Macroinvertebrate communities of karst springs of two river catchments in the Southern Limestone Alps (the Julian Alps, NW Slovenia)

The macroinvertebrate communities of 16 karst springs in the Southern Limestone Alps (Slovenia, SE Europe), were studied from May to September 1999, together with the major chemical parameters of the water and selected physical characteristics of the springs. The springs are located in an area of 800 km², between 410 and 955 m a.s.l., and drain into two rivers whose catchments are separated by mountain ranges up to 2864 m. The Soča river drains into the Adriatic Sea and the Sava into the Black Sea. Springs showed relatively large fluctuations in discharge and small variations in temperature (normally about 1 °C) during the period of study. Seventy macroinvertebrate taxa were collected. Chironomidae (Diptera), Limnephilidae (Trichoptera), Enchytraeidae (Oligochaeta), Tricladida (Turbellaria) and Belgrandiella kuesteri (Gastropoda) were the most widespread taxa, collected from the majority of sampling sites. The environmental characteristics showed a great heterogeneity, and there was a large variation in the taxonomic composition and relative abundance of taxa between springs and during seasons. The coexistence of taxa with highly divergent ecological requirements is characteristic of the spring communities studied. They range from stygobionts and spring-restricted species to ubiquitous species. The differences observed in spring fauna in the two river catchments were probably dictated by differences in local environmental parameters and not by the geographical location in the different river catchments.     

Macroinvertebrate assemblages in Pennsylvania (U.S.A.) springs

Five distinct macroinvertebrate assemblages were identified using cluster analysis of the rank-order abundances of 13 orders in 15 freshwater springs of central Pennsylvania, U.S.A. A principal components analysis of 20 environmental factors indicated that an insect-dominated assemblage occurred in low pH, softwater sandstone springs and an amphipod-dominated assemblage was associated with medium to hardwater springs with a silt to gravel substratum. Three other assemblages were found in hardwater limestone springs: the peracaridan-hydrobiid assemblage was characterized by dense macrophyte beds and a silt to cobble substratum, the peracaridan-triclad-glossosomatid assemblage by a rubbly, gravel-cobble substratum and relatively high discharge, and the peracaridan-hydrobiid-oligochaete assemblage by a silty substratum and dense mats of green algae.Most of the common macroinvertebrate species were associated with specific physical, chemical, and (or) vegetational factors, but abundance associations between species pairs were generally lacking. We infer that assemblages are primarily determined by the match between the environmental mosaic of individual springs and the ecological requirements of the available species, rather than by strong interspecific interactions.Species-environment associations were assessed in an additional study of five closely neighboring springs in which some environmental variables of the wider study were nearly constant.     

Physical and chemical differences in karst springs of Cantabria, northern Spain: do invertebrate communities correspond?

Benthic macroinvertebrate communities were studied and environmental variables were measured in six rheocrene springs in Cantabria, northern Spain. Principal component analysis revealed two different spring types according to their physical and chemical characteristics. Springs from group A (GA) had higher temperature and conductivity, while springs in group B (GB) had higher values of pH, altitude, mean water velocity, percentage of boulders and coarse particulate organic matter. Total number of invertebrate taxa and individuals were not different between GA and GB springs. However, Shannon diversity index was significantly higher for GB springs. Analysis of similarities (ANOSIM) and non-metric multidimensional scaling (NMDS) analysis indicated that invertebrate assemblages from GA and GB springs were different. The snails Theodoxus fluviatilis and Bythinella sp., and the amphipod Echinogammarus spp. had higher densities in GA springs, whereas ephemeropterans, plecopterans, trichopterans and chironomids were more important in GB springs. Higher water velocities in GB springs interacting with predation by Echinogammarus tarraconensis may be responsible for the observed patterns on invertebrate community structure and composition. The taxonomic resolution limited our ability to detect crenobiontic taxa. Sampling aquatic, semi-aquatic and semi-terrestrial habitats are needed to account for the biodiversity patterns of spring habitats.     

