María del Mar Sánchez-Montoya

Defining criteria to select reference sites in Mediterranean streams

The European Water Framework Directive establishes the need to define stream type-specific reference conditions to identify “high ecological status”. Methods for selecting reference sites using a priori criteria have been proposed by many authors. A review of these criteria revealed that the most relevant criteria for streams and rivers were those related to riparian vegetation, diffuse and point sources of pollution, river morphology and hydrological conditions and regulation. In this work, we propose 20 criteria that reflect the characteristics of Mediterranean streams and their most frequent disturbances for the selection of reference sites in Mediterranean streams in Spain. We studied 162 sites located in 33 Mediterranean basins belonging to five stream types. Of the locations, 57% were selected as a priori reference sites by having applied the proposed criteria. Reference sites were identified for all stream types except for “large watercourses” which includes the lower reaches of some rivers in this study area. This a priori selection of reference sites was subjected to validation using the macroinvertebrate community by applying of an IBMWP threshold, which is considered to be an indicator of undisturbed sites in Mediterranean streams. This approach determined that whole of this selection (100%) could be considered valid reference sites. Furthermore, we identified differences in the reference conditions for each stream type on the basis of macroinvertebrate assemblage composition.

Establishing physico-chemical reference conditions in Mediterranean streams according to the European Water Framework Directive

Type-specific physico-chemical reference conditions are required for the assessment of ecological status in the Water Framework Directive context, similarly to the biological and hydro-morphological elements. This directive emphasises that natural variability of quality elements in high status (reference condition) needs to be quantified. Mediterranean streams often present a marked seasonal pattern in hydrological, biological and geochemical processes which could affect physico-chemical reference conditions. This study establishes general physico-chemical reference conditions (oxygenation, nutrient, salinity and acidification conditions) for different Mediterranean stream types. 116 potential reference sites located in 23 Mediterranean catchments in Spain were sampled in spring, summer and autumn in 2003. All sites were subjected to a screening method for the selection of reference sites in Mediterranean streams (Mediterranean Reference Criteria) and classified using a pre-established stream typology that establishes five different stream types (temporary streams, evaporite-calcareous at medium altitude, siliceous headwaters, calcareous headwaters and large watercourses). Reference conditions (reference value and reference threshold equivalents to high-good class boundary) were calculated using two different methods according to the availability of reference sites: the reference site 75th percentile approach of all reference sites and the 25th percentile of the population approach. The majority of the studied potential reference sites (76 out of 116) were selected as reference sites. Regarding type-specific reference conditions, only siliceous headwaters could be considered different from the rest of stream types because lower conductivity and pH. All reference stream types presented seasonal differences as regards some parameters, except for temporary streams due to the high natural variation of this stream type. For those parameters which presented seasonal differences in a specific stream type, the least restrictive values were proposed as reference conditions.

The MEDiterranean Prediction And Classification System (MEDPACS): an implementation of the RIVPACS/AUSRIVAS predictive approach for assessing Mediterranean aquatic macroinvertebrate communities

In Spain, a national project known as GUADALMED, focusing on Mediterranean streams, has been carried out from 1998 to 2005 to implement the European water framework directive (WFD) requirements. One of the main objectives of the second phase of the project (2002–2005) was to develop a predictive system for the Spanish Mediterranean aquatic macroinvertebrate communities. A combined-season (spring, summer, and autumn) predictive model was developed by using the latest improvements on the selection of best predictor variables. Overall model performance measures were used to select the best discriminant function (DF) models, and also to evaluate their biases and precision. The final predictive model was based on the best five DF models. Each one of these models involved eight environmental variables. Final observed (O), expected (E), and O/E values for the number of macroinvertebrate families (NFAM) and two biotic indices (IBMWP and IASPT) were calculated by averaging their values, previously weighted by the quality of each DF model. Regression analyses among the final O and E values for the calibration dataset showed a high proximity to the ideal theoretical model, where the final E values explained 73–84% of the variation present in the macroinvertebrate communities of the Spanish Mediterranean watercourses. The ANOVA performed among the reference (calibration and validation) and test datasets showed clear differences for the O/E values. Finally, the assessments carried out by the predictive model were sensitive to anthropogenic pressure present in the study area and allowed the definition of five ecological status classes according to the WFD requirements.

