Maruxa Álvarez

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.

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.

Change in food ingestion induces rapid shifts in the diversity of microbiota associated with cutaneous mucus of Atlantic salmon Salmo salar

To test the hypothesis that a switch in diet might cause changes in the abundance and composition of mucous-dwelling microorganisms, a short-term experiment was conducted with Atlantic salmon Salmo salar. Fish were fed on three different diets: pelleted S. salar feed, macroinvertebrates or pellets supplemented with an antibiotic. A fourth group of fish was deprived of food throughout the trial. Seven days after manipulating diets, significant differences were found in microbial density and community composition (quantified by different morphologically distinct colonies), particularly between fed and unfed animals. Moreover, food deprivation caused a rapid decrease in the number of epidermal mucous cells of the lateral skin, which may indicate a decrease in mucous secretion and explain differences in the diversity of mucous-dwelling microbiota observed in the fish. This is the first report of an effect of feeding regime on the abundance of microbial communities associated with cutaneous mucus of fishes.

Factors controlling epilithon biomass in a temporary, karstic stream: the interaction between substratum and grazing

Abstract Factors affecting epilithon biomass were investigated at 3 sites along a temporary karst stream system on the Mediterranean island of Majorca. Two sites were on the mainstream axis (downstream site [T5], upstream site [T4]) and one site was in a spring-fed tributary (GB). The degree of abiotic control of periphyton biomass was examined at the 3 sites. Mean periphyton biomass during a surficial flow period (as chlorophyll a [chl a] and ash-free dry mass [AFDM]) was nearly 1 order of magnitude higher at T5 than at the other 2 sites. AFDM ranged from low values at GB to intermediate and very high levels at T4 and T5, respectively. Spatial variation in periphyton chl a was related to the coarser, and more stable, karstic substrata and higher discharge at T5 than at GB or T4. A significant proportion of the temporal variation in periphyton chl a at T4 and T5 was related to abiotic factors, but abiotic factors did not explain the temporal pattern at GB. A short-term field experiment was conducted at GB to determine the role of the dominant grazer, the glossosomatid caddisfly Agapetus quadratus, in regulating periphyton biomass. This experiment revealed that densities of A. quadratus may have controlled periphyton biomass at GB (where grazer densities were highest) because the relationship between A. quadratus density and chl a followed a negative exponential function. The grazer prevented an increase in chl a at intermediate and high densities (>4000 ind./m2), but no effect was found at lower grazer densities. Overall, the levels of calcareous deposition over substratum surfaces seemed to influence colonization by grazers, which, in turn, may have influenced periphyton accrual.

Do temporary streams of Mediterranean islands have a distinct macroinvertebrate community? The case of Majorca

Insular streams suffer from similar forms of environmental degradation that occur in continental systems. However, the intrinsically fragile nature of insular ecosystems, together with their exceptional conservation value, makes them particularly vulnerable. Streams on the Mediterranean island of Majorca (Spain) dry annually during summer, with the period of flow permanence ranging from a few (two - three) to several (six - seven) months. Moreover, humans have greatly influenced the landscape of this island, reducing the extent and quality of the riparian vegetation. This study investigates the associations between natural flow variability, riparian land use and water quality with the structure of macroinvertebrate communities of temporary streams of the island of Majorca. Overall, the patterns of faunistic similarity showed that land use and flow permanence are decisive factors shaping the structure of the macroinvertebrate communities inhabiting these streams. Taxa richness was higher in undisturbed spring-fed sites, surrounded by Mediterranean climatic vegetation, than in disturbed sites, affected by urban or rural uses. However, abundance of macroinvertebrates was highest in the sites with a longer period of flow permanence, independent of land use. We also conclude that the springs feeding streams with a longer flow period represent a potential refuge for a high number of the endemic and relict species of macroinvertebrates, particularly those located in well-preserved areas. In fact, in spite of the insularity and the absence of permanent running waters, macroinvertebrate taxa richness in these spring-fed streams compared well to that in similar mainland streams. These results indicate that even though insular assemblages are more distinct than those in continental systems, the temporary streams existing on Majorca are not especially depauperate in terms of taxa richness.

Nutrient limitation controls the strength of behavioral trophic cascades in high elevation streams

Indirect effects of predators on primary producers vary over space and time. Key components of the environmental context underlying that variability include the bottom-up supply of resources and the defense, mobility and foraging efficiency of primary consumers. We manipulated key resources that limit primary producers and tested the effects of system enrichment on the strength of top-down interactions between top predators (Salvelinus fontinalis), different types of grazing invertebrates and algae in high elevation streams of Colorado, USA. We added nitrogen and phosphorus to flow-through microcosms powered by stream water and tested the direct effects of chemical cues from brook trout on the behavior of sedentary, predator-resistant (caddisflies) and mobile, predator-vulnerable grazers (mayflies), and the indirect cascading effects of predators on: (1) the biomass of algae mediated by grazer behavior and (2) the impact of grazers on algal biomass accrual. Recognizing the value of whole-ecosystem experiments, we also tested the effects of supplementing nutrients to two headwater fishless streams on the impact of grazers on algae and the performance (growth rates) of four predominant grazer species compared to two adjacent reference streams with ambient (limited) nutrients. In both experiments, algal biomass increased with added nutrients where consumption by grazers was swamped by increased algal accrual due to enrichment. Results of the microcosm experiment were consistent with a context-dependent behavioral trophic cascade whereby predator cues increased the biomass of algae only in treatments with enriched nutrients and mobile, predator-vulnerable grazers (Baetis mayflies). Baetis showed risk-sensitive behavioral responses to fish cues, which resulted in cascading effects on algae only under enrichment. In contrast, behavior of sedentary, predator- resistant caddisflies (Allomyia) was unaffected by predator cues and a cascade was never observed. The whole ecosystem experiment revealed no aggregative responses by grazers but instead, strong developmental responses of all grazer species to nutrient enrichment. Growth rates of grazers increased with mobility, with the highest rates occurring for mobile, predator-vulnerable Baetis mayflies in enriched streams. Our results emphasize the importance of experimentally testing the context-dependency of trophic cascades, and are consistent with the hypothesis that nutrient limitation may reduce the strength of trophic cascades.

