MRA: Continental scale controls on instream and catchment contributions to greenhouse gas fluxes from rivers

The primary goal of this project is to derive a robust and defensible estimate of the greenhouse gases emitted to the atmosphere from the rivers of the United States and to build predictions of how these gas emissions are likely to change in coming decades. The greenhouse gases emitted from rivers are both delivered from their surrounding watersheds and produced by river organisms.

River and watershed characteristics such as river structure, groundwater exchange, algal production, respiration, temperature, and contributing land types can affect the rates at which rivers produce and release carbon dioxide and other greenhouse gases. However, scaling these known factors to continuous, widespread predictions of river greenhouse gas dynamics remains difficult because of limited sampling times and locations across this expansive suite of variables.

This data limitation has now been overcome for river monitoring sites included in the National Ecological Observatory Network (NEON). Using data collected from 26 NEON river sites across the United States, researchers will build models of continuous gas emissions. We will take advantage of the wide variation in watershed and channel characteristics across sites to build statistical models that allow us to estimate riverine greenhouse gas fluxes across the entire continent, and to identify those sites where gas emissions are most likely to respond strongly to rising temperatures or altered hydrology.

This project will also train early-career scientists, including underrepresented groups, in field ecology and the processing and interpretation of large data sets to make better predictions of global change.

This proposal seeks to answer three questions: (1) How do greenhouse gas (GHG) fluxes vary across streams and rivers of the continental U.S.? (2) What channel and catchment properties are associated with the highest riverine releases of carbon dioxide, methane, and nitrous oxide from rivers? and (3) what channel characteristics are associated with the highest instream contribution (e.g., primary production and respiration) to riverine GHG emissions? Using the diverse portfolio of hydrologic, biogeochemical, and geomorphic data collected by the National Ecological Observatory Network (NEON) at its 26 wadeable stream locations, we will leverage level 0 data (including discrete estimates of GHG concentrations and solute chemistry in the channel and in groundwater along with continuous measures of hydrology, pH, conductivity, and dissolved oxygen from the channel) to build robust estimates of annual GHG emissions from all 26 sites.

We will link these GHG fluxes with concurrent estimates of stream metabolism, rates of groundwater inflow, and groundwater-surface water GHG exchange, and use reaeration models to discriminate between instream and catchment derived GHG contributions to riverine GHG fluxes. We will refine and test our data-derived models with intensive field work at a subset of NEON sites and at a field site in North Carolina where we conduct detailed empirical research.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.