Temporary Mediterranean Streams Research Articles

Duration of water flow interruption drives the structure and functional diversity of stream benthic diatoms

Flow cessation affects river ecosystems submitted to low precipitation and increased water demand, and creates unfavourable conditions to aquatic biological communities. Diatoms are amongst the most sensitive biological groups to hydric stress, making them good indicators of preceding hydrological conditions. We here analyse the response of diatom assemblages to the duration and frequency of non-flow periods in 23 Mediterranean temporary streams. All of them experienced a strong decrease in water flow during summer, leading to a period of flow cessation. In addition, other ten permanent streams (zero dry days during the study period) were included in the study for comparative purposes. Temporary and permanent streams showed similar diatom species richness, evenness, and alpha diversity. However, beta diversity was higher in temporary than permanent streams, regardless rare taxa (<1% occurrence) had similar numbers in the two types of streams. Alpha diversity, richness, and evenness of diatom assemblages in the temporary streams changed with the duration of the non-flow period. Durations of 50–100 days were associated to higher alpha diversity, richness, and evenness, but longer non-flow periods caused their decrease. Diatoms thriving under the most extreme conditions were mostly aerophilic, pioneer, and motile taxa. The proportion of aerophilic diatom taxa increased beyond 100 dry days, particularly in those sites receiving more intense solar radiation. Overall, the taxonomical and functional composition of diatom assemblages mostly responded to the duration of the non-flow period, irrespectively of these being consecutive or not. This study shows that diatom assemblages from temporary streams contain taxa adapted to non-flow conditions, but remain vulnerable to further reductions of water flow associated to climate or global change. ‘Unimpacted’ watercourses in the Mediterranean region, independently of their flow regime, should be urgently protected and used as taxonomical and functional early indicators of climate change.

Responses of resources and consumers to experimental flow pulses in a temporary Mediterranean stream

Determining the resistance and resilience of resources and benthic invertebrates connected to instream refuges and species re-colonization in post-flood periods may help to elucidate mechanisms behind community recovery. This experiment simulated flow pulses in a small temporary stream in an extremely wet year, using upstream control and downstream flooded reaches at three sites in order to assess community resistance and resilience (benthos and drift), and analyse resources (periphyton and benthic organic matter) and invertebrates at pre- and post-flood time periods. The hyporheos was sampled in order to explore species exchanges with benthos. Fewer resources and benthic invertebrates at the beginning of the experiment were found than in previous studies when base flow conditions prevailed. Resource stocks and benthic invertebrates showed high resistance to the flow pulse. Interestingly, there was low resilience of benthic organic matter to natural seasonal flooding. Chlorophyll a did not recover after experimental floods; instead, it was reduced after floods, despite the more benign flow conditions and non-limiting irradiance levels, pointing to top-down control by consumers. Additionally, the experimental flood significantly disturbed only the invertebrate composition in the groundwater-fed stream, which was inhabited by the fewest adapted-to-flood-disturbances macroinvertebrates. Despite the low resilience observed, richness and densities of benthos increased during the study, evidencing progressive colonization. Around 50–60% of macroinvertebrates were present in both benthos and hyporheos. Richness increased after flooding, suggesting that the hyporheic zone could be the main source of colonizers. Three species traits characterized the hyporheos: small size, cocoons and feeders of microorganisms within the fine sediment matrix. The main results indicate that longer-than-normal flood periods in early spring may constrain invertebrate succession before the next summer drought in temporary streams. This study underscores the importance of hyporheic fauna as a significance source of colonizers, highlighting the importance of connectivity to the groundwater.

Impact of treated sewage effluent on the bacterial community composition in an intermittent mediterranean stream