On the influence of water conductivity,pH and climate on bryophyte assemblages in Catalan semi-natural springs

Bryophytes are some of the most sensitive biological indicators of environmental change. Springs have a significant presence of bryophytes and so are ideal habitats for studying their relationship with the environment. We tested whether bryophyte assemblages can be explained with macro-, meso- and micro-ecological variables (i.e. seasonal climate, altitude, water pH and conductivity) sampling bryophytes from 198 semi-natural springs distributed along montane regions in the north-eastern Iberian Peninsula. We tested the influence of environmental variables on bryophyte assemblages in springs using sparse Partial Least Squares. Our results show that variability in bryophyte assemblages is explained by seasonal climate (temperature and precipitation from winter, spring, summer and autumn and temperature and precipitation seasonality), altitude and water conductivity. The results obtained by the present study will be useful for predicting bryophyte diversity in springs using simple and easy to obtain variables such as climate, water pH and conductivity.     

Evolutionary history of Ash Meadows pupfish (genus <Emphasis Type="Italic">Cyprinodon</Emphasis>) populations inferred using microsatellite markers

Pupfish (genus Cyprinodon) persist in a series of isolated warm springs in Death Valley. Here we describe an analysis of microsatellite variation at six loci for nine populations encompassing three distinct taxa. Levels of genetic variation within populations and the pattern of relatedness among populations are best explained by spring elevation. Springs at higher elevations harbored less variation and exhibited greater among population divergence than lower elevation springs. This pattern reflects regional paleohydrological history showing a declining water table over the last 20,000 years. Continuing decline of the water table, a trend accelerated by local ground water mining, portends a future of increasing isolation and declining within-population variation.     

Associations among spring-dependent aquatic assemblages and environmental and land use gradients in a Mojave Desert mountain range

In many arid landscapes, springs provide the only reliable source of water. Accordingly, both native species and human land uses, including diversion of water, livestock grazing, and recreation, tend to concentrate around springs and spring‐fed riparian areas. We examined whether species richness and composition of aquatic macroinvertebrates at 45 springs in the Spring Mountains, an isolated mountain range in the eastern Mojave Desert (Nevada, USA), could be predicted using readily measured environmental gradients and estimates of disturbance intensity. The Spring Mountains is a focus of regional conservation planning, and managers are charged with prioritizing its springs for conservation and rehabilitation. Our results suggested that species richness of aquatic macroinvertebrates in the Spring Mountains system may be greatest at intermediate levels of natural and human disturbance. Discharge and springbrook length appeared to be only weakly correlated with species richness, whereas neither elevation, nor water temperature, nor electrical conductance was significantly associated with species richness. Nestedness analyses demonstrated that species present in relatively depauperate locations tended to be subsets of the species present in locations that were richer in species, but that pattern did not appear to be driven by either disturbance intensity or by the environmental variables we measured. Disturbance intensity was not associated with the extent to which species presences and absences were predictable. Although our results should not be interpreted to mean that major environmental gradients and disturbance intensity have no effect on distributional patterns of aquatic invertebrates in the Spring Mountains, the ability of these variables to serve as predictors of species richness and composition may be relatively low. Springs and other wetlands in arid landscapes are characterized by isolation and unpredictable disturbances, and faunal responses to environmental gradients may tend to be individualistic and taxon‐specific.     

