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Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32‐fold to 66‐fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting‐drying cycles on respiration and CO2emissions in stream networks. Sediment respiration in intermittent rivers and ephemeral streams increases substantially in response to rewettingRespiration pulses are driven by sediment properties, which, in turn, are influenced by climate and catchment characteristicsEffects of wetting‐drying cycles on respiration and CO2emissions in stream networks need consideration in upscaling and modeling efforts

Controls on the degradation of dissolved organic matter (DOM) are complex but key to understand the role of freshwaters in the carbon cycle. Both the origin and previous degradation history have been suggested to determine DOM reactivity, but it is still a major challenge to understand the links between DOM composition and biodegradation kinetics. An appropriate context to study these links are intermittent rivers, as summer drought naturally diversifies DOM sources and sinks. Here we investigated the biodegradation kinetics of DOM in the main aquatic environments present in a temporary river. During dark incubations we traced the dynamics of bulk DOM and its main chromatographic fractions defined using LC‐OCD: high molecular weight substances (HMWS), low molecular weight substances (LMWS), and humic substances and building blocks. Bulk DOM decay patterns were successfully fitted to the reactivity continuum (RC) biodegradation model. The RC parameters depicted running waters as the sites presenting a more reactive DOM, and temporary pools, enriched in leaf litter, as the ones with slowest DOM decay. The decay patterns of each DOM fraction were consistent throughout sites. LMWS and HMWS decayed in all cases and could be modeled using the RC model. Notably, the dynamics of LMWS controlled the bulk DOM kinetics. We discuss the mechanistic basis for the chromatographic fractions' kinetics during biodegradation and the implications that preconditioning and summer drought can have for DOM biodegradation in intermittent rivers. Inland waters, like rivers and lakes, are key in the global carbon cycle, as a significant amount of organic carbon is respired and emitted as CO2from these aquatic systems. Organic carbon originates from different sources, such as vegetation and organisms present in aquatic systems and their catchments and, might present different composition depending on the environment where its found. The results of the present study give information about the degradation speed of this organic carbon and its major molecular fractions. We could, for example, identify that the organic carbon present in rivers decays faster than the one in pools, although these process larger amounts of material mainly due to the accumulation of leaf litter by‐products. Also, we identify a faster and predictable decay of the compound classes grouping the smallest and biggest molecules. The result of this study can help to understand the changes in carbon processing in inland waters due to seasonal variability, as happens in intermittent rivers. Intermittent rivers, strongly affected by summer drought, might increase strongly in number with climate change and, as shown in our results, contribute to diversify the sources and decay speed of the organic carbon. Low and high molecular weight DOM fractions' decay fitted the reactivity continuum while humic substances presented nonmonotonic patternsBiodegradation patterns were consistent across sites, and the low molecular weight fraction drove the decay kinetics of bulk DOMVariability in DOM composition across sites was induced by summer drought and determined the biodegradation kinetics

AbstractThe Water Framework Directive EU2000/60/EC (WFD) was implemented for reservoirs at a regional scale (northeastern Spain). Twenty-one reservoirs were monitored quarterly over the course of a year. Using principal component analysis, the reservoirs were classified into types according to their geological and morphometric features. The Ecological Quality (EQ) of the reservoirs was assessed by integrating values of total chlorophyll a, cyanophyta chlorophyll aconcentration, fish metrics, Secchi depth, averaged hypolimnetic oxygen concentration and total phosphorus. For each reservoir type, a reference condition of quality was selected. When possible, this reference was the reservoir displaying the best EQ; otherwise expert judgment was used. To allow comparison of quality among reservoirs belonging to different types, thus identifying intrinsic differences, an Ecological Quality Ratio (EQR) was calculated by dividing the EQ value of each reservoir by that of its reference. According to EQR, the majority of the reservoirs accomplished the quality criteria of the WFD. This study identified a number of useful indicators for EQ assessment. Moreover, because the references were chosen among similar reservoirs, low EQR values are indicative of specific problems, such as untreated or wastewater spills or droughts. The results also demonstrate that expert judgment is a reasonable compromise when the low number of water bodies available for the study prevents statistical approaches.