Water quality monitoring is essential to safeguard human and environmental health. The advent of next-generation sequencing techniques in recent years, which allow a more in-depth study of environmental microbial communities in the environment, could broaden the perspective of water quality monitoring to include impact of faecal pollution bacteria on ecosystem. In this study, 16 S rRNA amplicon sequencing was used to evaluate the impact of wastewater treatment plant (WWTP) effluent on autochthonous microbial communities of a temporary Mediterranean stream characterized by high flow seasonality (from 0.02 m3/s in winter to 0.006 m3/s in summer).Seven sampling campaigns were performed under different temperatures and streamflow conditions (winter and summer). Water samples were collected upstream (Upper) of the WWTP, the secondary effluent (EF) discharge and 75 m (P75) and 1000 m (P1000) downstream of the WWTP.A total of 5,593,724 sequences were obtained, giving rise to 20,650 amplicon sequence variants (ASV), which were further analysed and classified into phylum, class, family and genus. Each sample presented different distribution and abundance of taxa. Although taxon distribution and abundance differed in each sample, the microbial community structure of P75 resembled that of EF samples, and Upper and P1000 samples mostly clustered together. Alpha diversity showed the highest values for Upper and P1000 samples and presented seasonal differences, being higher in winter conditions of high streamflow and low temperature.Our results suggest the microbial ecology re-establishment, since autochthonous bacterial communities were able to recover from the impact of the WWTP effluent in 1 km. Alpha diversity results indicates a possible influence of environmental factors on the bacterial community structure. This study shows the potential of next-generation sequencing techniques as useful tools in water quality monitoring and management within the climate change scenario.

Understanding divergences between ecological status classification systems based on diatoms

A large number of diatom-based classification systems have been developed worldwide in recent years. These new systems, together with the oldest, emerged on the need to assess the water quality of rivers, but knowledge on possible divergences resulting from their simultaneous application within a territory is limited. This study aimed to compare the ecological status classification provided by conceptually different methodological approaches, of use or potential use within the same region. 402 monitoring samples were collected from Atlantic siliceous streams (NW-Iberian Peninsula) and temporary Mediterranean streams (Balearic Islands, Spain). Two multimetric indices specifically developed for these areas (MDIAT and DIATMIB, respectively) were calculated, as well as the Specific Polluosensitivity Index (IPS). Multimetric indices were more sensitive methods at diagnosing degradation than IPS since they took directly account of abundance (i.e. chlorophyll a in DIATMIB) or indirectly by its proved inverse relationship with Chl a (MDIAT), together with their use of the regional reference diatom community. Alteration gradients were identified in both studied regions based on the distribution of diatoms, with the first axis of distance-based redundancy analyses (dbRDA) being related to nutrient enrichment and organic loads. Threshold Indicator Taxa ANalysis (TITAN) performed on diatoms sampled along environmental (dbRDA axis 1 and phosphate) and biological gradients (as Ecological Quality Ratio scores of classifications), pointed to lower than current Good/Moderate boundaries for phosphate maximum values (e.g. 22.5 and 71.6 μg L−1 for Galicia and the Balearic Islands, respectively) as well as for higher Good/Moderate boundaries for the MDIAT and IPS classifications. A ‘transition group’ of species was classified as sensitive or as tolerant depending on the regional nutrients range. Findings of the present study highlight the need to perform auto-ecological studies to increase the knowledge on regional diatoms and their optimal survival ranges across regions prior to adopt other non-regional diatom indices.

Legacy of Summer Drought on Autumnal Leaf Litter Processing in a Temporary Mediterranean Stream

Temporary Mediterranean streams dry out in summer due to low precipitation and high temperatures, thus affecting biota and ecosystem functioning. The consequences of exposing leaf litter to an extreme summer drought, that is, high solar radiation, low humidity, and high temperatures, were evaluated. It was hypothesised that litter exposure to summer drought conditions in a dry streambed would change litter chemical composition, affecting decomposition and associated benthic communities after autumnal flow resumption. Recently fallen black poplar leaves were enclosed in litterbags and separated into two sets: (1) control leaves, kept air-dry in the dark, and (2) drought leaves, exposed to harsh summer conditions in a simulated dry streambed. Both sets were then immersed in a temporary Mediterranean stream in autumn, and leaf litter decomposition monitored. As expected, summer drought exposure increased nitrogen, total fibre, lignin, and cellulose content in drought leaves. After litterbag immersion, higher cellulose-degrading enzyme activity was observed in drought leaves while lignin-degrading enzyme activity was greater in the control leaves. Fungal and bacterial assemblages clearly differed between treatments. Inter-replicate variability was lower in drought leaves for most of the measured variables, suggesting that drought homogenised decomposer and detritivore communities. Despite these drought-related effects, decomposition rates did not vary, apparently due to counterbalancing effects (that is, drought treatment enhanced recalcitrant compounds and nutrient content in leaf litter) probably related to terrestrial microbial colonisation and photodegradation. Our study demonstrates that summer drought has bottom-up critical legacy effects in temporary Mediterranean streams, affecting the structure and function of decomposer communities.

