Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter.

Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico-chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%-98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events.

estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds.
In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%-98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events.

K E Y W O R D S
biofilms, leaching, leaf litter, rewetting, sediments, temporary rivers

| INTRODUC TI ON
Human activities and climate change cause global-scale alterations in the flow regimes of rivers, which in turn are tightly linked to biogeochemical processes such as carbon processing (Arnell & Gosling, 2013;Bernhardt et al., 2018;Tonkin, Merritt, Olden, Reynolds, & Lytle, 2018). Currently, more than half of the global river network length is represented by intermittent rivers and ephemeral streams (IRES) -systems that cease to flow at some point in time and space (Acuña et al., 2014;Datry, Larned, & Tockner, 2014). Anthropogenic pressures alter the hydrological regime of perennial rivers toward intermittency, although the opposite can also happen at some locations. On the one hand, flow regulation, water diversion, groundwater extraction, and land-use alteration promote the prevalence of river flow intermittence both spatially and temporally (Datry, Bonada, & Boulton, 2017;Pekel, Cottam, Gorelick, & Belward, 2016).
On the other hand, naturally intermittent rivers turn permanent due to effluents from wastewater treatment plants or artificially enhanced discharge required for livestock and irrigation (Chiu, Leigh, Mazor, Cid, & Resh, 2017).
Despite their widespread distribution and distinct role in biogeochemical cycling, IRES are notably missing in current analyses of global carbon budgets and other biogeochemical processes such as cycling of nutrients and DOM (Datry et al., 2018). Still, research on IRES is based primarily on studies spanning fine spatial extents (Leigh et al., 2016), which limits our understanding of their roles in ecosystem processes at the global scale Skoulikidis, Sabater et al., 2017;von Schiller et al., 2017; but see Datry et al., 2018;Soria, Leigh, Datry, Bini, & Bonada, 2017). The contribution of IRES particularly to biogeochemical processes must be understood and quantified to correctly estimate carbon and nutrient fluxes. Studies indicating altered distribution of IRES in the future due to climate change (e.g. Milly, Dunne, & Vecchia, 2005) also emphasizes the need to adjust future river monitoring and conservation strategies.
The quantity and quality of dissolved substances leached from the channel beds of IRES during the rewetting process, and the environmental variables associated with variation in differences in leached amounts, has been little studied. However, such knowledge is essential for disentangling the role of IRES in biogeochemical processes under different scenarios of climate change. In the present study, we experimentally simulated pulsed rewetting events under controlled standardized laboratory conditions using substrates collected from 205 IRES located in 27 countries in five continents and covering five major climate zones. We aimed (a) to compare the amounts of nutrients and DOM, and the quality of DOM leached from leaf litter, biofilms, and bed sediments accumulated on dry IRES beds at the global scale as well as in different climate zones, (b) to explore and identify the environmental variables related to the variability in leached amounts, and (c) to estimate the potential areaspecific fluxes (per m 2 of bed surface) of nutrients and OM leached during pulsed rewetting events. We focused on common nutrient and DOM species, which control essential ecosystem processes such as primary production and microbial respiration (Conley et al., 2009;Elser et al., 2007). Furthermore, we estimated the size categories and optical properties of released DOM as proxies of its quality.
Our first hypothesis was that in comparison with mineral substrates (sediments), leachates from organic substrates (biofilms and leaves) contain higher amounts of nutrients and DOM relative to the content of the respective element (carbon or nitrogen) in the substrate. In addition, substrates of organic origin also have a higher variability in the composition of leachates due to a higher species richness and compositional heterogeneity. Within our second hypothesis we expected that significant differences in the amounts of leached substances are observed among substrates sampled across different climate zones, with the highest amounts of nutrients and OM leached in the continental climate zone compared to others due to high litter quality (Boyero et al., 2017). In combination with the highest mass of litter observed (Datry et al., 2018) we expect this to result in the highest nutrient and OM fluxes from a representative area of dry river bed in the continental zone. Finally, we hypothesized that quantitative and qualitative composition of leachates will depend on substrate characteristics, which in turn are expected to correlate with environmental variables sampled at the study sites.