Regional and local drivers of macroinvertebrate assemblages in boreal springs

Aim To identify the most important environmental drivers of benthic macroinvertebrate assemblages in boreal springs at different spatial scales, and to assess how well benthic assemblages correspond to terrestrially derived ecoregions. Location Finland. Methods Benthic invertebrates were sampled from 153 springs across four boreal ecoregions of Finland, and these data were used to analyse patterns in assemblage variation in relation to environmental factors. Species data were classified using hierarchical divisive clustering (twinspan ) and ordinated using non‐metric multidimensional scaling. The prediction success of the species and environmental data into a priori (ecoregions) and a posteriori (twinspan ) groups was compared using discriminant function analysis. Indicator species analysis was used to identify indicator taxa for both a priori and a posteriori assemblage types. Results The main patterns in assemblage clusters were related to large‐scale geographical variation in temperature. A secondary gradient in species data reflected variation in local habitat structure, particularly abundance of minerogenic spring brooks. Water chemistry variables were only weakly related to assemblage variation. Several indicator species representing southern faunistic elements in boreal springs were identified. Discriminant function analysis showed poorer success in classifying sites into ecoregions based on environmental than on species data. Similarly, when classifying springs into the twinspan groups, classification based on species data vastly outperformed that based on environmental data. Main conclusions A latitudinal zonation pattern of spring assemblages driven by regional thermal conditions is documented, closely paralleling corresponding latitudinal patterns in both terrestrial and freshwater assemblages in Fennoscandia. The importance of local‐scale environmental variables increased with decreasing spatial extent. Ecoregions provide an initial stratification scheme for the bioassessment of benthic macroinvertebrates of North European springs. Our results imply that climate warming, landscape disturbance and degradation of spring habitat pose serious threats to spring biodiversity in northern Europe, especially to its already threatened southern faunistic elements.     

Factors governing macrozoobenthic assemblages in perennial springs in north-western Switzerland

Springs are important freshwater habitats that provide refuge for many rare species. In this study, the fauna and abiotic parameters of 20 perennial springs in north-western Switzerland were investigated. Correlation of abiotic and macrozoobenthos data showed that physicochemical parameters had little impact on macrozoobenthic composition, whereas specific substrate parameters strongly influenced the composition of the macrofauna. Surprisingly, nonmetric multidimensional scaling did not reveal a grouping of springs with similar substrate composition or macrozoobenthic assemblages. However, discharge was identified as the factor significantly determining substrate and the composition of macroinvertebrate assemblages. This justifies the hypothesis that, variation in discharge is the disturbance factor governing the macrofaunal composition temporally and spatially within and between patches. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

External morphology of the eggs of <Emphasis Type="Italic">Asplanchnopus multiceps</Emphasis> (Schrank, 1793) (Rotifera): solving the 150-year-old case of mistaken identity

Degradation of groundwater-dependent ecosystems has raised a need for their restoration, but ecological responses to restoration are largely unknown. We evaluated the effectiveness of spring restoration using data from near-natural, restored, and human-impacted springs, the major impact being degradation of spring hydrology by forest drainage. We used both taxonomic (bryophytes, macroinvertebrates, and leaf-decomposing fungi) and functional (leaf breakdown) measures of restoration success. We expected that by reducing surface water input, restoration will improve spring hydrology and place spring ecosystems in a trajectory towards more natural conditions. Restored springs were thermally more stable than impacted springs and the contribution of surface water was greatly reduced. Bryophytes were more abundant in restored than in impacted springs but did not differ among restored and natural springs. Similarly, macroinvertebrate communities differed between restored and impacted springs whereas no difference was detected between restored and natural sites. Species diversity and functional attributes showed weaker responses to restoration. Our results suggest that restoration enhances spring habitat quality, and the first signs of biodiversity enhancement were also detectable only a few years post-restoration. Restoration clearly bears great promise as a conservation tool for the protection of this valuable component of regional freshwater biodiversity.     

