The contribution of aquatic metabolism to CO2 emissions from New Hampshire streams

Abstract:

Fluvial networks represent a significant source of carbon dioxide (CO₂) to the atmosphere. Recent evidence has highlighted the ubiquity of CO₂ supersaturation in streams, rivers, and lakes worldwide, yet our understanding of how the source of this CO₂ flux (e.g. in situ aquatic production versus soil and groundwater sources within the catchment) varies in time and across different aquatic systems remains limited. In this study we used continuous, high-frequency measurements of dissolved oxygen (DO) and CO₂ to model stream metabolism and CO₂ emissions for five stream sites across New Hampshire that vary in size, nutrient loading, and landscape context, with the goal of quantitatively partitioning the aquatic CO₂ flux into catchment and aquatic sources, respectively. Spectral analysis of the DO and CO₂ time series indicates that these gases often deviated from the pure inverse behavior that would be expected if CO₂ flux originated solely from in-stream biological activity. Across all streams, the estimated contribution of aquatic net ecosystem production (NEP) to stream CO₂ flux varied from approximately 0% to 50%. For each site, the proportion of CO₂ flux supported by aquatic NEP was lower at higher discharge, perhaps due to increased CO₂ transport from soils to streams during wetter periods, and/or due to effects of scouring flows and carbon removal on stream metabolism. Our data provides evidence that catchment sources represent substantial contributions to aquatic CO₂ flux across temperate streams, but that the proportion of CO₂ flux originating from net in situ production and carbon transformation is variable throughout the growing season.