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Biofilm communities are exposed to long periods of desiccation in temporary streams. We investigated how water flow intermittency affected the bacterial community structure colonizing three different streambed compartments in a Mediterranean stream. Massive parallel sequencing revealed different bacterial communities in biofilms from sand sediments and cobbles. Bacterial communities were similar (62% of shared operational taxonomic units) in the epipsammic and hyporheic biofilms, and more diverse than those in the epilithic biofilms. The non-flow phase caused a decrease of bacterial diversity in the biofilms, when communities included only bacterial taxa assumed to be adapted to water stress. The most sensitive bacterial communities to flow intermittency were in the epilithic, where the exposure to physical stress was the highest. In sand sediments a wide group of bacterial taxa was tolerant to desiccation. During non-flow the proliferation of opportunistic taxa in the superficial compartments evidenced the biological link with the terrestrial environment. Bacterial communities better tolerate rewetting than desiccation, since a major number of taxa tolerant to rewetting occurred in all biofilms. Overall, bacterial communities in sandy compartments showed higher resistance to flow intermittency than those in epilithic biofilms.

Temporary streams make up the majority of river networks in many regions around the world. Although they are known to have non-flow periods, it is uncertain in what ways the temporal components of the non-flow period affect stream ecosystems. We analyzed how duration and frequency of the non-flow period influence the biofilm metabolism of 33 Mediterranean streams in NE Iberian Peninsula. Selected streams ranged from perennial to ephemeral, and their hydrology was characterized during a period of 150 days before the sampling. Cobbles were collected from the streams, for which the total biofilm biomass (ash-free dry mass and chlorophyll-a) and metabolism (community respiration and gross primary production) were measured. Metabolic differences were observed between both permanent and temporary streams, as well as within temporary streams. Among these, the frequency of the non-flow period did not affect biofilm biomass or metabolism, but the duration did significantly decrease autotrophic biomass and gross primary production. Severity of the non-flow period (solar radiation and maximum streambed temperature) also affected gross primary production negatively. Thus, 80% of the observed gross primary production variability among all temporary streams was explained by the total duration and the severity of the non-flow period. In contrast, community respiration in the streams was not affected by the temporal components of the non-flow period. Our results highlight the effects of different temporal components of the non-flow period on autotrophic and heterotrophic processes, indicating that longer durations of the non-flow period or high severity conditions might decrease gross primary production promoting heterotrophy.

Increased duration and frequency of droughts under global change may affect both temporary and formerly permanent watercourses. Streambed desiccation, cooccurring solar radiation, and high temperature all may affect the composition and metabolism of photoautotrophic organisms in benthic biofilms. To test these effects, we characterized the composition of pigments when biofilms were exposed to dry conditions, and compared this to pigments measured after flow resumed, as well as to others measured from permanent streams in the same region. This comparison revealed important differences in pigmentary composition based on flow conditions. Active chlorophylls decreased exponentially with the duration and severity of the nonflow period, while chlorophyll degradation products and protective carotenoids increased. Streams experiencing longer and more severe nonflow periods showed the highest dissimilarities in their biofilm pigmentary composition, and this pattern was maintained after flow resumed. Our results pointed to duration of nonflowing periods as the major driver of pigmentary composition, regardless of whether these were interrupted or not by short intervals of water flows. Multivariate changes in pigmentary composition between sampling periods highlighted the influence of the long-term hydrological history on the relative abundance of different taxonomic groups and their associated pigments. Overall, pigmentary fingerprints are informative of the biofilm capacity to resist water flow interruption and of their ability to recover after water flow returns.

