Thinking like a consumer: Linking aquatic basal metabolism and consumer dynamics

The increasing availability of high-frequency freshwater ecosystem metabolism data provides an opportunity to identify links between metabolic regimes, as gross primary production and ecosystem respiration patterns, and consumer energetics with the potential to improve our current understanding of consumer dynamics (e.g., population dynamics, community structure, trophic interactions). We describe a conceptual framework linking metabolic regimes of flowing waters with consumer community dynamics. We use this framework to identify three emerging research needs: (1) quantifying the linkage of metabolism and consumer production data via food web theory and carbon use efficiencies, (2) evaluating the roles of metabolic dynamics and other environmental regimes (e.g., hydrology, light) in consumer dynamics, and (3) determining the degree to which metabolic regimes influence the evolution of consumer traits and phenology. Addressing these needs will improve the understanding of consumer biomass and production patterns as metabolic regimes can be viewed as an emergent property of food webs

Carbon fluxes and productivity regimes in Alpine streams

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

Streamwater pCO2in an Alpine Headwater Catchment

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

High‐Resolution Spatial Sampling Identifies Groundwater as Driver of CO 2 Dynamics in an Alpine Stream Network

Inland waters are major sources of CO2 to the atmosphere. The origin of this CO2 is often elusive, especially in high‐altitude streams that remain poorly studied at present. Here we study the spatial and seasonal variations in streamwater CO2, its potential sources and drivers in an Alpine stream network (Switzerland). High‐resolution sampling combined with stable isotope analysis and mixing models enabled us to capture the fine‐scale spatial heterogeneity in streamwater pCO2 as the stream network expanded and contracted during seasons. We identified soil respiration as a major source of CO2 to the stream. We also identified a major groundwater upwelling zone as an ecosystem ‘control point’ that disproportionately influenced stream biogeochemistry. This was particularly pronounced when the stream network expanded during snowmelt, when it covered a five times larger area compared to winter (35 300 m2 compared to 7 100 m2). Downstream from this ‘control point’, CO2 evaded rapidly owing to high gas transfer velocity. The stream network was a net source of CO2 to the atmosphere with an average areal evasion flux of 30.1 (18.0–43.1) μmol m‐2 s‐1 and a total flux at network scale ranging from 237 (141–339) kg C d‐1 in winter to 1793 (1069–2565) kg C d‐1 during spring snowmelt. Our study highlights the role of stream network dynamics and ‘control points’ for the CO2 dynamics in high‐altitude streams

Alpine Glacier Shrinkage Drives Shift in Dissolved Organic Carbon Export From Quasi‐Chemostasis to Transport Limitation

The export of dissolved organic carbon (DOC) from catchments is considered as an important energy flux through streams and a major connection between terrestrial and aquatic systems. However, the impact that predicted hydrological changes due to glacier retreat and reduction in snow cover changes will have on DOC export from high‐mountain streams remains unclear. In this study, we measured daily runoff and DOC yield during 1 year in Alpine streams draining catchments with different levels of glacier coverage. DOC yield showed a varied response to runoff across the catchments and varied seasonally as a function of the degree of glaciation and vegetation cover. Using space‐for‐time substitution, our results indicate that the controls on DOC yield from Alpine catchments change from chemostasis to transport limitation as glaciers shrink