Spatial scale effects on taxonomic and biological trait diversity of aquatic macroinvertebrates in Mediterranean streams

We examined the effect of spatial scale on aquatic macroinvertebrate communities in Mediterranean streams from six basins distributed across southern Europe, including Spain, France, Italy, and Greece. We classified the studied streams according to their long-term aquatic regime into the three following types: (i) permanent (P), (ii) intermittent with summer pools (I-P), and (iii) intermittent with summer dry channels (I-D). For each stream type, we analyzed taxonomic and trait diversity, as well as the composition of the macroinvertebrate community, following a spatially nested design at three spatial scales of analysis: microhabitat (substratum patches), mesohabitat (pools vs. riffles), and macrohabitat (streams). In order to assess intrinsic seasonal variability in streams from the Mediterranean region, 20 Surber samples were taken from each stream according to mesoand microhabitat frequency in the wet and the dry season during 2010. Given the need for adaptation to specific hydrological conditions and the fact that microhabitats should encompass the niche requirements of particular taxa, we hypothesized that this spatial scale would have a greater influence on macroinvertebrate taxa composition and biological traits than the other two larger spatial scales in intermittent streams. We observed that patterns in the relative importance of variance components across hierarchical spatial scales changed with time because low flow or droughts altered both mesohabitat prevalence and microhabitat composition. Our results confirm the importance of the microhabitat scale in I-P streams in the wet season but not in the dry one, when a loss of microhabitat diversity occurred. Stream-to-stream variability was more important in P and I-D streams. Our study also explored the relationships between traits and aquatic regimes. We found that aquatic macroinvertebrates inhabiting permanent streams exhibited traits related to the longer duration of life cycles in these rivers (e.g. large size of adult stages) and adaptations to flowing conditions (e.g. modes of aquatic dispersal), whereas aquatic macroinvertebrates inhabiting intermittent streams with summer pools had traits adapted to depositional conditions and ecological preferences for confined habitats (i.e. disconnected pools without flow). Finally, aquatic macroinvertebrates from intermittent streams with summer dry channels had adaptations conferring ability to survive periods with no water (e.g. modes of aerial dispersal, resistant stages). These results indicate that microhabitat conditions should not be neglected as they can play an important role in certain situations.

A biological tool to assess flow connectivity in reference temporary streams from the Mediterranean Basin

Many streams in the Mediterranean Basin have temporary flow regimes. While timing for seasonal drought is predictable, they undergo strong inter-annual variability in flow intensity. This high hydrological variability and associated ecological responses challenge the ecological status assessment of temporary streams, particularly when setting reference conditions. This study examined the effects of flow connectivity in aquatic macroinvertebrates from seven reference temporary streams across the Mediterranean Basin where hydrological variability and flow conditions are well studied. We tested for the effect of flow cessation on two streamflow indices and on community composition, and, by performing random forest and classification tree analyses we identified important biological predictors for classifying the aquatic state either as flowing or disconnected pools. Flow cessation was critical for one of the streamflow indices studied and for community composition. Macroinvertebrate families found to be important for classifying the aquatic state were Hydrophilidae, Simuliidae, Hydropsychidae, Planorbiidae, Heptageniidae and Gerridae. For biological traits, trait categories associated to feeding habits, food, locomotion and substrate relation were the most important and provided more accurate predictions compared to taxonomy. A combination of selected metrics and associated thresholds based on the most important biological predictors (i.e. Bio-AS Tool) were proposed in order to assess the aquatic state in reference temporary streams, especially in the absence of hydrological data. Although further development is needed, the tool can be of particular interest for monitoring, restoration, and conservation purposes, representing an important step towards an adequate management of temporary rivers not only in the Mediterranean Basin but also in other regions vulnerable to the effects of climate change.