How Does an Invasive Cyprinid Benefit from the Hydrological Disturbance of Mediterranean Temporary Streams?

The Iberian Peninsula has been subjected to numerous fish introductions and the colonization of new areas by non-native species is constantly reported. However, there is a lack of knowledge about many aspects of the bio-ecology of these species and their invasive success within the environmental context of Mediterranean temporary rivers. This study aimed to address the following questions: (i) what are the main regional and local environmental drivers influencing fish assemblages and differentiating native from non-native species, particularly A. alburnus?; (ii) what are the environmental and anthropogenic disturbance factors responsible for the occurrence and abundance of A. alburnus?; (iii) is there a pattern in the spatiotemporal distribution of A. alburnus size classes, along the tributaries of reservoirs where the species occurs? Data on fish species, environmental variables, and anthropogenic disturbance were collected in 77 sites of the Guadiana and Sado river basins in the south of Portugal. Additionally, a seasonal sampling was performed along an upstream-downstream gradient of several tributaries from three reservoirs in these river basins. A multivariate analysis and a multi-model approach were used in data analyses. Native and non-native fish assemblages showed different environmental drivers and responses to anthropogenic disturbance levels, though A. alburnus has revealed similarities with native species. The occurrence of A. alburnus was mainly determined by hydrological and morphological disturbances driven by anthropogenic activities. Furthermore, this species apparently performed seasonal movements along the tributaries of several reservoirs, profiting from these lentic habitats as a stepping-stone for further invasions. This study highlighted the wide ecological plasticity of A. alburnus, as it benefits from the anthropogenic hydrological disturbance (induced by reservoirs), and is also able to cope with the natural hydrological disturbance (resulting from the intermittency of these streams), to guarantee and enhance its invasive success in Mediterranean intermittent streams. It also gives a sound contribution to understand the spread of A. alburnus in these vulnerable freshwater ecosystems, and to delineate management measures, namely by identifying critical points in the river network along with prioritizing river restoration measures that benefit native species.

Source ascription in bed sediments of a Mediterranean temporary stream after the first post-fire flush

First flushes can be crucial to sediment transport in fluvial systems of drylands, as temporary streams are a characteristic feature of Mediterranean basins. After a wildfire, storm flows may enhance runoff delivery to channels, so increasing the first-flush effect. 137Cs and 210Pbex were used as tracers for recognizing the first post-fire flush effect on the source ascription of bed sediments temporarily stored in a Mediterranean temporary stream severely affected by a wildfire. Thirty sediment source samples were collected along one of the tributaries of a catchment (4.8 km2) located in Mallorca during a field campaign some weeks after the wildfire. The sample collection took into account effects of the wildfire and distinguished between soil surface and channel bank. To measure the source contribution temporarily stored with the bed sediment, five sediment samples—deposited during the first storm, occurring 3 months after the wildfire—were collected from the bed of the main channel. The 137Cs and 210Pbex concentrations were measured by gamma spectrometry. Then, a linear mixing model was used to establish the contribution of each source type to the bed sediments along the main stream. First post-fire flush effect was generated by a torrential event with a suspended sediment concentration peak of 33,618 mg L−1, although transmission losses under a very low runoff coefficient (1%) promoted sediment deposition. Significant differences were observed in fallout radionuclide concentrations between burned surface soil and burned channel bank samples (p 0.05). Source ascription in bed sediments upstream shows that 67% was generated from burned hillslopes, reaching 75% in the downstream part, because downstream propagation of the sediment derived from the burned area. Bed sediments were mostly generated on burned surface soils because of the fire’s effect on soil and sediment availability, high-intensity rainfall, and the limited contribution of channel banks, because these are fixed by dry-stone walls. This hydro-sedimentary response indicates an association between these factors driving erosion and sediment delivery, generating effective slope-to-channel sediment connectivity. The integration of the short-term response with the medium- and long-term analysis will allow for the analysis of the evolution of catchment sediment sources in future studies, determining if fire modifies the catchment sensitivity to that specific disturbance.