Environmental Constraints Underpin the Distribution and Phylogenetic Diversity of <Emphasis Type="Italic">nifH</Emphasis> in the Yellowstone Geothermal Complex

Biological nitrogen fixation is a keystone process in many ecosystems, providing bioavailable forms of fixed nitrogen for members of the community. In the present study, degenerate primers targeting the nitrogenase iron protein-encoding gene (nifH) were designed and employed to investigate the physical and chemical parameters that underpin the distribution and diversity of nifH as a proxy for nitrogen-fixing organisms in the geothermal springs of Yellowstone National Park (YNP), Wyoming. nifH was detected in 57 of the 64 YNP springs examined, which varied in pH from 1.90 to 9.78 and temperature from 16°C to 89°C. This suggested that the distribution of nifH in YNP is widespread and is not constrained by pH and temperature alone. Phylogenetic and statistical analysis of nifH recovered from 13 different geothermal spring environments indicated that the phylogeny exhibits evidence for both geographical and ecological structure. Model selection indicated that the phylogenetic relatedness of nifH assemblages could be best explained by the geographic distance between sampling sites. This suggests that nifH assemblages are dispersal limited with respect to the fragmented nature of the YNP geothermal spring environment. The second highest ranking explanatory variable for predicting the phylogenetic relatedness of nifH assemblages was spring water conductivity (a proxy for salinity), suggesting that salinity may constrain the distribution of nifH lineages in geographically isolated YNP spring ecosystems. In summary, these results indicate a widespread distribution of nifH in YNP springs, and suggest a role for geographical and ecological factors in constraining the distribution of nifH lineages in the YNP geothermal complex.     

Community concordance between bryophyte and insect assemblages in boreal springs: a broad-scale study in isolated habitats

1. We examined species–environment relationships and community concordance between aquatic bryophytes and insects in boreal springs. We sampled bryophytes (Marchantiophyta and Bryophyta), benthic macroinvertebrates and environmental variables in 138 springs in Finland, spanning a latitudinal gradient of 1000 km. Macroinvertebrates were subdivided into two groups: Ephemeroptera, Plecoptera, Trichoptera and Coleoptera (EPTC taxa) and chironomid midges (Diptera; Chironomidae). Our aim was to test whether EPTC taxa could be used as surrogates in biodiversity surveys and bioassessment for the two less-well known organism groups, chironomids and bryophytes.
2. Bryophyte assemblages were clearly differentiated along gradients in thermal conditions and water chemistry (pH, conductivity). Chironomids and EPTC were also differentiated in relation to thermal conditions and, to a lesser extent, physical habitat variables, but were only weakly associated to spring water chemistry. Chironomid and EPTC assemblages were more concordant with each other than with bryophytes, but all concordances were relatively weak.
3. Our results suggest that even if the overall compositional patterns of the three taxonomic groups were significantly concordant, the relative importance of environmental drivers underlying their community compositions differed strongly. The results thus imply that spring bryophytes and insects are relatively poor surrogates for each other. The proportion of spring specialists was highest in bryophytes, promoting their primacy for spring bioassessment and biodiversity conservation. We suggest that adequate variation in water chemistry be assured to protect spring bryophyte biodiversity, whereas preserving the physical variation of springs is more important for macroinvertebrates.     

Does predator abundance influence species diversity of equilibrium macroinvertebrate assemblages in spring fens?

1. Predation may significantly control number and density of coexisting species. The effects of predation on species diversity have traditionally been tested in experiments and theoretical models of simple trophic systems. In complex natural ecosystems, however, disentangling multiple sources of variation is difficult. In groundwater-fed environments, a significant effect of predation can be expected due to the relatively stable environmental conditions; however, it has never been properly examined.
2. We analysed species diversity and total abundance of macroinvertebrate assemblages in 48 Western Carpathian spring fens, separately for whole sites and mesohabitat/season, and partitioned the effects of predation intensity from those of environmental variables in robust models using a bootstrapping technique. We verified our results by accounting for taxa resistant to predation.
3. The assumption that predation-mediated coexistence of species is the main mechanism responsible for the relatively species-rich assemblages in the Western Carpathian spring fens was not supported. However, predation may significantly influence abundance of non-predatory species and, under some conditions, it may contribute to explaining patterns in species diversity.
4. The effect of predation did not differ between the mesohabitats with different stability. However, we found higher environmental control in spring and a stronger effect of predators in autumn, which suggests that different mechanisms influence fen assemblages in different seasons.
5. Our study provides a new robust approach how to test the effect of predation on natural macroinvertebrate assemblages. The importance of predation was lower than expected in equilibrium assemblages but it may vary in time.

