Spatiotemporal Drivers of CO2 Dynamics and Evasion Fluxes from Mountain Streams

Inland waters are key components of the global carbon cycle, emitting a substantial amount of carbon dioxide (CO2) to the atmosphere. Climate change and global warming affect mountain catchments significantly, leading to glacier retreat and changing precipitation patterns. What this means for the streams in the mountains, and for CO2 sources, dynamics and fluxes from those streams, is poorly understood. Generally, mountain streams have low CO2 concentrations, largely due to low organic carbon content in catchments above the tree line and poorly developed soil horizons. However, it is yet unclear from where the streamwater CO2 is coming, what drives the CO2 dynamics, and the magnitudes of the CO2 evasion fluxes from the stream surfaces to the atmosphere.

This PhD thesis aims to fill this knowledge gap. By using a combination of field sampling, sensor monitoring stations, and analyses of stable isotopes, I identify potential drivers of CO2 concentrations and fluxes from mountain streams across different spatial and temporal scales. Moreover, in combination with recent insights of gas exchange from mountain streams, this thesis comprises new quantifications of CO2 evasion fluxes from Swiss mountain streams, as well as mountain streams worldwide.

Within the frame of this thesis, two major field studies were carried out. The first field study consisted of repeated seasonal sampling campaigns every 50 meter across an Alpine stream network. We found large seasonal and spatial variations in streamwater CO2 concentrations, and identified soil respiration as the major source of streamwater CO2. With spring snowmelt, and increased catchment connectivity, more respiratory CO2 could be transported to the stream, however due to high gas transfer velocities, the CO2 was rapidly evaded downstream.

The second major field study consisted of high-frequency (10 minutes) monitoring of streamwater CO2 in 12 streams located in 4 Alpine catchments. This enabled us to further explore the role of snowmelt for CO2 dynamics in mountain streams. We identified different responses in CO2 concentration and CO2 evasion fluxes to increasing runoff across different temporal scales. On annual time scales, increasing runoff led to lower streamwater CO2 concentration. However, during the onset of snowmelt we found increasing CO2 concentrations with higher runoff, leading to higher CO2 evasion fluxes.

To better contextualize our results and estimate CO2 evasion fluxes from mountain streams, we combined insights gained from the two field studies with recent insights of gas exchange in turbulent streams. We developed a simple CO2 prediction model, and modelled the CO2 evasion fluxes from all small mountain streams in Switzerland, as well as all mountain streams worldwide. We estimate that mountain streams worldwide emit 167 ± 1.5 Tg C yr-1. This estimate is unexpectedly high given the small total surface area of small mountain streams.

This thesis brings new light on the CO2 dynamics in mountain streams, not only by highlighting the role of spatiotemporal catchment dynamics and in particular the important role of snowmelt for facilitating transport of catchment-derived CO2 to the streams, but also by providing one of the first global estimates of the quantity of CO2 that is evaded from mountain streams every year.

Carbon fluxes and productivity regimes in Alpine streams

Marta Boix Canadell

High-altitude catchments have a major role in the transport of organic matter to streams due to the storage of dissolved organic carbon (DOC) in soils and glacier ice and the subsequent mobilization during the melting processes. Yet, stream function goes beyond the conveying downstream of the terrestrial and glacier derived DOC since they are also highly active in the mineralization, retention and production of organic matter. Stream ecosystem metabolism integrates the processes that regulate the conversion between the organic and inorganic forms of carbon in streams and is a fundamental measure to determine whether carbon accumulates or it is lost within the ecosystem. In a global warming scenario, high-mountain stream ecosystems are faced to modifications in their structure and function as a result of climate-driven hydrological changes. Thus far, despite the active role of alpine streams in the global carbon cycle, studies on the impacts from changes in snowmelt, glacier ice melt and groundwater contribution to streamflow have been largely focusing on alterations in hydrological regimes, water availability, geomorphology and biodiversity. To fill this gap, the aim of this thesis is to examine DOC fluxes dynamics and productivity regimes across a range of glacierized and non-glacierized alpine catchments to anticipate the possible consequences of projected hydrological changes on alpine stream biogeochemistry and ecosystem functioning. This thesis is supported by the collection and analysis of high-frequency time series of physicochemical parameters, geomorphological data and streamwater samples from different Alpine streams with contrasting glacier coverage. The first part of the thesis investigates the response across all the streams of the annual DOC export to runoff primarily driven by snowmelt and glacier melt. In the second part, with the use of dissolved oxygen time series, the ecosystem energetics regimes in a glacier-, groundwater- and snowmelt-fed stream are explored during two years and related to their physical template. The third and last part is focused on providing estimates of gross primary production (GPP) rates for the energetic regimes established in the three Alpine stream types. The obtained results show a varied response of DOC export to runoff across the catchments which was related to the degree of glaciation and vegetation cover. Our findings also reveal different stream ecosystem energetic regimes among the streams and highlight discharge as the major modulator of drivers, such as light, gas exchange rate or stability, on the seasonal and daily dissolved oxygen dynamics. Lastly, the magnitude and the temporal patterns of ecosystem GPP are the result of the light and streambed disturbance conditions that are largely determined by the hydrological and turbidity regime. I argue that glacier shrinkage together with changes in snowmelt hydrology will alter the response of DOC yield to discharge, with consequent impact on the timing and magnitude of the lateral DOC fluxes from terrestrial to stream ecosystems. Also, an eventual reduction in glacier runoff and snowpack duration and content will alter the physical template for primary producers, which may lead to a greater production of autochthonous organic matter across high-mountain streams. Overall, I assume a shift in the magnitude and temporal patterns of energetic inputs, with consequences for the stream biogeochemistry and function.

EPFL2020

Streamwater pCO2in an Alpine Headwater Catchment

Tom Ian Battin, Marta Boix Canadell, Asa Lovisa Viktoria Horgby, Amber Joy Ulseth

Boreal and alpine streams are highly sensitive to climate change. Previous studies have shown the relevance of streams for carbon cycling in boreal landscapes. Less work has been done on alpine streams, and we do not understand the contributions from these ecosystems to the regional and global carbon cycles. The objective of this study is to assess critical carbon fluxes in an Alpine stream network, by quantifying the partial pressure of carbon dioxide, pCO2, and potential sources. We sampled every 50 meter along the stream network in a small Swiss Alpine catchment, during the four different seasons of the year (winter, spring, summer and autumn of 2016). By doing this, we identify high resolution spatial patterns along the stream network, together with seasonal patterns to capture different hydrological signatures (snowmelt, glacier melt and groundwater). We found that the streamwater was supersaturated in CO2 and the δ13C of dissolved inorganic carbon (DIC) indicated weathered bedrock materials as a major source of CO2, where groundwater transfers carbon to the stream. Moreover, the pCO2 increased with enriched stable isotopes of water, indicating a shift in CO2 sources from upstream to downstream, where the more snowmelt dominated upstream tributaries were diluted with groundwater inflow downstream. These results are yet preliminary. However, the supersaturation of CO2 shows that the contributions of CO2 from Alpine streams may be significant when assessing CO2 outgassing from inland waters. Further analysis of current results is needed, including quantifying the in-stream respiratory contribution to the stream pCO2.

2017

Carbon fluxes and productivity regimes in Alpine streams

Marta Boix Canadell

EPFL2020