Discriminating permanent from temporary rivers with traits of chironomid genera

Chironomidae are diverse and present a wide variety of ecological preferences. Thus, they have high potential in establishing reference conditions for river bioassessment and in providing functional information, especially when other macroinvertebrates are poorly represented. However, because of taxonomic difficulties and poor knowledge of traits, they are neglected in bioassessment programmes and kept at coarser taxonomic levels, reducing the discrimination power of invertebrate-based diagnostic tools. Here, we compared the efficiency of Chironomidae at the subfamily and genus levels and their biological traits (Eltonian and morphological) in the distinction between permanent (medium altitude and lowland) and temporary Mediterranean streams. We established a priori predictions on the expected Chironomidae trait categories in each stream type, conferring the best adaptations to particular environmental constraints. Genus composition (not subfamily) and respective trait categories differed among the three stream types. Both biological traits identified differences between stream types. Among Eltonian traits, diapause stages segregated permanent medium altitude from lowland and temporary stream assemblages, reflecting adaptations to temperature and flow regime variations. Substrate relation and size distinguished temporary from permanent stream assemblages, interpreted as an adaptation to unstable sediments and irregular flow regimes. Morphological traits associated with mobility and foraging (e.g. , body setae, Lauterborn sensory organs) discriminated temporary from permanent stream assemblages, reflecting differences in temporal and spatial stream heterogeneity. The use of these morphological characteristics relying on the observation of few characteristics, offers an alternative to the use of Chironomidae Eltonian traits in bioassessment and ecological studies, without a need for high taxonomic expertise.

Indicator macroinvertebrate species in a temporary Mediterranean river: Recognition of patterns in binary assemblage data with a Kohonen artificial neural network

Current classifications used in bioassessment programs, as defined by the Water Framework Directive (WFD), do not sufficiently capture the variability present in temporary Mediterranean streams. This may result in inaccurate evaluation of the water quality biological metrics and difficulties in setting reference conditions. The aim of the study was to examine if aquatic invertebrate data of increased taxonomical resolution but expressed on a binary abundance (frequent/rare) scale and referring to good bioindicator species only suffice to indicate clear gradients in water courses with high natural variability such as intermittent Mediterranean streams. Invertebrate samples were collected from 74 sites in the Quarteira River basin, located in southern Portugal. Their classification with the use of a Kohonen artificial neural network (i.e., self-organising map, SOM) resulted in five categories. The variables that drove this categorization were primarily altitude, temperature and conductivity, but also type of substrate, riparian cover and percentage of riffles present. According to the indicator species analysis (ISA), almost all the studied taxa were significantly associated with certain SOM categories except for the category that included sites with disrupted flow regime. The SOM and ISA allowed us to effectively recognize biotic and abiotic patterns. Combined application of both methods may thus greatly enhance the effectiveness and precision of biological surveillance and establish reference sites for specific channel units in streams with high natural variability such as intermittent Mediterranean streams.

Detecting the impact of bank and channel modification on invertebrate communities in Mediterranean temporary streams (Sardinia, SW Italy)

We hypothesized that reach-scale, bank and channel modification would impact benthic communities in temporary rivers of Sardinia, when pollution and water abstraction are not relevant. A range of variables were considered, which include both artificial structures/alterations and natural features observed in a stream reach. Multivariate regression trees (MRT) were used to assess the effects of the explanatory variables on invertebrate assemblages and five groups, characterized by different habitat modification and/or features, were recognized. Four node variables determined the splits in the MRT analysis: channel reinforcement, tree-related bank and channel habitats, channel modification and bank modification. Continuity of trees in the river corridor diverged among MRT groups and significant differences among groups include presence of alders, extent of channel shading and substrate diversity. Also, the percentage of in-stream organic substrates, in particular CPOM/Xylal, showed highly significant differences among groups. For practical applications, thresholds for the extent of channel reinforcement (40%) and modification (10%) and for bank alteration (≈30%) were provided, that can be used to guide the implementation of restoration measures. In moderately altered river reaches, a significant extent of tree-related habitats (≈5%) can noticeably mitigate the effects of morphological alteration on aquatic invertebrates. The outcomes highlight the importance of riparian zone management as an opportune, achievable prospect in the restoration of Mediterranean temporary streams. The impact of bank and channel modification on ecological status (sensu WFD) was investigated and the tested benthic metrics, especially those based on abundance data, showed legible differences among MRT groups. Finally, bank and channel modification appears to be a potential threat for the conservation of a few Sardo-Corsican endemic species. The introduction of management criteria that openly use information on endemic species seems crucial to reduce the risk of improving overall ecological status while depleting biodiversity.

