Carbon dioxide fluxes increase from day to night across European streams

Globally, inland waters emit over 2 Pg of carbon per year as carbon dioxide, of which the majority originates from streams and rivers. Despite the global significance of fluvial carbon dioxide emissions, little is known about their diel dynamics. Here we present a large-scale assessment of day- and night-time carbon dioxide fluxes at the water-air interface across 34 European streams. We directly measured fluxes four times between October 2016 and July 2017 using drifting chambers. Median fluxes are 1.4 and 2.1 mmol m−2 h−1 at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. We attribute diel carbon dioxide flux variability mainly to changes in the water partial pressure of carbon dioxide. However, no consistent drivers could be identified across sites. Our findings highlight widespread day-night changes in fluvial carbon dioxide fluxes and suggest that the time of day greatly influences measured carbon dioxide fluxes across European streams

Photosynthetic pigment changes and adaptations in biofilms in response to flow intermittency

14 páginas, 5 figuras, 2 tablas.Among the environmental factors affecting benthic algae and cyanobacteria in streams, the one often producing the largest effects is flow intermittency. This study aimed to characterize the responses of algal assemblages to flow intermittency in a Mediterranean intermittent stream during the drying, non-flow (112 days), and rewetting phases. Algae growing in the epilithic, epipsammic and hyporheic streambed compartments were analyzed for pigment composition, and for the existence of structural changes in cells. Chlorophyll-a concentrations decreased between 60 to 90 % during the non-flow phase, indicating low resistance of algal assemblages to desiccation. In contrast, fast recoveries of Chlorophyll-a when flow resumed indicated high resilience. Pigment composition revealed that the epilithic algal assemblage was considerably different than the epipsammic and hyporheic ones. These differences were mainly attributed to the physical conditions prevailing on each streambed compartment that allowed the growth of different algal assemblages. During the non-flow phase, the synthesis of protective carotenoids (i.e. echinenone and scytonemin) and the occurrence of cell resistance structures (i.e. enlarged membrane thickness and resistant spores) enhanced resistance of the epilithic biofilm. The resistance observed in the epilithic biofilm might also be related to the tightly adhered growth-form of algae on this substratum. Main results suggest that algal assemblages in the epilithic compartment, which were the most exposed to desiccation, were structurally and functionally better adapted to flow interruption than those colonizing other streambed compartments, and that this compartment plays a crucial role in maintaining ecosystem functions under varying flow periods.Xisca Timoner was recipient of a PhD fellowship from the Spanish Ministry of science and technology (AP- 2007-01945), and Teresa Buchaca was partially supported by the Spanish Government project Invasive fish (427/2011). This research was funded by the projects SCARCE (CONSOLIDER-INGENIO CSD2009-00065), and CARBONET (CGL2011-30474-C02-01) of the Spanish Ministry of Science and Innovation.Peer reviewe

Hydrological transitions drive dissolved organic matter quantity and composition in a temporary Mediterranean stream

18 páginas, 7 figuras, 1 tablaThe implications of stream flow intermittency for dissolved organic matter (DOM) are not well understood despite its potential significance for water quality and ecosystem integrity. We combined intensive sampling with liquid chromatography and spectroscopic techniques to follow changes in DOC and DON concentrations as well as in DOM size fractions and spectroscopic properties in a temporary stream during an entire contraction–fragmentation–expansion hydrological cycle. DOC and DON concentrations remained low (range = 1.4–5.2 mg C L-1 and 0.05–0.15 mg N L-1) during hydrological contraction and fragmentation, with concomitant increases in the proportion of high molecularweight substances (HMWS)during contraction and of DOM aromaticity during fragmentation. DOC and DON concentrations abruptly increased (up to 8.8 mg C L-1 and 0.37 mg N L-1) at the end of the fragmentation phase, with a concomitant increase in the non-humic, microbial and aquatic character of DOM. Upon rewetting, the DOC and DON concentrations reached their highest values (up to 12.7 mg C L-1 and 0.39 mg N L-1), with concomitant increases in the proportion of HMWS and in the humic, aromatic and terrestrial character of DOM. Subsequently, DOC and DON concentrations recovered to values similar to those at the contraction phase, while DOM composition variables indicated the prevalence of a DOM of humic and terrestrial character during the whole expansion phase. Overall, our results emphasize the importance of hydrological transitions forDOMdynamics in temporary streams, and point to the potential response of perennial streams under future water scarcity scenarios.This studywas funded by theEuropean Union through the MIRAGE project (FP7 ENV 2007 1). Additional funds were provided by the Spanish Ministry of Economy and Competitiveness through the Consolider-Ingenio projects SCARCE (CSD2009-00065) and GRACCIE (CSD2007- 00067). D. von Schiller was supported by a DAAD-‘‘laCaixa’’ fellowship and a ‘‘Juan de la Cierva’’ postdoctoral grant (JCI- 2010-06397).Peer reviewe

Contraction, fragmentation and expansion dynamics determine nutrient availability in a Mediterranean forest stream.