Characterizing disturbance regimes of mountain streams

Abstract: Characterizing biologically relevant stream disturbance regimes is challenging, but necessary to answer questions about disturbance effects on ecological processes. No universally accepted approach exists for characterizing stream regimes. Our goal was to evaluate approaches that can be applied to test effects of disturbance on benthic organisms. We defined disturbance as events or environmental conditions caused by changes in stream discharge that affect the stability or habitability of the stream bed. We used several metrics to describe disturbance regimes of mountain streams that were not permanently gauged in 1 catchment, and considered the trade-off between effort required to obtain the data and the quality of information gained. We used an innovative photographic method to assess substrate particle movement empirically as a benchmark for comparison to other indicators of channel stability and to metrics describing hydrologic variability relevant to streambed stability. We used a model selection procedure to choose the best combination of individual variables to explain variation in substrate particle movement and included those variables in a multivariate axis of disturbance that can be applied to evaluate effects of disturbance on benthic organisms. Individual variables with the highest explanatory power were maximum daily increase in discharge and the Pfankuch index of channel stability. Substrate particle size and stream size (drainage basin area) were related to the multivariate index of disturbance, but channel gradient was not. Protocols used to measure substrate stability and to obtain the multivariate index of disturbance were labor intensive, but our analyses indicate it may be reasonable to use more easily measured variables (e.g., Pfankuch index) to estimate disturbance to benthic organisms at local scales, although explanatory power may be reduced. Our analyses provide a menu of options to estimate variation in local disturbance regimes of ungauged mountain streams that may not be adequately explained by extrapolation from hydrographs of gauged streams.

Chironomidae assemblages of temporary streams of the Mediterranean island of Majorca (Spain)

The effect of flow permanence and land use as well as the biogeography of the chironomid (Diptera) communities inhabiting temporary streams was studied in the Mediterranean island of Majorca. A total of 52 species were identified from exuviae and pupae, comprising the subfamilies Orthocladiinae (58%), Chironominae (29%) and Tanypodinae (13%). Forty-six species of chironomids were first records for the island of Majorca, seven of which had not been recorded in the Iberian Peninsula before. Most of the genera found have a Holarctic distribution and contrary to what was expected, typical Mediterranean assemblages were not recorded, nor did the chironomid communities inhabiting these streams reflect water temporality. The presence of common species characteristic of permanent running waters where more forms typical of temporary waters were expected, reflect that the streams are only dry for short periods, thus, many species may survive these episodes, either finding refuge as larvae in the damp substrata or by colonising the springs.

Characterization of an Atlantic salmon Salmo salar stream at the southern limit of its eastern Atlantic distribution

Ten reaches of an Atlantic stream located in north-west Spain were sampled intensively during one summer to characterize the conditions where Atlantic salmon Salmo salar have been re-introduced along the stream. Fish species richness and diversity showed a downstream increase, which was mainly attributed to the higher number of cyprinid species found in the lower reaches. Moreover, except for brown trout Salmo trutta that appeared to be the most ubiquitous species, the densities of the other species was higher in the lower than in the upper stream reaches. Redundancy analysis showed that the pattern of fish assemblages observed along the studied stream was mainly related to the expected gradient observed in the levels of dissolved oxygen, discharge and mean current speed. There was a significant differentiation between midstream and downstream reaches, both in terms of the composition of their fish assemblages and the freshwater habitat. This study emphasizes the importance of describing the variations in fish assemblages and habitat characteristics along a river to explore its relation to potential changes in the survival of fish populations. In particular, the development of habitat-fish relationships may be a useful tool for water managers to assess the effects of development and restoration projects on the habitat of S. salar.

Disturbance legacies and nutrient limitation influence interactions between grazers and algae in high elevation streams

Debate about control of interaction strength among species is fueled by variation in environmental contexts affecting food webs. We used extensive surveys and two field experiments to test the individual and interactive influences of variation in the assemblages and associated traits of grazers as shaped by the legacy of disturbance, nutrient limitation and the presence of top predators on the accrual of basal resources. We quantified hydrologic variation and streambed movement to describe the legacy of disturbance and sampled biota of 20 streams over five years in a high-elevation catchment in Colorado, USA. Grazer assemblages switched from caddisfly-dominated to mayfly-dominated as disturbance increased. We manipulated the composition of grazer assemblages and the availability of nutrients (N and P) within flow-through mesocosms assembled adjacent to 10 streams, and also deployed larger in-stream channels manipulating the presence of top predators (brook trout) in five streams varying in disturbance reg...