| Sampling sites, substrate collection, and environmental variables
A total of 205 IRES, located in 27 countries and spanning five major Köppen-Geiger climate classes, were sampled during dry phases, following the standardized protocol of the 1,000 Intermittent Rivers Project  irstea.fr, Figure 1). Five major climate zones were assigned to sites based on their location: arid (merging Köppen-Geiger classes BSh, BSk, BWh and BWk, n = 29), continental (Dfb, Dfc, n = 13), temperate (Cfa, Cfb, Csa, Csb, Cwa, n = 142), tropical (As, Aw, n = 19), and polar (ET, n = 1). Differences in sample size resulted from the occurrence of IRES and accessibility of sampling sites by researchers involved in the sampling campaign. A larger sample size increases the variability of the results while increasing the precision of the mean/ median values, that is, reducing the variability of the sample mean/ median. This needs to be considered in data evaluation and interpretation. For each river, one reach was selected and sampled for leaf litter (further referred as leaves), epilithic biofilms (biofilms), and sediments (details on material collection are provided in Supporting Information). After collection, field samples were further processed in the laboratory. Leaves and biofilms were oven-dried (60°C, 12 hr) to achieve constant mass, reduce variability from fluctuations in water content (Boulton & Boon, 1991), and ensure cellular death of the leaf tissue. Oven-drying mainly affects volatile and oxidizable compounds, which were not in the focus of our study. However, oven-drying may increase the amount of leached substances from leaves and biofilms (e.g. Gessner & Schwoerbel, 1989). Bed sediments were sieved (2 mm) and air-dried for 1 week. The dry material was placed in transparent plastic bags, shipped to laboratories responsible for further analyses (see Acknowledgements), and stored in a dry and dark room until processing and analysis.
Nine environmental variables were selected to analyze their association with leachate characteristics ( Table 1). The variables were selected based on a conceptual understanding of the leaching process. As proxies of a regional-scale influence, we used the aridity index and potential evapotranspiration (PET) extracted from the Global Aridity and PET database (for details see Datry et al., 2018). River width, riparian cover (%, visually estimated as the proportion of river reach covered by vegetation), dry period duration (estimated either with water loggers or repeated observations, precision: 2 weeks), altitude, and land cover (%) of pasture, forest, and F I G U R E 1 Location of the sampling sites (N = 205) across five climate zones. Climate zones according to Köppen-Geiger classes are marked with different colors [Colour figure can be viewed at wileyonlinelibrary.com] urban areas within the catchment were selected as proxies of local influence. These local-scale parameters (apart from land cover) were recorded in situ by participants of the 1,000 Intermittent Rivers Project. Land cover was derived using GIS maps. For details on the environmental variables sampled and substrate characteristics, see Table S1.
TA B L E 1 Overview of the variables included in the partial least squares (PLS) regression models and transformations applied to meet assumptions of analysis

| Leaching experiments
Rewetting was simulated in the laboratory by exposing dried substrates to leaching solutions as a proxy for their exposure in situ to river water during first flush events. Leaves were cut into approximately 0.5 cm × 0.5 cm pieces and homogenized in glass beakers using a spoon. If the sample contained conifer-needles (approximately 30% of samples), these were cut into fragments of approximately 4 ± 0.5 cm length. From each sample, 0.5 ± 0.01 g were

| Physical and chemical characterization of substrates and leachates
Organic carbon (C) and total nitrogen (N) content of substrates (%C and %N, respectively) were determined using elemental analyzers (for details see Supporting Information