13 páginas, 6 figuras.Temporary streams are a dominant surface water type in the Mediterranean region. As a consequence of their hydrologic regime, these ecosystems contract and fragment as they dry, and expand after rewetting. Global change leads to a rapid increase in the extent of temporary streams, and more and more permanent streams are turning temporary. Consequently, there is an urgent need to better understand the effects of flow intermittency on the biogeochemistry and ecology of stream ecosystems. Our aim was to investigate how stream nutrient availability varied in relation to ecosystem contraction, fragmentation and expansion due to hydrologic drying and rewetting. We quantified the temporal and spatial changes in dissolved nitrogen (N) and phosphorus (P) concentrations along a reach of a temporary Mediterranean forest stream during an entire contraction–fragmentation–expansion hydrologic cycle. We observed marked temporal changes in N and P concentrations, in the proportion of organic and inorganic forms as well as in stoichiometric ratios, reflecting shifts in the relative importance of in-stream nutrient processing and external nutrient sources. In addition, the spatial heterogeneity of N and P concentrations and their ratios increased substantially with ecosystem fragmentation, reflecting the high relevance of in-stream processes when advective transport was lost. Overall, changes were more pronounced for N than for P. This study emphasizes the significance of flow intermittency in regulating stream nutrient availability and its implications for temporary stream management. Moreover, our results point to potential biogeochemical responses of these ecosystems in more temperate regions under future water scarcity scenarios.. This study was funded by the European Union through the Mediterranean Intermittent River ManAGEment (MIRAGE) project (ref: FP7 ENV 2007 1, http://www.mirage-project.eu). Additional funds were provided by the project CGL2007-65549/BOS and the Consolider-Ingenio projects CSD2009-00065 (SCARCE) and CSD2007-00067 (GRACCIE) of the Spanish Ministry of Science and Innovation. D. von Schiller was supported by a fellowship of the German Academic Exchange Service and ‘‘laCaixa’’ Foundation.Peer reviewe

Utilisation du phosphate par des communautés planctoniques fluviales: variations spatio-temporelles de l'activité phosphatase

International audienceRegulation of large rivers for human purposes (e.g. hydroelectricity production, flood prevention, recreation activities) may broken down the longitudinal patterns on microbial planktonic communities, altering their role in organic matter production and nutrient recycling in river ecosystems. The present study aims to analyse the relationships between phosphorus nutrient sources and phosphatase activity in the particulated and dissolved water fractions of the Ebro river (NE Spain). Whit this purpose, we measured the nutrient content and phosphatase activity from the two water fractions in 6 sites upstream and 6 downstream of a reservoirs system during three years. Algal and bacterial densities and phytoplankton community composition were analysed throughout the study. Our working hypothesis was that the temporal patterns (seasonality) would mainly determine the phosphatase activity between water fractions, though spatial differences between river sections (upstream and downstream of reservoirs) may interact in its regulation. Results showed higher phosphatase activity in spring-summer than in autumn-winter which agreed to the increases in plankton cell densities along the two sections. It was during spring-summer, when strong correlation between water SRP and phosphatase activity in the two water fractions were observed. Instead, these trends shifted during autumn-winter months when SRP-phosphatase correlations were only consistent in the dissolved fraction. Irrespectively of the study period, relationships between SRP-phosphatase in the upstream section were stronger in the particulated fraction while in the downstream section stronger for the dissolved fraction. These differences could be explained by the differences in microbial cell densities and phytoplankton species composition found between sections. The present research shows that phosphatase activity patterns in large regulated rivers did not follow a progressive increase to the lower sections. The uncoupling between nutrients and phosphatase activity in the particulated fraction (phytoplankton dominated) might evolve on a decrease in authochthonous organic matter production in lower water courses

Influence of flow intermittency on nutrient availability in a Mediterranean temporary stream

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