     

The conservation genetics of Ash Meadows pupfish populations. I. The Warm Springs pupfish <Emphasis Type="Italic">Cyprinodon nevadensis pectoralis</Emphasis>

The Warm Springs pupfish (Cyprinodon nevadensis pectoralis) inhabits several low flow (<0.15 cfs) springs in an otherwise dry Mojave Desert landscape. Increasing demands on groundwater coupled with predictions of decreased precipitation from global warming make this species a sentinel for biodiversity dependent on springs. Here we examine mitochondrial DNA haplotypes and nuclear microsatellite genotypes for individuals sampled in 1998 and 2007 as a means of inferring the historical demography of the subspecies. Estimates of genetic effective population size from comparison of allele frequencies over time underscores that all spring populations support small populations, typically <100 individuals. Such small population sizes suggest spring populations may have heightened probabilities of extirpation. Despite small population sizes, all springs harbor relatively high levels of genetic variation for both nuclear and mitochondrial DNA. Unexpectedly high levels of variation may be explained by recent declines in population size coupled with rare episodes of gene flow between springs suspected of occurring during flooding events. The inferences gained from the genetic data provide the basis for evaluating current and future restoration plans. The genetic data suggest there is a need to balance the predicted positive effects of restoring hydrological connectivity with the potential negative impacts of providing avenues for the spread of exotic species detrimental to pupfish populations.     

Small‐Scale Distribution of Aquatic Macroinvertebrates in Two Spring Fens with Different Groundwater Chemistry

We examined responses of macroinvertebrate assemblages to environmental and temporal variations along spring source‐spring brook transects in two fen habitats, sharply differing in groundwater chemistry, and compared the patterns among individual taxonomical groups. We hypothesised a different importance of environmental heterogeneity and seasonal changes primarily linked to strong tufa precipitation, which causes stronger environmental filtering in the calcareous fen. In concordance, we observed that assemblages of the more homogenous calcareous fen primarily changed over time, due to seasonal shifts in source availability and favourable conditions. Their spatial distribution was determined by the amount of CPOM, tufa crusts and temperature variation, but a substantial part of the assemblage exhibited spatial uniformity (Plecoptera, Clitellata, and especially Trichoptera and Diptera). The assemblages of the more heterogeneous Sphagnum ‐fen were primarily driven by water pH and substrate and the season was a notably weaker predictor. We found that different macroinvertebrate groups can display various responses to the measured variables shaping the overall pattern obtained based on the whole community. Further, greater environmental heterogeneity can result in temporally stable species distribution patterns even at very small spatial scales within a single site. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Can environmental tolerances explain convergent patterns of distribution in endemic spring snails from opposite sides of the Australian arid zone?

Patterns of distribution are influenced by species environmental requirements and limits, but experimental tests are needed to discern whether correlates of abundance directly affect survival and success. Springs in Australia’s arid interior support a wide diversity of gastropods only found in springs, and these species show dichotomous patterns of distribution. “Amphibious” species are broadly distributed across many springs and microhabitats, and “aquatic” species confined to the deepest pool areas within large springs. This pattern appears to be driven by the interaction between different environmental conditions in different microhabitats and the environmental tolerances of each endemic snail species. Factorial experiments were used to test whether conditions in the environmentally extreme and variable tail area of springs (considering pH, conductivity, temperature and desiccation potential, alone and in synergistic scenarios) elicited lethal or sub-lethal responses in spring snails endemic to springs on opposite sides of the Australian arid zone. All species restricted to spring pools were able to endure 24 h exposed to the average tail conditions, alone and in combination, but most suffered mortalities when subjected to extremes, and mortalities occurred sooner in the most restricted species when elevated pH and conductivity were experienced in combination. Responses of species from different locations are similar, but pattern of distribution in the field were not correlated with tolerance of environmental extremes—with the “amphibious” species from the sub-tropics being far more sensitive than its arid counterpart. These findings suggest that environmental variance within springs can influence patterns of distribution and abundance, particularly when extremes are experienced simultaneously over sustained time periods. But despite similarities in responses across species from these two spring complexes, no simple generalisations linking distribution and tolerance were discernible.     

