Rosa Gómez

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.

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.

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.

Nitrogen retention in natural Mediterranean wetland-streams affected by agricultural runoff

Nitrogen retention efficiency in natural Mediterranean wetland-streams affected by agricultural runoff was quantified and the effect of the temporal variability and hydrological/chemical loading was examined from March 2007 to June 2008 in two wetland-streams located in Southeast Spain. Nitrate-N (NO−3 -N), ammonium-N (NH + 4 -N), total nitrogen-N (TN-N), total organic nitrogen-N (TON-N) and chloride (Cl ) concentrations were analyzed to calculate nitrogen retention efficiencies. These wetland-streams consistently reduced water nitrogen concentration throughout the year with higher values for NO −3 -N (72.3%), even though the mean value of inflow NO −3 -N concentrations was above 20 mg l−1. Additionally, they usually acted as sinks for TONN (8.4%), but as sources for NH +4 -N. Over the entire study period, the Taray and Parra wetland-streams were capable of removing on average 1.6 and 0.8 kg NO −3 -N a day −1, respectively. Retention efficiencies were not affected by temperature variation. NO−3 -N retention efficiency followed a seasonal pattern with the highest retention values in summer (June–September). The temporal variability for NO −3 -N retention efficiency was positively and negatively explained by the hydrologic retention and the inflow NO −3 -N concentration ( R2 adj=0.815,p <0.01), respectively. No significant regression model was found for TON-N and NH +4 -N. Finally, the conservation of these Mediterranean wetlandstreams may help to not only improve the surface water quality in agricultural catchments, but to also achieve good ecological status for surface waters, this being the Water Framework Directive’s ultimate purpose. Correspondence to: V. Garćıa-Garćıa (viquigar@um.es)

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.

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.

Diel variations in physical and chemical parameters in a semi-arid stream in Spain (Chicamo Stream)

Rivers and streams fluctuate both spatially and temporally. Diel, seasonal and annual variations are well documented for temperate forest rivers (e.g. CHESTERIKOFF et al. 1991, REBSDORF et al. 1991), for desert streams and pools (e.g. SCHOLNICK 1994, JONES et al. 1995) and prairie streams (e.g. ZALE et al. 1989) but few data exist on diel variations in arid and semi-arid streams in the Mediterranean region (e.g. MARTI et al. 1994, GUASCH et al. 1998). However, these fluctuations are important for the aquatic community (e.g. JACKSON 1988) and may influence their life cycle. This study focused on diel variations in some physical and chemical parameters in a Mediterranean stream, as part of a major project, the principal objective of which was to examine some aspects of the metabolism of the Chicamo Stream, a semi-arid Mediterranean stream located in south-east Spain. The objectives of this study could be summarized as: (1) the analysis of diel variations of seven physico–chemical parameters in six sampling sites of the Chicamo Stream, during winter, spring, summer and autumn, and (2) a comparison of the seasonal and diel fluctuations with the variations obtained during an annual cycle.

The effect of water salinity on wood breakdown in semiarid Mediterranean streams

Saline streams occur naturally and they are distributed worldwide, particularly in arid and semiarid regions, but human activities have also increased their number in many parts of the world. Little attention has been paid to assess increasing salt effects on organic matter decomposition. The objectives of this study were to analyse wood breakdown rates and how salinity affects them in 14 streams that exemplify a natural salinity gradient. We also analysed the effect of this gradient on changes in wood chemical composition, fungal biomass and microbial activity. Our results showed low breakdown rates (0.0010-0.0032 d(-1)), but they fell within the same range as those reported in freshwater streams when a similar woody substrate was used. However, salinity had a negative effect on the breakdown rates and fungal biomass along the salinity gradient, and led to noticeable changes in wood composition. Water salinity did not affect microbial activity estimated using hydrolysis of fluorescein diacetate. Variation in breakdown rates and fungal biomass across streams was mediated mainly by salinity, and later by stream discharge. Despite the role of fungi in stick breakdown, the potential wood abrasion by salts must be analysed in detail to accurately understand the effect of increasing salinity on organic matter breakdown. Finally, our results indicate that increased salinity worldwide by human activities or by the global warming would imply organic matter breakdown and mineralisation slowing down, even in natural saline streams. However, because many variables are implicated, the final effect of climatic change on organic matter decomposition in streams is difficult to predict.