| Calculation of the total areal flux of nutrients and OM
Total areal flux of nutrients and OM per square meter of the riverbed was calculated based on information about the mass of leaves and biofilm accumulated on the dry riverbeds (Datry et al., 2018), as well as on average mass of sediment per square meter of surface area. For the latter, we assumed an average density of sediments of 1.6 g/cm 3 (Hillel, 1980) and the depth of the sediments potentially affected by a rewetting event to be 10 cm (see Merbt et al., 2016), which also corresponds to the depth of the sampled sediment layer according to the sampling protocol. We acknowledge that this assumption should be considered with caution as high variability in sediment densities can be found in nature (e.g. Boix-Fayos et al., 2015) and contribution of sediment layers within 10 cm depth to leaching also may differ (e.g. Merbt et al., 2016).
Overall, the total areal flux is the sum of nutrients and OM leached from all substrates found within the dry riverbed. To execute a global comparison of total areal fluxes, samples from 157 reaches were selected for which a complete set of nutrients and OM concentrations (except DON) were available. Reaches for which one or more chemical measurements were identified as technical outliers after exploration with boxplots and Cleveland dotplots (Zuur, Ieno, & Elphick, 2010) were excluded. We assume these calculations reflect spatial differences in surface fluxes of nutrients and OM across a range of sampled IRES. TA B L E 3 Ranking of environmental variables and substrates characteristics that explain variance in quantitative composition (A) and qualitative characteristics (B) of leachates at global and regional scales according to their value of VIP (variable influence on projection) in the PLS analysis. VIP > 1 indicate highly influential predictors (dark grey), 1 > VIP > 0.8 indicate moderately influential variables (medium grey), VIP < 0.8 -variables of low influence (light grey)  In order to identify the environmental variables and substrate characteristics driving the quantitative (amounts of nutrients and OM) and qualitative (DOM quality) characteristics of the leachates partial least squares (PLS) regression models were applied (Wold, Sjöstrom, & Eriksson, 2001). This approach allows exploration of the relationship between collinear data in matrices X (independent variable) and Y (dependent variable). An overview of the components to be included in the models is given in In order to partition the variance in quantitative and qualitative From each PLS-regression model, the explained variance R 2 Y was calculated and used to calculate the fraction of variance explained by each set of predictors separately (Borcard et al., 1992). For the PLS regression analysis, we selected the complete set of variables for which the required data (all predictors and response variables,

| Total and relative leaching rates
The total leached amounts (mg/g dry mass) of nutrients (except N-NO 3 − ) and DOM were highest for leaves, followed by biofilms, and sediments ( Figure 3; Table S2).  (Table S2).
The relative leached amounts of DOC and phenolics (mg/g C) and DON (mg/g N) were highest for leaves, followed by biofilms and sediments ( Figure 3; Dunn's test, p < 0.001; Figure 3; Table S2).
For all substrates, we observed large variations in the total and relative leached amounts of nutrients and DOM (Figure 3, Table   S2). The highest variability in total and relative leached amounts of DOC, N-NO 3 − , and SRP was observed for biofilms, which was up to 10 times higher than for sediments and leaves. Sediments had the highest variability in the total leached amounts of DON and relative leached amounts of N-NH 4 + and phenolics. For leaves, the highest variability was found in the relative leached amounts of DON.
F I G U R E 2 Variance partitioning among variables that influence leaching of nutrients and organic matter from substrates accumulated in intermittent rivers and ephemeral streams. In all leachates, HS was the dominant fraction of DOM followed by BP and LMWS ( Figure 5; Table S3

| Differences in amounts of leached substances and DOM quality across climate zones
Cross-climate differences in amounts of leached substances and qualitative characteristics of DOM depended on the type of substrate (

