Patterns and Drivers of Dissolved Gas Concentrations and Fluxes Along a Low Gradient Stream

Abstract

Abstract Freshwater ecosystems are globally significant sources of greenhouse gases (GHGs) to the atmosphere. Previous work has indicated that GHG flux in headwater streams is dominated by terrestrially derived gases, with in situ production limited by short organic matter residence times and high dissolved oxygen concentrations due to turbulent reaeration. However, low‐gradient headwater streams that contain pool structures with longer residence times may be conducive to the in situ production of GHG. These streams, and the longitudinal heterogeneity therein, are seldom studied. We measured continuous ecosystem metabolism alongside concentrations and fluxes of carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) in a low‐gradient third order stream in the North Carolina Piedmont. From autumn to the following spring, we characterized spatial and temporal patterns of GHG along an 8 km segment in the context of channel geomorphology, hydrology, and ecosystem metabolic rates using linear mixed effects models. We found that stream metabolism was responsible for most of the CO 2 flux over this period, and that in‐channel aerobic metabolism was a primary predictor of both CH 4 and N 2 O fluxes as well. Long water residence times, limited reaeration, and substantial organic matter from terrestrial inputs foster conditions conducive to the in‐stream accumulation of CO 2 and CH 4 from microbial respiration. Streams like this one are common in landscapes with low topographic relief, making it likely that the high contribution of instream metabolism to GHG fluxes that we observed is a widespread yet understudied behavior of many small streams.