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Enhanced Carbon Metabolism Downstream of a Stream Confluence Mixing Zone

Parameters: CDOM, Turbidity

Congratulations to Stephen Plont who was awarded a Travel Stipend for his presentation at the 2019 Society for Freshwater Science featuring our Cyclops and AquaFluor Fluorometers.


Stream confluences are ubiquitous features in freshwater networks and are known for distinct hydrogeomorphic characteristics relative to upstream tributaries and downstream reaches. However, the role of confluences as control points, locations that disproportionately influence ecosystem function (e.g., carbon metabolism, nutrient uptake), is still unknown. We might expect enhanced biological processes at confluences due to the delivery and mixing of carbon and nutrients. To test the potential influence of confluence mixing zones on dissolved organic carbon (DOC) removal in stream networks, we estimated DOC uptake using pulse injections of roasted barley leachate (standardized DOC source) and NaCl in two tributary reaches and downstream of their confluence. We used high-frequency fluorescent dissolved organic matter sensors (fDOM; Turner Cyclops 7F) to estimate changes DOC concentration as the roasted barley pulse moved passed our sampling sites, and calculated DOC uptake both from water samples taken every 2 minutes, and from fDOM sensor data (sampling interval = 5 seconds). DOC uptake velocity (vf-DOC) in each tributary was 0.843 and 0.378 mm/min. vf-DOC downstream (0.94 mm/min) was higher than vf-DOC predicted from a mixing model of upstream vf-DOC and proportional flow contributions of tributaries (0.75 mm/min). This discrepancy in measured and predicted vf-DOC is also supported by bioassay experiments which found increased DOC uptake within the confluence mixing zone.. These results suggest that DOC uptake downstream of confluences cannot be estimated from tributary DOC uptake alone. Confluences may play a distinct role in carbon cycling. >>View Presentation

Author: Stephen Plont, Caitlin Miller, Erin Hotchkiss

Institution: Virginia Tech, Virginia, USA

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