Temporary streams are characterized by the alternation of dry and wet hydrological phases, creating both a harsh environment for the biota as well as a high diversity of opportunities for adaptation. These systems are eminently microbial-based during several of these hydrological phases, and those growing on all solid substrata (biofilms) accordingly change their physical structure and community composition. Biofilms experience large decreases on cell densities and biomass, both of bacteria and algae, during dryness. Algal and bacterial communities show remarkable decreases in their diversity, at least locally (at the habitat scale). Biofilms also respond with significant physiological plasticity to each of the hydrological changes. The decreasing humidity of the substrata through the drying process, and the changing quantity and quality of organic matter and nutrients available in the stream during that process, causes unequal responses on the biofilm bacteria and algae. Biofilm algae are affected faster than bacteria by the hydric stress, and as a result the ecosystem respiration resists longer than gross primary production to the increasing duration of flow intermittency. This response implies enhancing ecosystem heterotrophy, a pattern that can be exacerbated in temporary streams suffering of longer dry periods under global change.

The 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⁻¹and 0.05–0.15 mg N L⁻¹) during hydrological contraction and fragmentation, with concomitant increases in the proportion of high molecular weight substances (HMWS) during contraction and of DOM aromaticity during fragmentation. DOC and DON concentrations abruptly increased (up to 8.8 mg C L⁻¹and 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⁻¹and 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 for DOM dynamics in temporary streams, and point to the potential response of perennial streams under future water scarcity scenarios.

River and stream biofilms in mediterranean fluvial ecosystems face both extreme seasonality as well as arrhythmic fluctuations. The hydrological extremes (droughts and floods) impose direct changes in water availability but also in the quantity and quality of organic matter and nutrients that sustain the microbial growth. This review analyzes how these ecological pulses might determine unique properties of biofilms developing in mediterranean streams. The paper brings together data from heterotrophic and autotrophic community structure, and extracellular enzyme activities in biofilms in mediterranean streams. Mediterranean stream biofilms show higher use of peptides during the favorable period for epilithic algae development (spring), and preferential use of cellulose and hemicellulose in autumn as a response to allochthonous input. The drying process causes the reduction in bacterial production and chlorophyll biomass, but the rapid recovery of both autotrophs and heterotrophs with rewetting indicates their adaptability to fluctuations. Bacteria surviving the drought are mainly associated with sediment and leaf litter which serve as “humid refuges”. Some algae and cyanobacteria show resistant strategies to cope with the drought stress. The resistance to these fluctuations is strongly linked to the streambed characteristics (e.g., sediment grain size, organic matter accumulation, nutrient content).

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.

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.

Intermittent water flow regimes characterize streams in many world regions, especially those with arid and semiarid climates. During cease to flow conditions, biofilms on streambed sediments may be exposed to desiccation. Environmental conditions and resource availability change with desiccation and may influence biofilm functioning and whole stream ecosystem processes. Rainfall events during the nonflow phase can rehydrate streambed sediments, but the effect of these pulses on biofilm functioning is unclear. This study aimed to analyze the effects of a rehydration event on biofilm functional diversity during the nonflow period in a subtropical Australian stream. Biofilms from three different stream pools on the same reach; one permanently water-covered and the other two differing in their desiccation time were studied. Biofilms initially differed owing to the time they were exposed to dry conditions but rehydration events significantly increased biofilm functional diversity, producing a “reset” effect on the desiccation exposure, as after that bacterial functioning decreased again because of the new dry conditions. The observed rapid biofilm responses to rehydration during flow intermittency might be essential in sustaining biofilm functional diversity in intermittent streams.

Intermittent water flow regimes characterize streams in many world regions, especially those with arid and semiarid climates. During cease to flow conditions, biofilms on streambed sediments may be exposed to desiccation. Environmental conditions and resource availability change with desiccation and may influence biofilm functioning and whole stream ecosystem processes. Rainfall events during the nonflow phase can rehydrate streambed sediments, but the effect of these pulses on biofilm functioning is unclear. This study aimed to analyze the effects of a rehydration event on biofilm functional diversity during the nonflow period in a subtropical Australian stream. Biofilms from three different stream pools on the same reach; one permanently water-covered and the other two differing in their desiccation time were studied. Biofilms initially differed owing to the time they were exposed to dry conditions but rehydration events significantly increased biofilm functional diversity, producing a \"reset\" effect on the desiccation exposure, as after that bacterial functioning decreased again because of the new dry conditions. The observed rapid biofilm responses to rehydration during flow intermittency might be essential in sustaining biofilm functional diversity in intermittent streams.