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Modifying the most common design for the on-line coupling of a precolumn to reversed phase LC with diode array detection has resulted in reduction of the broadening of the peaks which results when the compounds of interest are strongly retained by a highly hydrophobic sorbent. The modification consists of the desorption of the analytes trapped on the precolumn solely by the organic solvent used to modify the solvent strength of the mobile phase. Results obtained using this design were compared with those obtained with the conventional design, with C18and PLRP-S precolumns. The performance of the system was also tested with a highly cross-linked styrene-divinylbenzene copolymer (ENVI-chrom P) precolumn for the determination of phenolic compounds in real samples. The advantages and disadvantages are discussed. Ion-pair solid phase extraction is used in order to increase the breakthrough volumes of more polar compounds, mainly phenol. The use of the new design enables phenolic compounds to be determined at the low µg L−1level with limits of detection ranging between 0.1 and 2 µg L−1in tap water when a 10 mL sample was analyzed.

Modifying the most common design for the on-line coupling of a precolumn to reversed phase LC with diode array detection has resulted in reduction of the broadening of the peaks which results when the compounds of interest are strongly retained by a highly hydrophobic sorbent. The modification consists of the desorption of the analytes trapped on the precolumn solely by the organic solvent used to modify the solvent strength of the mobile phase.

A direct method for the simultaneous determination of tartaric, malic, lactic, acetic, citric, shikimic, fumaric and succinic acids in fruit juices and wines by isocratic reversed phase HPLC is reported.

A method for determining histamine in wine by precolumn derivatization with PITC (phenylisothiocyanate) with reversed-phase HPLC and UV detection is reported. Histamine can be determined together with the 24 amino acids within 40 min, or separately in a shorter time (less than 4 min) if a prior solid phase extraction clean-up is used.

A method to determine free amino acids by pre-column derivatization with phenylisothiocyanate is discussed. The method has been applied to determine free amino acids in wine samples, and the results have been compared with those obtained by means of an automatic orthophthal-aldehyde-9-fluorenylmethyloroformate (OPA-FMOC) method.

A method for determining sixteen PAHs, which are included in the US Environmental Protection Agency list of priority pollutants is described. The procedure involves column liquid chromatography with fluorescence and UV detection and off-line concentration with solid-phase extraction membranes. In this study, two different membranes are tested, C18 and SDB. Different steps for cleaning and conditioning the membranes to prevent interfering peaks in the chromatogram were studied. The use of an organic solvent and Brij-35 as surfactant to prevent the analytes from being adsorbed on inner walls or surfaces is studied. The method enables these compounds to be determined in tap and river water samples at low ng 1-1 levels.

Summary: A group of pesticides has been determined by liquid chromatography with diode-array detection (DAD). The coupling of a particle-beam (PB) mass spectrometer to the outlet of the DAD enabled the identities of the pesticides to be confirmed from mass spectra obtained by electron impact ionization. On-line trace enrichment of 200 mL of sample with a PLRP-S precolumn enabled most pesticides to be determined in real samples at levels between 0.2 and 5 mgrg L–1 and with limits of detection (LOD) between 0.05 and 0.5 mgrg L–1. In the analysis of real samples, one of the pesticides studied was determined by DAD and its identity was confirmed by comparison of PB mass spectra. The use of PB-MS also enabled the detection of non-target compounds.

Summary: A comparative study of high-performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE) for determination of eleven EPA priority phenols in water samples is discussed. The repeatability, linearity and detection limits of both methods are compared. A solid-phase extraction method using a highly cross-linked styrene-divinylbenzene copolymer sorbent is used for the off-line preconcentration process in order to determine these compounds in water at low levels. Both methods are used to determine phenolic compounds in Ebro river and industrial waste water. Identification was achieved by comparison of the UV spectra corresponding to the different peaks, acquired with a diode-array detector, with those in a spectral library.