Influence of water chemistry and environmental degradation on macroinvertebrate assemblages in a river basin in south-east Brazil

Benthic macroinvertebrate assemblages, water chemistry variables and environmental degradation were investigated in an Atlantic Forest region in Brazil. Seven sites of the Guapimirim river basin were studied during three sampling periods based on the rain regime: end of wet season (May 1998), dry season (August 1998), and wet season (January 1999). Four substrates were collected at each site: sand, stony substrates, litter in pool areas and litter in riffle areas. Relationships between macroinvertebrate assemblages, water chemistry variables and environmental degradation were examined using canonical correspondence analysis (CCA). According to CCA, concentrations of dissolved oxygen and chloride, and the environmental degradation, measured by the Riparian Channel Environment index, exhibited the strongest relationship to macroinvertebrate assemblages. Overall, the loss of community diversity measured by the Shannon Index along the degradation gradient was observed. Some taxa were shown to be sensitive to water pollution, especially among Plecoptera, Trichoptera, Coleoptera and some Ephemeroptera, while others such as Simuliidae, Odonata and molluscs were tolerant to moderate levels of pollutants. The Chironomidae were the only group tolerant to a high level of pollutants and degradation.     

Some factors affecting distribution of diatom assemblages in Pyrenean springs

SUMMARY.

• 1 The composition and abundance of diatom assemblages were studied together with physiographical and physicochemical variables in twenty-eight springs in the Central Pyrenees during the period 1987–88.
• 2 Characteristic groupings of diatom taxa are revealed by Principal Components Analysis. One group, made up of assemblages of the taxa Navicula cincta, Caloncis spp. and Niizschia elliptica, is characteristic of springs with waters of high ionic strength. Certain diatom taxa are characteristic of hard water springs, either in quiet waters (Denticula tenuis, Achnanthes minutissima, Eunotia spp.) or fast flowing ones (Fragilaria spp., Diatoma hiemale). Other taxa are representative of relatively quiet, softwater springs: Anomoeoneis bracfiysira, Tabellaria flocculosa, Aulacoseira distans and Fragilaria vaucheriae.
• 3 Five different spring types were identified, with respect to diatom assemblages, using discriminant analysis. The first consisted of high mountain springs with cold, fast waters and low ionic strength. The second group, distributed throughout the Pyrenees, had similar environmental characteristics to the first group but with intermediate water velocity and slightly higher mineral content. The third contained springs in calcareous regions of the Pyrenees, mainly slow-flowing and with hard waters. The fourth was composed of softwater springs, whilst the fifth consisted of two springs which were subject to stressful conditions (limitation of light, abundance of nitrates or high salinity).
• 4 Ionic strength and current velocity appeared to be the dominant environmental factors affecting diatom distribution in Pyrenean springs. These factors also seem to determine diatom communities in some other geographical areas. In some springs, however, peculiar environmental factors (dim light, salinity, and high temperature) affect the structure of diatom assemblages.

     

Different roles of environmental variables and spatial factors in structuring stream benthic diatom and macroinvertebrate in Yangtze River Delta,China

