The role of inland waters for the global carbon cycle is now recognized and evidence increasingly suggests that stream ecosystems disproportionately contribute to the carbon cycle. Understanding the dynamics and drivers of stream water partial pressure of CO 2 (pCO 2 ) and CO 2 evasion fluxes from streams to the atmosphere is imperative for assessing the role of climate change on the carbon cycle in stream ecosystems. Monitoring pCO 2 over 3 years, we here report on the seasonal, diurnal, and event-driven dynamics of pCO 2 in the hyporheic zone and stream water of an Alpine stream and assess possible drivers of these dynamics. Our findings suggest that both catchment-derived CO 2 delivered by shallow groundwater into the stream and in-stream respiration continuously build up pCO 2 in the hyporheic zone. Depending on stream water temperature and assumedly on primary production (inferred from photosynthetically active radiation), hyporheic CO 2 contributes to stream water pCO 2 and ultimately to CO 2 outgassing to the atmosphere. Diurnal patterns of stream water pCO 2 increasingly built up during extended base flow and streambed-scouring storms caused the collapse of these diurnal patterns. Post storm recovery of the diurnal pCO 2 patterns was generally rapid. Our findings suggest that decreasing gas exchange velocity related to receding discharge drives recovery dynamics. We found that average CO 2 outgassing fluxes during night exceeded those during day by up to 1.8 times. Our study highlights temperature and hydrology-key components of climate change-as major drivers of pCO 2 dynamics in Alpine streams. They also underscore the necessity to consider day-night differences in CO 2 outgassing fluxes to properly establish carbon budgets and regional estimates of CO 2 outgassing to the atmosphere.

Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. In light of climate and global change it is crucial to understand the temporal dynamics of DOM concentration and composition, and its export fluxes from headwaters to larger downstream ecosystems. We monitored DOM concentration and composition based on a diurnal sampling design for 3 years in an Alpine headwater stream. We found hydrologic variability to control DOM composition and the coupling of DOM dynamics in the streamwater and the hyporheic zone. High‐flow events increased DOM inputs from terrestrial sources (as indicated by the contributions of humic‐ and fulvic‐like fluorescence), while summer baseflow enhanced the autochthonous imprint of DOM. Diurnal and seasonal patterns of DOM composition were likely induced by biological processes linked to temperature and photosynthetic active radiation (PAR). Floods frequently interrupted diurnal and seasonal patterns of DOM, which led to a decoupling of streamwater and hyporheic water DOM composition and delivery of aromatic and humic‐like DOM to the streamwater. Accordingly, DOM export fluxes were largely of terrigenous origin as indicated by optical properties. Our study highlights the relevance of hydrologic and seasonal dynamics for the origin, composition and fluxes of DOM in an Alpine headwater stream.

Lakes are an inherent component of the global carbon cycle. They receive dissolved organic matter (DOM) from the catchment, which is stored, transformed and respired, or delivered downstream. In this study, we show that a subalpine lake shifts its role from DOM “transporter” to “transformer” depending on season and climate. We monitored dissolved organic carbon (DOC) concentration and DOM optical properties at the inlet and outlet of subalpine Lake Lunz (Austria) at high frequency during two contrasting years: an extreme drought in 2015, and regular precipitation regime in 2016. During both years, the DOC mass balance revealed that inflowing and outflowing DOC loads were nearly balanced (+6.57% and +1.70% DOC production in 2015 and 2016, respectively). However, DOM optical properties revealed an in‐lake turnover of DOM compounds, so that the terrestrial and aromatic signature of inflowing DOM was modified into autochthonous, protein‐like DOM. The magnitude of this transformation varied seasonally, being maximal in summer and minimal in winter, presumably following periods of high and low primary production and photo‐degradation. Inter‐annually, we found that drought further increased DOM transformation during summer by extending the lake water residence time. Finally, our results demonstrate a rapid response of DOM dynamics to hydrological and meteorological changes at both seasonal and inter‐annual scales, suggesting that carbon cycling in clear‐water mountain lakes may be highly sensitive to hydrological variation.

Agriculture delivers significant amounts of dissolved organic matter (DOM) to streams, thereby changing the composition and biodegradability of the aquatic DOM. This study focuses on the interactive effects of DOM quality and biofilm composition on the degradation of DOM in a laboratory flume experiment. Half of the flumes were exposed to light to stimulate algal growth, the other half was shaded. Leachates of deciduous leaves, maize leaves, and cow dung were added to the flumes in a single pulse and changes of DOC (dissolved organic carbon) and nutrient concentrations, DOM composition (absorbance and fluorescence data), chlorophyll-a concentrations, bacterial abundances, and enzymatic activities were recorded over a week. DOM was taken up with rates of 50, 109, and 136 µg DOC L−1 h−1 for dung, leaf, and maize leachates, respectively, in the light flumes and 37, 80, and 170 µg DOC L−1 h−1 in the dark flumes. DOC uptake correlated strongly with initial SRP (soluble reactive phosphorus) and DOC concentrations, but barely with DOM components and indices. Algae mostly stimulated the microbial DOC uptake, but the effects differed among differently aged biofilms. We developed a conceptual model of intrinsic (DOM quality) and external (environmental) controlling factors on DOM degradation, with the microbial community acting as biotic filter.

Streams and rivers metabolize dissolved organic matter (DOM). Although most DOM compounds originate from natural sources, recreational use of rivers increasingly introduces chemically distinct anthropogenic DOM. So far, the ecological impact of this DOM source is not well understood. Here, we show that a large music festival held adjacent to the Traisen River in Austria increased the river’s dissolved organic carbon (DOC) concentration from 1.6 to 2.1 mg L –1 and stream ecosystem respiration from −3.2 to −4.5 mg L –1 . The DOC increase was not detected by sensors continuously logging absorbance spectra, thereby challenging their applicability for monitoring. However, the fluorescence intensity doubled during the festival. Using parallel factor analysis, we were able to assign the increase in fluorescence intensity to the chemically stable UV-B filter phenylbenzimidazole sulfonic acid, indicating organic compounds in sunscreen and other personal care products as sources of elevated DOC. This observation was confirmed by liquid chromatography coupled with mass spectrometry. The elevated respiration is probably fueled by anthropogenic DOM contained in beer and/or urine. We conclude that intense recreational use of running waters transiently increases the anthropogenic DOM load into stream ecosystems and alters the fluvial metabolism. We further propose that chemically distinct, manmade DOM extends the natural range of DOM decomposition rates in fluvial ecosystems.