| Effects on the amounts of leached nutrients and DOM
On a global scale, 25% of the variance in the amounts of nutrients and DOM leached from sediments could be explained by selected variables (fraction [a + b + c]), which was more than twice that for leaves (11%) (Figure 6a approximately an equal percentage of variance, 8% and 6% respectively, which was much lower than that explained for sediments. Environmental variables and substrate characteristics accounted for 3% of variance in the quantitative composition of leaf leachates. For both substrates, the most influential variables (VIP >1) were C fraction, N fraction, PET, and in the case of leaves, C:N and pasture cover within the river catchment (Table 3).
For both sediments and leaves, the highest percentage of variance in amounts of leached nutrients and DOM was explained for the continental and tropical zones (59% and 46% for sediments, 39% and 40% for leaves respectively, Figure 6a). Substances leached from sediments from these regions were explained mostly by the environmental variables and their effect on substrate characteristics. High VIP was found for the dry period duration, N fraction and textural classes (both zones), river width and forest cover (continental), PET, urban cover, and fraction of C (tropical). In contrast, for leaves in these zones, most of the variance was explained by environmental variables alone and not by their effect on the substrates. Environmental variables with high VIP in these zones were PET and aridity (in both), river width and altitude (in the continental zone), as well as pasture cover and dry period duration (in the tropical zone) (Table 3).
For the temperate zone, the results of variance partitioning were available for all analyzed substrates. Here, the total variance in leachates was best explained for biofilms (48%) followed by sediments (30%) and leaves (15%). In contrast to sediments and leaves, the variance in biofilm leachates was better explained by environmental variables (VIP >1 for aridity and altitude) than by substrate characteristics.

| Effects on qualitative characteristics of DOM
For sediments and leaves, the percentage of variance that was explained for qualitative characteristics of DOM on the global-scale was much lower (around 7% for each of the substrates) than that for the amounts of leached substances (Figure 6b). The contribution of environmental variables, substrate characteristics, and effect of environmental variables on substrate characteristics to the total variance was approximately equal (Figure 6). Influential variables with VIP >1 were altitude and C fraction (for both substrates), PET and texture (for sediments), and river width and urban cover (for leaves).
F I G U R E 4 Qualitative characteristics of dissolved organic matter leached from leaves (L), biofilms (B), and sediments (S) of IRES globally. Box: median, interquartile range (25%-75%), and outliers (i.e. values that exceed 1.5 interquartile range). For parameter acronyms see Table 1. Letters in parentheses on the xaxis indicate that the difference between leachates from specified substrates was nonsignificant (p > 0.0167, Dunn test for post-hoc comparison; see Section 2) For sediments, as in the case of amounts of leached substances, the variance across sampling sites was explained best in the tropical (58%) and continental (53%) zones, and was driven mainly by the environmental variables and their effect on substrate characteristics.
Variables with VIP >1 in both zones were sediment texture (fraction of silt and clay) and, additionally PET, aridity, and urban cover in samples from the tropical zone, and pasture and forest cover, riparian cover, aridity, and dry period duration in samples from the continental zone (Table 3). For sediments in the arid zone, the explained variance was around 28% and the share of groups of variables that explained the observed variance was different. In particular, almost all variance explained by environmental variables was due to the effect of environmental variables on substrates (VIP >1 for texture, %C, %N, and forest cover). This was the opposite for leaf leachates, where the variance was explained mainly by the effect of environmental variables alone (PET, aridity, and dry period duration).
In samples from the temperate zone, variance of leachate quality was best explained for biofilms (27%) followed by leaves (13%) and sediments (6%) ( Table 3)
Leaves provided the second highest contribution to the total areal flux. In contrast to sediments and leaves, the relative contribution of biofilms to area-specific flux rates was very low for all substances (in average: <0.1%), but slightly higher for N-NO 3 − (1.5 ± 7%) (values above 100% or lower than 0% reflect deviation and not the real data).
The highest fluxes were estimated from riverbeds in the continental zone (Table 4), whose areal flux of N-NH 4 + and phenolics was three times higher than that of the arid zone, four times higher for