Exploring the relative contribution of spatial factors and environmental variables in shaping communities is of widespread interest in biodiversity conservation and environmental management. Stream communities are hierarchically regulated by environmental variables over multiple spatial scales, and the reaction of different organisms to stressors are still equivocal. We sampled both macroinvertebrates and diatom at 80 sites and additional 10 sites for macroinvertebrates, field measured and laboratory analyzed environmental variables, from the tributaries of Qiantang River, Yangtze River Delta China in 2011. We used PCNM (principal coordinates of neighbor matrices) to generate spatial predictors. We applied redundancy analysis and variation partitioning procedures to identify key spatial and environmental factors, and to quantify their relative roles in shaping diatom and macroinvertebrate assemblages. Our results demonstrated the role of spatial and environmental variables differed in shaping benthic diatom and macroinvertebrate. Diatom assemblage variations were better explained by spatial factors, however macroinvertebrate assemblage variations were better explained by environmental variables. In terms of environmental variables, catchment scale variables (e.g., land use estimators, land use diversity) played the primary role in determining the patterns of both diatom and macroinvertebrate assemblages, whereas the influence of reach-scale variables (e.g., pH, substrates, and nutrients) appeared less. However, nutrients were the stronger factors influencing benthic diatom, whereas physical habitat (e.g., substrates) played more important role than water chemistry in structuring macroinvertebrates. Our results provided more evidence to the incorporation of spatial factors interpreting spatial patterns of stream organisms, and highlighted the useful of multiple organisms and environmental variables at different spatial scales in diagnosing mechanism of stream degradation and in building a sound stream conditions monitoring program for Yangtze River Delta.     

Substrate specificity,environmental degradation and disturbance structuring macroinvertebrate assemblages in neotropical streams

Structure and composition of benthic macroinvertebrate assemblages were investigated in seven sampling sites with a gradient of environmental integrity and water quality conditions. Composite samples of the four most representative substrates were collected in order to characterize the riffle-pool dynamic in each sampling site. Spatial and temporal variability of macroinvertebrate assemblages were analyzed at two scales: using substrates and grouping samples for comparing sampling sites. Distribution of macroinvertebrates was influenced primarily by substrate type, but also by environmental integrity, water quality and sampling period. Species occurrence was highly dependent on substrate type. At local spatial scale, environmental degradation measured by the Riparian Channel Environmental Inventory and water chemistry were the determinants of assemblage patterns. We evaluated to which extent the substrates were influenced by environmental integrity and water chemistry, and we found that degradation influenced significantly the macroinvertebrate fauna on the four substrate types, although they were not responding to the same variables. Our results show that qualitatively communities were not influenced by seasonal changes, but abundance was stochastically dependent on rainfall.     

Analysis of the reproductive strategy of <Emphasis Type="Italic">Jenynsia multidentata</Emphasis> (Cyprinodontiformes,Anablepidae) with focus on sexual differences in growth,size, and abundance

The habitat heterogeneity generated and sustained by the connectivity of floodplain habitats, the seasonal flood pulse, and the variability of the physical structures typically found in floodplains of large rivers results in a variable space–time mosaic of water sources that results in a high biodiversity of the river-floodplain system. In order to assess the implications of natural connectivity and the heterogeneity on the patterns of macroinvertebrate assemblages at different spatial scales, monthly samplings in six different mesohabitats (lakes with different hydrological connection and secondary channels with permanently and intermittent flow) of the Paraná River floodplain were performed from April 2005 to March 2006. The mesohabitats had different granulometry and detritus composition of their bottom sediments. They also had different conductivity, transparency, and depth in relation to the different connectivity degrees. Mesohabitats differed in the abundance of macroinvertebrates of different taxonomic groups and diversity. The environmental variables were correlated to the patterns of macroinvertebrate abundance, with dominance of different species of annelids and mollusks at the patch, mesohabitats, and island scales. An alpha diversity gradient from the isolated lake (65 taxonomic units) to the secondary channels (25 taxonomic units) was obtained. The analyzed mesohabitats showed a high taxa turnover, with high values not only among the mesohabitats located in the different islands, but also among the mesohabitats in relation to different connectivity degrees. The mesohabitats showed negative co-occurrence of macroinvertebrate assemblages. The spatial heterogeneity, sustained by the connectivity degree, played a key role in structuring benthic assemblages at different scales, positively influencing the regional diversity.