Implications of flow intermittency on sediment nitrogen availability and processing rates in a Mediterranean headwater stream

Most streams draining to the Mediterranean basin are temporary. As a result of their hydrological regime, temporary streams are affected by drying and rewetting periods. Drying can alter in-stream nitrogen (N) availability and reduce N processing rates and subsequent retention after re-wetting. We sought to determine if hydrologic drying modifies reach-scale sediment chemical properties and constrains the response of N processing to rewetting. We compared different abiotic characteristics of sediments and nitrification and denitrification rates between a perennial and intermittent reach in the same stream over a wet period, when surface water flowed in both reaches, and a dry period, when the intermittent reach dried up. We analyzed N processing rates by incubating sediments with stream water, thereby simulating a rewetting when sediments from the intermittent reach were dry. We found that drying increased the sediment nitrate (NO3−) content. Conversely, drying did not reduce the recovery of N processing rates to pre-dry levels after simulated flooding conditions. Our results suggest that dry reaches may act as a potential NO3− source by releasing downstream NO3− pulses after stream flow recovery. Given the European Water Framework Directive requirements to assess stream ecological status, these N pulses following rewetting should be considered when designing management plans in temporary streams. Our study highlights the rapid response of in-stream N processing to rewetting period following a drought. This high resilience to process N should be seen as a vital ecosystem service provided by temporary streams despite annual dry periods.

Seasonal and interannual variability of macroinvertebrate reference communities and its influence on bioassessment in different Mediterranean stream types.

We investigated the seasonal changes in macroinvertebrate reference communities in four Mediterranean stream types (temporary, evaporite calcareous at medium altitude, siliceous headwaters at high altitude, and cal- careous headwaters at medium and high altitudes) and the interannual changes in the two headwaters stream types in Spain. Eighty-eight seasonal reference sites distributed into 23 basins were sampled on three occasions (spring, summer and autumn of 2003), and 18 interannual reference sites distributed in 6 basins were sampled in the autumn of 2003, 2004 and 2005 to examine this temporal variability. Interannual reference sites were a subset of seasonal reference sites. The analysis of similarity (ANOSIM) performed on Bray-Curtis similarity distances, using pres- ence-absence data, showed no seasonal or interannual changes in the macroinvertebrate communities. The infl uence of seasonal and interannual variability was also tested in all the stream types using 18 macroinvertebrate metrics classifi ed as richness, index, multimetric index, tolerance/intolerance and diversity. ANOVAs showed no seasonal differences in any of the studied metrics for temporary and evaporite calcareous at medium altitude and most of the metrics in the two headwaters stream types. This suggests the suitability of using a single season approach for the biomonitoring purposes of these metrics. Conversely, the seasonal differences detected in the metrics related with EPT (Ephemeroptera, Plecoptera and Trichoptera) and OCH (Odonata, Coleoptera and Heteroptera) taxa in calcareous headwaters indicate possible differences in the relative presence of macrohabitats (riffl es and pools) as a result of fl ow variation. No interannual changes were detected in any of the metrics except EPT/OCH in siliceous headwaters. However, the large variability in the annual rainfall in this study area suggests that this three-year study period may be too short to assess the effect of climatic variations on the ecological status assessment. In general, the lower temporal variability (measured as seasonal and interannual coeffi cients of variation) of the taxon richness metric (S) and the two studied indices (IBMWP and IASPT), compared with the other metrics, make them a priori robust indicators to assess ecological status in Mediterranean streams.