| Rewetting events in IRES in the context of global biogeochemical cycles
Our globally comparable assessment of nutrient and DOM leaching in rewetted IRES shows that the quantity and quality of leached nutrients and DOM are substrate-and climate-specific, with the highest amounts leached in continental climate and with sediments contributing most to the total areal flux from dry river beds. These data provide a basis on which to develop models of biogeochemical cycling in river networks including IRES.
According to our first hypothesis, we found a high variability in the amount of leached substances and the quality of leachates from organic, but also from inorganic substrates, mainly as a consequence of inherent substrate properties and their modification during the drying period. Leaching from organic materials (leaves and biofilms) was relatively enriched in P vs N in contrast to sediments. Due to their higher mass within the riverbeds, sediments were the main contributors to the areal fluxes. Sediments leached high amounts of N-NO 3 -, the accumulation of which in dry riverbeds is promoted by aerobic conditions (Amalfitano et al., 2008;Arce et al., 2014;Borken & Matzner, 2009;Merbt et al., 2016). Considering quality of leached DOM, we found that depending on the proportion of each substrate within the riverbed, different ecosystem processes can be affected.
For example, leachates from biofilms with a high proportion of biopolymers may play a key role as sources of bioavailable DOM in IRES and are more likely to be retained within the riverbed upon rewetting (Romani, Vazquez, & Butturini, 2006;von Schiller et al., 2015).
A high proportion of LMWS leached from leaves suggests that such leachates can trigger ecosystem processes in downstream surface waters and groundwaters, as molecules of this size fraction can easily be transported through the hyporheic zone with limited immobilization (Romani et al., 2006). DOM leached from sediments was mainly of microbial origin, suggesting its high potential bioavailability (Marxsen, Zoppini, & Wilczek, 2010;Schimel et al., 2007). Overall, we suggest that rewetting of sediments is key for understanding biogeochemical cycles in fluvial networks with IRES, and that leaves and biofilms can introduce regional variabilities in the global scale patterns depending on the accumulated amount of these substrates in the channel during the dry phase. Indeed, accumulation of plant litter on the dry riverbed ranges from 0 to 963 g/m 2 depending on aridity, river width, catchment area, riparian cover, and drying duration (Datry et al., 2018 and Table S1). In our study, accumulations of biofilms were very common in the temperate zone and ranged from 0.3 to 327 g/m 2 (Table S1).
We also found differences in the amounts of leached substances among climate zones, in accordance with our second hypothesis, but only for sediments. Initially, we expected cross-climate differences to be more pronounced for leaves due to climatic effects on vegetation composition and leaf litter quality (e.g. Aerts, 1997;Boyero et al., 2017), rather than for sediments whose composition is controlled mainly by geology and geomorphology. The absence of significant differences among climate zones for leaves could be explained by the considerable variability we observed among leaf material collected within climate zones, both in terms of species composition and drying history. Although we did not assess the site-specific composition of riparian vegetation, previous studies indicated that up to 40% of variation in leaf traits at a given site can be explained by small-scale spatial and temporal environmental heterogenity in environmental factors such as hydrology and disturbance regime (Cornwell et al., 2008).
High concentrations leached in the continental climate zone suggest that nutrient loads to freshwaters will increase with the projected increase in the extent of IRES in such regions. In the arid zone where terrestrial primary production is severely constrained by water availability (Austin et al., 2004), rewetting events are expected to stimulate stream ecosystem productivity not only due to water availability, but also because the potential bioavailability of leachates is particularly high in this climate zone. However, despite a high potential bioavailability of DOM, leachates from the arid zone were characterized by low amounts of nutrients, probably resulting from leaf traits that reflect adaptation to dry conditions (Cornwell et al., 2008).
Comparison of fluxes from 1 m 2 of IRES within the 4 hr duration of the experiment with the annual flux from 1 m 2 of watersheds (  (Fierer & Schimel, 2002). Given the predicted increase in the duration of droughts, the exacerbation of extreme low-flow conditions, and the intensity of storm events (De Girolamo, Bouraoui, Buffagni, Pappagallo, & Lo Porto, 2017;Huntington, 2006;IPCC, 2014), the results of this study emphasize the need to integrate IRES in global carbon cycles and budgets, from which they are currently excluded (Raymond et al., 2013; although see Datry et al., 2018).