Heterogeneity in leaf litter decomposition in a temporary Mediterranean stream during flow fragmentation

In temporary Mediterranean streams, flow fragmentation during summer droughts originates an ephemeral mosaic of terrestrial and aquatic habitat types. The heterogeneity of habitat types implies a particular ecosystem functioning in temporary streams that is still poorly understood. We assessed the initial phases of leaf litter decomposition in selected habitat types: running waters, isolated pools and moist and dry streambed sediments. We used coarse-mesh litter bags containing Populus nigra leaves to examine decomposition rates, microbial biomass, macroinvertebrate abundance and dissolved organic carbon (DOC) release rates in each habitat type over an 11-day period in late summer. We detected faster decomposition rates in aquatic (running waters and isolated pools) than in terrestrial habitats (moist and dry streambed sediments). Under aquatic conditions, decomposition was characterized by intense leaching and early microbial colonization, which swiftly started to decompose litter. Microbial colonization in isolated pools was primarily dominated by bacteria, whereas in running waters fungal biomass predominated. Under terrestrial conditions, leaves were most often affected by abiotic processes that resulted in small mass losses. We found a substantial decrease in DOC release rates in both aquatic habitats within the first days of the study, whereas DOC release rates remained relatively stable in the moist and dry sediments. This suggests that leaves play different roles as a DOC source during and after flow fragmentation. Overall, our results revealed that leaf decomposition is heterogeneous during flow fragmentation, which has implications related to DOC utilization that should be considered in future regional carbon budgets.

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

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−1 and 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−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 for DOM dynamics in temporary streams, and point to the potential response of perennial streams under future water scarcity scenarios.

Macroinvertebrate indicators of ecological status in Mediterranean temporary stream types of the Balearic Islands

In the last years the Water Framework Directive (WFD) has encouraged the development of classification systems to assess the ecological status in permanent rivers and streams, but in temporary streams development has been scarce. Here, we constructed an Invertebrate Multimetric index (INVMIB) to assess the ecological status of temporary streams in Mediterranean Islands based on benthic invertebrates following the WFD guidance. We studied 60 temporary streams in different seasons (n=270 samples), corresponding to three different stream types that were a priori established using topographical and valley information: lowland, canyon and mountain streams. We collected invertebrates with a multihabitat-based macroinvertebrate sampling standardised for a sampling area of 2.5m2, and stream water to analyse most relevant water chemistry variables. The analyses of benthic macroinvertebrate assemblages confirmed our stream typology (ANOSIM, R=0.363, p<0.001). The reference community of lowland streams was defined by taxa belonging to the order Diptera, the gastropod Ancylus fluviatilis, and several beetles and EPT taxa. Canyons showed a high diversity strongly dominated by dipterans and aquatic mites together with mayflies. Finally, the amphipod Echinogammarus sicilianus-monomerus, many dipterans of the Chironomidae family, together with beetles, mayflies, stoneflies and caddisflies were highly represented in the reference communities from mountain streams. The developed INVMIBs consisted of a combination of individual metrics that included diversity, tolerance/sensitivity and composition measures. The metrics and developed type-specific multimetric indices responded significantly to the gradients of pressures obtained from PCAs (i.e., general and organic degradation), as an integrated response to the physico-chemical variables, hydromorphological parameters and land uses. Indeed, the classification system obtained with the INVMIBs reflected the ecological quality of temporary streams of the Balearic Islands. The multimetric indices are considered a valid and accurate tool for the assessment of temporary Mediterranean streams, being currently used by the water authorities of the Balearic Islands.

Towards sustainable management of Mediterranean river basins: policy recommendations on management aspects of temporary streams

In 2011, the European Council stressed the significance of water quality for sustainable development in Europe and emphasized the need for better integration of the water policy objectives into the Common Agriculture Policy reform in rural areas. Since 2000, the Water Framework Directive (WFD) has required the EU Member States to target good ecological status for their water bodies. However, the implementation of the Directive for the numerous Mediterranean temporary streams has been delayed indicating the need for elaboration of the tools and methods that address the special characteristics of such water bodies. This requirement has been addressed by the recently completed MIRAGE project. In the context of the recent publication of the European Commission's Blueprint to Safeguard Europe's Waters, the MIRAGE-proposed framework for the characterization of the eco-hydrological dynamics and the systematic description of the measured impact for temporary rivers could bring considerable added value to the EU revision of all relevant water policies. The project recommends additions to WFD articles including an explicit definition of temporary rivers, adaptation of environmental objectives to their peculiarities and establishment of a proper method to determine the initial status and specific actions in River Basin Management Plans.