A comparative study of a chemically modified polymeric resin which has a benzoyl group and several commercial sorbents, such as PLRP-S, Envi-chrom P and LIChrolut EN, for the solid-phase extraction of various phenolic compounds and pesticides was carried out. Selectivity and breakthrough volumes for these compounds with different sorbents were studied by coupling an on-line solid-phase extraction system with a modified liquid chromatograph equipped with a UV detector. After determining the chromatographic conditions, linearity range and detection limits, the method was applied to the determination of these compounds in drinking and surface water. For 25 ml of sample, the recovery was >70% for all the compounds, except for 4-nitrophenol (62%), and detection limits were between 0.2 and 0.8 μg l−1.

A comparative study of the solid phase extraction of phenolic compounds in water was carried out. Three different sorbents, C18, CH and PLRP-S, were tested for the preconcentration process in order to determine phenolic compounds in water at μg l−1 levels by reversed-phase LC separation and diode-array detection. The addition of an ion-pair reagent in the extraction process was also studied in order to increase the breakthrough volume of some compounds. Using the optimized conditions the limit of detection was less than 0.5 μg l−1, the maximum admissible concentration (MAC) in drinking water. The method was applied to determine phenolic compounds in Ebro river water however no phenols were found within the detection limits of the method.

Two packing materials, C18 and PLRP-S, are studied for on-line trace enrichment of phenolic compounds in water. Various precolumns of different internal diameter are also tested and the addition of an ion-pair reagent to increase retention and thus, breakthrough volumes of phenolic compounds, is studied. Best results are obtained when a PLRP-S precolumn is coupled on-line with a C18 analytical column and DAD detector. Addition of TBA considerably increases breakthrough volumes. In contrast, when a C18 precolumn is used, breakthrough volumes are lower and it is impossible to determine TCP and PCP, under the experimental conditions used, because of interference of other nonpolar compounds in the samples. The performance of the system is evaluated with river and tap water and the preconcentration of 10 ml of sample in a PLRP-S precolumn involves a linear range between 1 μg 1−1 and 20 μl−1 and limits of determination between 0.5 μg l−1 and 1 μg l−1 are obtained.

Ion-pair solid-phase extraction was on-line coupled to a chromatographic system with a PLRP-s precolumn and the results were compared with those obtained when C18 sorbents are used. After optimization, the method was used to determine these compounds in two river waters. A sample of 20 ml of river water was preconcentrated with good recovery values for all the compounds studied and detected at 220 nm with detection limits between 0.25 and 0.1 μ1-1. Some of the compounds were provisionally determined by diode-array UV detection.

Heat uptake is a key variable for understanding the Earth system response to greenhouse gas forcing. Despite the importance of this heat budget, heat uptake by inland waters has so far not been quantified. Here we use a unique combination of global‐scale lake models, global hydrological models and Earth system models to quantify global heat uptake by natural lakes, reservoirs, and rivers. The total net heat uptake by inland waters amounts to 2.6 ±3.2 ×1020J over the period 1900–2020, corresponding to 3.6% of the energy stored on land. The overall uptake is dominated by natural lakes (111.7%), followed by reservoir warming (2.3%). Rivers contribute negatively (‐14%) due to a decreasing water volume. The thermal energy of water stored in artificial reservoirs exceeds inland water heat uptake by a factor ∼10.4. This first quantification underlines that the heat uptake by inland waters is relatively small, but non‐negligible. Human‐induced emissions of CO2and other greenhouse gases cause energy accumulation in the Earth system. Oceans trap most of this excess energy, thereby largely buffering the warming of the atmosphere. However, the fraction of excess energy stored in lakes, reservoirs, and rivers is currently unknown, despite the high heat capacity of water. Here we quantify this human‐induced heat storage, and show that it amounts up to 3.6% of the energy stored on land, while covering 2.58% of the land surface. The increase in heat storage from 1900 to 2020 is dominated by warming of lakes. The thermal heat contained in the water stored in man‐made reservoirs is about ten times larger. Our study overall highlights the importance of inland waters–next to oceans, ice and land–for buffering atmospheric warming, especially on regional scale. We use a unique combination of lake models, hydrological models, and Earth System models to quantify global heat uptake by inland watersHeat uptake by inland waters over the industrial period amounts up to 2.6 ×1020J, or 3.6% of the continental heat uptakeThe thermal energy of the water trapped on land due to dam construction (26.8 ×1020J) is  10.4 times larger than inland water heat uptake