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

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 (CO2), methane (CH4), and nitrous oxide (N2O) 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 CO2 flux over this period, and that in‐channel aerobic metabolism was a primary predictor of both CH4 and N2O 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 CO2 and CH4 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. Plain Language Summary: Stream ecosystems play a role in producing greenhouse gases (GHGs) and transporting them from groundwater to be released into the atmosphere. Some of these gases are produced through the breakdown of organic matter by microbes in the stream. We don't know how important this microbial production is compared to gases coming from soil and groundwater, but comparing it to rates of ecosystem metabolism may help us learn about it. We measured GHG and metabolism along a stream and found that metabolism is directly responsible for the production of carbon dioxide and is a good predictor of methane, meaning that microbial production in the stream is likely important. N2O production was limited by competition between microbes for nitrogen, as a result, the stream was removing N2O from the atmosphere rather than releasing it. Key Points: Patterns in carbon dioxide are almost entirely accounted for by instream metabolism except during a storm eventMethane is highly variable in space and covaries with aerobic respirationThe stream shifts from a source of nitrous oxide to a sink during peak autumn respiration, likely due to nitrogen limitation [ABSTRACT FROM AUTHOR]