°C water), March (2°C), and May (17°C) during ebb tide, with discrete monitoring of CH4, CO2, and CH4 δ13C. Methanogenesis was limited in our anoxic samples, largely due to its inhibition by the presence of sulfate reducing bacteria and the lack of sediment in sample vessels, although there was some evidence for the hydrogenotrophic mechanism of production. Oxidation occurred in 7°C and 17°C oxygenated outlet samples, as well as 17°C marsh porewater samples, though only after a lag period of at least 30 days. The lag likely reflects the presence of facultative methanotrophs in Narragansett Bay, in which methane oxidation occurs only after preferential substrates are consumed. Rates of CH4 oxidation and microbial respiration, through observed production of CO2, were significantly correlated with temperature indicating seasonal changes in methane cycling, also evidenced by the lack of activity in all 2°C samples and greater abundance of methanotrophs at 17°C, determined by quantitative PCR. Higher methane oxidation rates in marsh outlet samples reflected the greater degradability of marine organic sources over marsh organic material, indicating that flood tide is crucial for the input of bioavailable organic material to salt marsh microbial communities.^ The spatial and temporal distribution of methane was investigated through shipboard samplings from the Providence River to the mouth of Narragansett Bay and the implementation of a time-series study monitoring methane at a fixed location from May to July, respectively. Methane concentrations varied inverse to the salinity gradient, with high concentrations (96nM) at the river mouth suggesting a significant freshwater source. Freshwater methane was likely the results of wastewater treatment plant effluent and increased methanogenesis due to an absence of sulfate reducing bacteria. Methane concentrations throughout Narragansett Bay were found to be supersaturated with respect to the atmosphere due to freshwater input (including wastewater) and marsh and sediment methanogenesis influence. Principal component analysis of time-series data confirmed the seasonality of methane in Narragansett Bay, as methane concentrations increased with temperature from Spring into Summer as wind speed decreased. Tidal influence was also evident in time-series data, with an observed anticorrelation between methane and both tidal height and salinity. ^ The contribution of salt marshes to Narragansett Bay was determined to be 2.4 × 105 mol year-1, computed from data collected continuously by a moored CTD at Fox Hill Salt Marsh outlet. With the limited methanotrophy that occurred in incubation samples, it is probable that the majority of the methane contribution from salt marshes is emitted rather than oxidized in Narraganset Bay.^
Christopher James McAleer,
"Environmental Factors Affecting Methane Cycling in Narragansett Bay"
Dissertations and Master's Theses (Campus Access).