Freshwater systems are an important component of the global carbon cycle as they outgas disproportionately large quantities of carbon compared to the terrestrial landscape. Of particular importance are headwater streams, which represent roughly 90% of the channel network by length and have been conservatively estimated to outgas roughly 36% of the carbon dioxide (CO2) that is evaded from rivers and streams globally. We investigated carbon fluxes of a second order headwater stream that drains a 96 ha forested watershed in western Oregon, USA. Total inorganic carbon imports and exports were estimated to be 1294 kg C yr-1 (130 kg C ha-1yr-1). Influx from hillslope runoff and groundwater was measured to be 65.6 kg C ha-1yr-1, 50% of total imports. The remaining imports were split between stream metabolism at 26% (33.8 kg C ha-1yr-1) and near stream riparian sources at 23% (29.9 kg C ha-1yr-1). Exports of inorganic carbon as CO2 from the stream to the atmosphere were estimated to be 59% (76.9 kg C ha-1yr-1) of total exports. Streamflow exported the remaining 41% (53.1 kg C ha-1yr-1) of basin-scaled flux. Results highlight the importance of both external and internal carbon sources to the stream carbon budget.
Aeration rate is an integral parameter for the measurement of CO2 evasion. It is also needed to measure instream metabolic processing. Common field methods to estimate the aeration rate have strengths and weaknesses, and researchers continue to search for better techniques, particularly for steep streams with high rates of gas exchange and low productivity.
We developed the oxygen carbon (OC) method for calculating gas-exchange rates from simultaneous measurement of oxygen (O2) and dissolved inorganic carbon (DIC). Gas-exchange rates are calculated by solving the combined stream transport equation for O2 and DIC. The output is a time series of aeration rates at the same sampling frequency as the input O2 and carbon (C) data. Field tests in a fourth order montane stream in Oregon, USA, were a success. The OC method estimated the aeration rate to 3.25 h-1, which agreed well with the value from direct gas injection of 3.22 h-1. Sensitivity analysis indicated that application of the OC method is limited to reaches with a suitable change in combined O2 and CO2 concentration ≥ ~4 μmol/L and combined O2 and CO2 saturation deficits ≈ 4 μmol/L. The OC method was then applied in a second order headwater stream over a wide range of flow conditions, allowing for development of a site-specific regression between discharge and aeration rate.
This research was made possible through support from the National Science Foundation (1417603)
H. J. Andrews Experimental Forest research program, funded by the National Science Foundation (NSF) LTER (DEB 1440409), US Forest Service Pacific Northwest (USFS PNW), and Oregon State University (OSU).