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

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.

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.

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Tom Ian Battin

Subalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream-side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.

2018

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 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.

2017

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