Denitrification rates and controlling factors in two agriculturally influenced temporary Mediterranean saline streams

In this study, we tested the hypothesis that agriculture, through its influence on water NO3−-N availability, would control denitrification in agriculturally influenced temporary saline streams, and that water salinity would not affect this process. We also tested the effect of summer drought on the denitrification process. We approached these objectives by estimating sediment denitrification (using the acetylene inhibition technique) in two temporary Mediterranean streams following an increased natural water salinity and agricultural gradient under pre- and post-drought conditions. During the pre-drought conditions, the water NO3−-N concentration was the main predictor of denitrification rates. Together with the water NO3−-N concentration, sediment redox conditions and water salinity appeared to be significant predictors, the latter showing a negative effect. During the post-drought, denitrification rates dropped significantly in both streams and no abiotic factors seemed to significantly influence this process. Our results suggest that high water salinity and drought affected negatively the stream-denitrifying capacity. This study highlights that stressors such as water salinity and hydrological intermittency should be considered in future stream management plans in order to preserve the role of streams on controlling the NO3−-N export, especially in the context of a warmer and drier climate.

Diatom communities as indicators of ecological status in Mediterranean temporary streams (Balearic Islands, Spain)

Diatoms are widely used for the bioassessment of permanent rivers and streams due to their broad distribution and their ability to integrate changes occurring in water composition and quality, but they have rarely been investigated in temporary streams. Water and epilithic diatom samples were collected from 60 sites in temporary streams distributed among the three largest Balearic Islands (Majorca, Minorca and Ibiza). The sampling design included only temporary streams that have water for at least 4–5 months during the annual period. The selected sites included minimally disturbed (reference sites) and other sites influenced by different degrees of human pressures. Each study site was sampled at least twice in different seasons to take into account the temporal variability in diatom assemblages ( n = 255). Non-metric multidimensional scaling (NMDS) and analyses of similarities (ANOSIM) were used to assess variation in the communities between a priori defined types and along to the degradation gradient, and to study the species tolerance to pollution. A diatom multimetric index (DIATMIB) was developed to assess the ecological status of temporary streams, as a combination of three metrics: abundance of sensitive taxa, abundance of tolerant taxa and a metric of chlorophyll a values. Individual diatom metrics and the multimetric DIATMIB were correlated in a significant way with phosphate, nitrate, ammonia, silicon dioxide, ash free dry mass and water electric conductivity. Cymbella vulgata, Diploneis oblongella and Encyonopsis microcephala appeared only in reference sites and in high and good status classes. These taxa were replaced by others such as Navicula veneta, Nitzschia inconspicua, N. frustulum and Planothidium frequentissimum that appeared in moderate, poor and bad status classes, which were characterized by high levels of organic/nutrient pollution. Our study validates the application of a diatom multimetric index as a good approach to classify the ecological status of temporary streams due to changes in the structure of diatom communities. We concluded that the multimetric index DIATMIB is a valuable tool for the assessment and future monitoring of temporary Mediterranean streams.

Do seasonal changes in habitat features influence aquatic macroinvertebrate assemblages in perennial versus temporary Mediterranean streams?

In this study we examined the importance of seasonal changes in habitat features and aquatic macroinvertebrate responses in temporary and perennial streams from two different catchments in the Western Mediterranean region in Spain. Macroinvertebrate sampling was spatially intensive to account for the relative frequency of meso- (i.e., riffles and pools) and micro-habitats (i.e., different mineral and organic-based substrata) at each site. Samples were collected at two distinctly different phases of the hydrograph: (1) during the flowing period, when pool-riffle sequences were well-established, and (2) during the dry phase, when only isolated pools were expected to occur in the temporary streams. During the dry season, both a reduction in the available total habitat and in microhabitat diversity in all sites studied was observed. As a result, taxon richness decreased in all streams, but more dramatically at temporary stream sites and particularly so in the infrequently remaining discontinuous riffles. Macroinvertebrate assemblages differed among catchments (i.e., geographical identity) and sites (perennial vs. temporary). Invertebrate differences were also strong within and among meso- and micro-habitats, particularly mineral and organic microhabitat patches, and differences were due to both loss of taxa from some habitats and some taxa exhibiting certain habitat affinities.