Intra-Annual Variation in Benthic Organic Matter in A Saline, Semi-Arid Stream of Southeast Spain (Chicamo Stream)

During 1994 and 1998–1999, temporal changes in the benthic organic matter (BOM) and its fractions (CBOM = coarse; FBOM = fine; UBOM = ultrafine) were studied in a 354-m reach of Chicamo stream, a saline (9.5 g l−1), temporary semi-arid stream located in southeast Spain. Both time periods differed in their frequency and intensity of spates occurring before sampling. BOM at all sites was dominated by FBOM, followed by UBOM and CBOM. Significant differences in total BOM and its fractions occurred among sampling sites, years, months and dates, illustrating the complex temporal variation of benthic organic matter. Positive correlations were found for FBOM and TBOM with discharge, for CBOM with water depth, and between total BOM, FBOM and UBOM and biomass of periphyton. Principal Component Analysis and Cluster Analysis revealed five different groups, each characterised by different patterns of dominance by fractions of BOM.

Functional response of aquatic invertebrate communities along two natural stress gradients (water salinity and flow intermittence) in Mediterranean streams

Functional trait diversity can provide insight into ecosystem function beyond that provided by species diversity measures. The relationship between functional diversity and natural stressors has received less attention compared to anthropogenic stressors. In this study, we investigated how two natural stressors, water salinity and flow intermittence, affect functional richness and functional redundancy of aquatic invertebrate communities using seven biological traits and 39 modalities. For this purpose, we characterized these functional diversity measures in 22 Mediterranean streams with a gradient of natural salinity and flow intermittence. Our findings showed that both functional richness and functional redundancy decreased with increased stress by water salinity and flow intermittence for all the studied traits but more rapidly for the former, suggesting that water salinity is a stronger environmental stressor than flow intermittence. Our study also described an antagonistic interaction of the two study stressors, in which the net effect of both is less than the sum of their independent effects. This study emphasizes that in saline streams, characterized by lower functional richness and functional redundancy, the loss of any taxon can have a huge impact on community functioning. In particular, the functional singularity of saline intermittent streams makes them extremely sensitive to additional anthropogenic impacts. In the context of future global change scenarios, which predict higher flow intermittence and water salinity, this study gives a better understanding of the functional features of these types of ecosystems.

Understanding the effects of predictability, duration, and spatial pattern of drying on benthic invertebrate assemblages in two contrasting intermittent streams

In the present study, we examined the effects of different drying conditions on the composition, structure and function of benthic invertebrate assemblages. We approached this objective by comparing invertebrate assemblages in perennial and intermittent sites along two intermittent Mediterranean streams with contrasting predictability, duration, and spatial patterns of drying: Fuirosos (high predictability, short duration, downstream drying pattern) and Rogativa (low predictability, long duration, patchy drying pattern). Specifically, we quantified the contribution of individual taxa to those differences, the degree of nestedness, and shifts in the composition, structure and function of benthic invertebrate assemblages along flow intermittence gradients. We observed greater effects of drying on the benthic invertebrate composition in Fuirosos than in Rogativa, resulting in a higher dissimilarity of assemblages between perennial and intermittent sites, as well as a lower degree of nestedness. Furthermore, a higher number of biotic metrics related to richness, abundance and biological traits were significantly different between perennial and intermittent sites in Fuirosos, despite a shorter dry period compared to Rogativa. At the same time, slightly different responses were detected during post-drying (autumn) than pre-drying (spring) conditions in this stream. In Rogativa, shifts in benthic invertebrate assemblages along increasing gradients of flow intermittence were found for three metrics (Ephemeroptera, Plecoptera and Trichoptera (EPT) and Odonata, Coleoptera and Heteroptera (OCH) abundances and aerial active dispersal. Furthermore, we demonstrated that combined gradients of dry period duration and distance to nearest perennial reach can generate complex, and different, responses of benthic invertebrate assemblages, depending on the flow intermittence metric. Our study advances the notion that special attention should be paid to the predictability, duration and spatial patterns of drying in intermittent streams in order to disentangle the effects of drying on benthic invertebrate assemblages, in particular in areas subject to high spatial heterogeneity and temporal variability in drying conditions.