Lost In Transition – aquatic carbon evolution along a headwater stream network

Large amounts of carbon (C) are transported through Earth’s aquatic conduit every year. While the C transported to freshwater systems as dissolved organic C (DOC) and dissolved inorganic C (DIC) has been investigated for some time, the dynamics controlling how much DIC gets evaded to the atmosphere (as CO₂) remains uncertain and an area of active investigation. Recent technological advancement using in-situ dissolved CO₂ (pCO₂) sensors increased previously established global C evasion estimates but sensor networks that inform this estimate have an inherent spatial bias towards major tributaries. Contributions from headwater streams where pCO₂ values tend to be higher are less well constrained and tend to be biased towards daytime measurements. This research seeks to highlight how much C (as DOC and pCO₂) is transported at multiple locations along a headwater to 2^nd order stream system, along with an investigation of local dynamics controlling DOC and DIC transformation within the stream network.

Headwater monitoring stations were installed in University of British Columbia’s Malcolm Knapp Research Forest in the North American Pacific Coastal temperate rainforest (PCTR) region - an aquatic C hotspot due to its high productivity rainforest ecosystems and steep elevation gradient. This research reports trends of aquatic DOC dynamics and CO₂ evasion fluxes where continuous DOC measurements [s::can UV-Vis Spectrolyzer] are validated monthly with the Shimadzu Total Organic Carbon analyzer and time series data obtained from dissolved CO₂ probes [Vaisala GMP221] are corrected using gas chromatography [Agilent 7890A]. Monitoring stations (DOC and pCO₂) installed over several kilometers in the same stream permitted an investigation into how aquatic carbon evolves between organic and inorganic phases, as well as CO₂ transfer between the dissolved phase and the atmosphere.

Preliminary data indicates that over the 2 km reach, pCO₂ decreased by an average of 22.8%, while DOC declined by 2.35 mg/L between upstream and downstream sites - a 74.9% reduction in DOC concentration over this distance. Ongoing research seeks to disentangle dilusion vs. aquatic metabolism as controlling factors of the observed DOC concentration reduction. In this presentation I will also discuss temporal dynamics (e.g., high flow vs. low flow conditions and hysteresis behaviour), DOC characterization (as SUVA254 and spectral slopes), and evasion flux calculations at the two locations. Results from this study will help establish transformation pathways that connect DOC and DIC with contributions to biogeochemical understanding of catchment carbon cycle and aid in identifying the role of PCTR headwater streams in the global evasion estimate context.