The influence of extreme climatic events and human disturbance on macroinvertebrate community patterns of a Mediterranean stream over 15 y

We analyzed long-term changes in macroinvertebrate communities in a Mediterranean temporary stream in southern Portugal over 15 y (1993–2008) at 10 locations with 3 degrees of physicochemical disturbance (reference, high disturbance, and mild disturbance). We related year-to-year variation of macroinvertebrate communities to long-term (59 y) information on precipitation and temperature. Our goals were to: 1) determine the stability of macroinvertebrate communities in the stream, 2) establish the influence of physicochemical disturbance on community stability, 3) assess the influence of climate change on the macroinvertebrate communities, and 4) assess the interactive effects of climate change and disturbance level on macroinvertebrate communities. Community structure varied naturally from year to year, but changes in taxon richness and evenness were much stronger and more unpredictable in disturbed than in other sites. In the long term, the more diverse (reference) and the poorest (disturbed) communities were stable, whereas communities affected by mild disturbance slowly decreased in taxon richness (slope = −0.07, r2 = 0.38). This decrease could be a response to the continuous stress or to climate change. The multivariate patterns over time of invertebrate communities at mildly disturbed sites were the only patterns significantly correlated with climatic patterns. In the past 59 y in this Mediterranean area (southeastern Europe), winter temperature has increased 1°C and precipitation has decreased 1.5 mm/d. Marked changes in community composition (70–80% Bray–Curtis dissimilarity) occurred only in years of extremely low precipitation or temperature. In years of climatic extremes and at chemically disturbed sites, Orthocladiinae and Simuliidae became dominant. In this stream, a shift in community equity occurs before species elimination. This shift might be useful as an early warning for biodiversity loss because of disturbance or climate change. We recommend continued sampling of reference sites for monitoring purposes so that effects of climate change can be established and so that contemporary human disturbance can be assessed relative to an adjusted reference condition.

Homage to the Virgin of Ecology, or why an aquatic insect unadapted to desiccation may maintain populations in very small, temporary Mediterranean streams

In temporary streams, the annual constriction of drying is associated with high local extinction risk. To survive in such habitats, organisms with no specific biological traits for coping with dry periods should experience high colonisation rates from permanent reaches of the same basin or from other basins. Hydropsyche siltalai is a widespread caddisfly common in permanent and temporary headwaters reaches in the Mediterranean climate region of the Iberian Peninsula. In this study, we used genetic analyses to test if populations of H. siltalai in temporary streams are resettled from populations of the same basin or from other basins. The geographical distribution of H. siltalai was surveyed in 97 temporary and permanent reaches across four basins; larvae were found in 22 reaches (12 temporary and 10 permanent). Population genetic analyses of 11 selected reaches (6 temporary and 5 permanent) revealed low genetic diversity and no genetic population structure among and within basins. Overall, H. siltalai appeared to disperse well among basins independent of stream temporality. Permanent reaches from different basins act as a source of the individuals that recolonise temporary reaches after local extinctions, indicating a metapopulation structure at regional scale. Moreover, our results support other studies that showed that dispersal among basins is a recurrent pattern in aquatic insects.

Chironomid species richness of a permanent and a temporary Mediterranean stream: a long-term comparative study

AbstractPatterns of chironomid species richness were compared between 1 permanent and 1 temporary Mediterranean mountain stream in the same basin. From 1991 to 1995, pupal exuviae were collected at irregular intervals in the permanent stream, but the temporary stream was dry because of a long-lasting drought in the region. From March 1996 to March 2000, pupal exuviae were collected fortnightly in both streams. In contrast to other comparative studies that found more species in permanent than in temporary streams, our 2 streams harbored similar temporal γ and α diversity during the 1996 to 2000 study period. This result might be explained by the relatively long time the temporary stream was flowing. In addition, the temporary stream showed higher temporal β diversity than the permanent stream, a result that might be attributable to the greater variability in temperature and substrates in the temporary stream. Spates scarcely affected α diversity and favored colonization by new species in both streams, a re...