CLIMATE-INDUCED CHANGES IN SPRING SNOWMELT IMPACT ECOSYSTEM METABOLISM AND CARBON FLUXES IN AN ALPINE STREAM NETWORK

Although stream ecosystems are recognized as an important component of the global carbon cycle, the impact of climate-induced hydrological changes on carbon fluxes in stream networks remain unclear. We will discuss the effects of changes in snowmelt hydrology during the anomalously warm winter of 2013/2014 on gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) in an Alpine stream network. We estimated ecosystem metabolism across 12 study reaches of a 254 km2 subalpine catchment located in the Ybbs River Network (YRN), Austria, for 18 months. GPP peaked in 10 of our 12 study reaches during spring snowmelt, regardless of the amount of snow the previous winter. However, the shift from snow to rain precipitation following the low snow winter in 2013/2014 increased spring ER in upper elevation catchments, thereby shifting spring NEP from autotrophy to heterotrophy. Our findings suggest that the YRN evolved from transient sinks to sources of carbon dioxide (CO2) in spring as snowmelt hydrology differed following the high snow versus low snow winter. This shift towards increased heterotrophy during spring snowmelt following a warm winter has potential consequences towards annual ecosystem metabolism, as spring GPP contributed on average 33% to annual GPP fluxes compared to spring ER, which averaged 21% of annual ER fluxes. We propose that Alpine headwaters shunt less organic carbon to downstream ecosystems, but will emit more within-stream respiratory CO2 to the atmosphere annually, as the climate gets warmer.

Climate-Induced Changes in Spring Snowmelt Impact Ecosystem Metabolism and Carbon Fluxes in an Alpine Stream Network

Tom Ian Battin, Enrico Bertuzzo, Amber Joy Ulseth

Although stream ecosystems are recognized as an important component of the global carbon cycle, the impacts of climate-induced hydrological extremes on carbon fluxes in stream networks remain unclear. Using continuous measurements of ecosystem metabolism, we report on the effects of changes in snowmelt hydrology during the anomalously warm winter 2013/2014 on gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) in an Alpine stream network. We estimated ecosystem metabolism across 12 study reaches of the 254 km2 subalpine Ybbs River Network (YRN), Austria, for 18 months. During spring snowmelt, GPP peaked in 10 of our 12 study reaches, which appeared to be driven by PAR and catchment area. In contrast, the winter precipitation shift from snow to rain following the lowsnow winter in 2013/2014 increased spring ER in upper elevation catchments, causing spring NEP to shift from autotrophy to heterotrophy. Our findings suggest that the YRN transitioned from a transient sink to a source of carbon dioxide (CO2) in spring as snowmelt hydrology differed following the highsnow versus low-snow winter. This shift toward increased heterotrophy during spring snowmelt following a warm winter has potential consequences for annual ecosystem metabolism, as spring GPP contributed on average 33% to annual GPP fluxes compared to spring ER, which averaged 21% of annual ER fluxes. We propose that Alpine headwaters will emit more within-stream respiratory CO2 to the atmosphere while providing less autochthonous organic energy to downstream ecosystems as the climate gets warmer.

Springer Verlag2017

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

Land use controls stream ecosystem metabolism by shifting dissolved organic matter and nutrient regimes

Amber Joy Ulseth

Stream ecosystem metabolism integrates production and respiration of organic matter and plays a fundamental role in the global carbon (C) cycle. Several studies have identified distal and proximal physical controls, for example, land use and transient storage, or the effects of water chemistry, that is, organic matter and nutrient availability, on stream metabolism. In parallel, research on organic matter quality has identified conspicuous gradients of chemical composition, yet mostly without demonstrating any functional implications. 2. We hypothesise that organic matter holds a key position in a more comprehensive causal framework of stream ecosystem metabolism, and that a concurrent study can improve mechanistic understanding. Specifically, we here postulate that dissolved organic matter (DOM) quality, that is, its chemical composition, acts as a control of ecosystem respiration (ER) as much as it is a result of gross primary production (GPP). As such, DOM quality likely forms a central link between land use and stream metabolism, besides known physical controls including transient storage and light availability. 3. To examine these hypotheses, we studied 33 streams in north-eastern Austria, a region with diverse land use ranging from semi-natural, forested areas to agricultural areas and settlements. We analysed DOM composition by absorbance and fluorescence spectroscopy, including modelling excitation–emission matrices with parallel factor analysis. We then opposed these data to GPP and ER estimated by fitting a metabolism model to single-station diurnal oxygen records. 4. Structural equation modelling revealed land use as a control on light conditions, DOM composition and concentration and nutrient concentrations, which together ultimately shaped GPP and ER. In particular, humified, coloured and aromatic DOM of predominantly terrestrial origin was prevalent in coniferous forest catchments and increased stream ER. Agricultural and urban areas enriched streams with phosphorous and nitrogen, which increased ER and GPP. Besides nutrients, GPP seemed to be weakly correlated with light availability and – in contrast to our hypothesis – left only a weak imprint on DOM composition. 5. Land-use change is rated as the most pervasive human influence on natural ecosystems and our results highlight its impact on aquatic GPP and ER in streams. To understand the role of inland waters in the global C cycle will require mechanistic understanding of ecosystem metabolism, which notably includes organic matter quality as a hitherto underappreciated key player.

Wiley-Blackwell2017