Testing the Pulse-Shunt Hypothesis: In Situ Data Reveal Hydrological Extremes and Scaling Controls on Carbon Uptake in a River Network.

Abstract:

When and where river networks process organic matter as it flows from watersheds to oceans has emerged as a critical question in the study of the global carbon cycle. Headwater streams are viewed as important sites of carbon processing during baseflow conditions. New evidence suggests that the large pulses of organic matter that are transported from watersheds during storm events may overwhelm the processing capacity of headwaters, shifting the majority of carbon metabolism to larger channels downstream. We tested this hypothesis using a network of in situ water quality sondes to model dissolved organic matter flux and ecosystem metabolism – respiration and gross primary production – in the Connecticut River watershed. Probes were deployed in channels ranging from first order to the main stem of the Connecticut River (eight order) in the Farmington River (Connecticut) and Passumpsic River (Vermont) watersheds. Dissolved organic matter was estimated using fluorescence dissolved organic matter (fDOM) sensors and a hierarchical Bayesian model was applied to dissolved oxygen data to estimate ecosystem metabolism at a site.

We identified a positive relationship between ecosystem respiration and discharge across all sites with this effect being particularly strong in fourth and fifth order rivers, exceeding the rates observed at both headwater and Connecticut River main stem sites. Dissolved organic matter concentration and flux also responded positively to increased flows. Similar relationships between flow and metabolism were observed in both Connecticut and Vermont sites, indicating that the patterns observed can be generalized across the Connecticut River watershed. While processing of organic matter increased during storm events, the greater flux of organic matter during high discharge overwhelmed the increased processing capacity. Thus, while the proportion of organic matter metabolized increased, the absolute amount of organic matter shunted downstream remained far larger during high flows. Nonetheless, our results demonstrate that ‘middle-order’ rivers are important sites of carbon processing, particularly following precipitation events.