| Environmental variables correlated with release of nutrients and OM
Environmental variables that are prone to be affected by climate change (namely PET, aridity, dry period duration, land-use) correlated with amounts and quality of leachates, particularly for sediments.
For leaves, these correlations were less pronounced, suggesting that although not considered in the study, may also be responsible for the unexplained variance given that litter quality generally increases with latitude (Boyero et al., 2017).
The amounts of leached substances from both leaves and sediments were correlated with PET. This variable is expected to be intensified in the future (Milly & Dunne, 2016) and will most likely lead to fluctuations in moisture conditions in dry riverbeds. Low moisture level reduces litter decomposition and C consumption, thereby promoting the release of DOM upon rewetting (Abril et al., 2016;Aerts, 1997;Bruder et al., 2011;Gessner, 1991) and hence increasing the probability of negative consequences for stream ecosystems such as blackwater events leading to hypoxia (Hladyz et al., 2011).  (Abril et al., 2016), where soils generally contain less carbon and nitrogen compared to the continental zone (Table S1 and This suggests that modification of land use in the catchments with IRES can also affect their contribution to nutrient load due to changes in the composition of CPOM accumulating in dry riverbeds. TA B L E 4 Comparison of the areal fluxes (g/m 2 ) of the different nutrients and OM species across climate zones (for abbreviations see Table 1  Although dry period duration is an important factor affecting the amounts and quality of litter accumulations in IRES (del Campo & Gómez, 2016;von Schiller et al.,2017), we found its influence on the variance in leachates only in continental and tropical zones. This indicates that during the dry phase materials with different drying history (as affected by different climates) and potential to leach nutrients and OM can accumulate in IRES. This also suggests that dry period duration cannot invariably be used as a master proxy to assess potential impacts of nutrient loading from IRES upon rewetting. Under field conditions, other factors such as severity and timing of a rewetting event as well as presence/absence of plant material growing in dry channels can affect nutrient fluxes from riverbeds, and the fate of nutrients in ecosystems, as well as potential ecosystem impacts (e.g. eutrophication, mass mortality of aquatic organisms) in downstrean receiving waters and groundwater (Baldwin & Mitchell, 2000;Bernal et al., 2013;Cavanaugh, Richardson, Strauss, & Bartsch, 2006;Hladyz et al., 2011;Ocampo, Oldham, Sivapalan, & Turner, 2006). Substrate moisture content and variability in associated microbial communities can potentially be responsible for the unexplained part of the variance in the leachates, due to their effect on decomposition rates of accumulated CPOM, nutrient processing in sediments, release of DOM upon rewetting, and its modification by microbial communities (Abril et al., 2016;Arce et al., 2015;Dieter, Frindte, Krüger, & Wurzbacher, 2013;McIntyre, Adams, Ford, & Grierson, 2009;Meisner, Leizeaga, Rousk, & Bååth, 2017).

| Implications for freshwater ecosystems and future research
We identified IRES to function as pulsed biogeochemical reactors (sensu Larned et al., 2010) at a global scale even though the experiments were conducted under laboratory conditions and magnitudes of leached substances may differ in the natural environment. Our data serve also as a basis for further upscaling and modeling of the processes observed in the laboratory to address ecological implications of rewetting events at catchment scales. Potential implications for the functioning of rivers could be determined by the effect of leached substances on the degree of nutrient limitation of microorganisms downstream, and therefore community composition (Demi, Benstead, Rosemond, & Maerz, 2018) as well as on the fate of refractory substances and intensification of their decomposition through the so-called "priming effect" (Guenet, Danger, Abbadie, & Lacroix, 2010). The results of our study support the recent call for developing effective strategies for the management of IRES to avoid negative consequences for downstream ecosystems caused by excessive nutrient and OM load.

ACK N OWLED G EM ENTS
This work was carried out within the SMART Joint Doctorate Programme "Science for the MAnagement of Rivers and their