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Fish as bioindicators are a widely used tool for environmental monitoring of freshwater ecosystems worldwide. However, its adaptation to temperate Neotropical streams constitutes a challenge due to the presence of pauperised ichthyofauna with high tolerance to environmental degradation. We designed, evaluated, and validated an Index of Biotic Integrity (IBI) that uses fish as bioindicators of the ecological integrity of streams of the Pampa Plain, Argentina. An integral characterization of the environmental conditions (water quality, habitat structure, and riparian condition) and fish assemblages were performed in 17 study sites (natural grasslands, urban, and livestock reaches). The IBI was designed with 10 metrics and showed to be sensitive to different environmental conditions due to significant differences between study sites and land uses. The IBI also showed to be responsive due to its significant associations with water quality, habitat structure variables, and some multimetric indexes. Moreover, when validated against a recently fish database, it demonstrated to be sensitive for use in other temperate streams exposed to similar environmental conditions. An integral environmental assessment as the one designed can lead to optimized strategies for the management and conservation of these fragile ecosystems. [ABSTRACT FROM AUTHOR]

In the context of a main project that aims to recover modern data on diatom distribution applicable to paleosalinity reconstructions in coastal areas of Southern South America, the composition and distribution of dead diatom assemblages in the littoral zone of the Quequén Salado estuary (Argentina) were studied. Diatom zones were defined along the estuarine gradient by cluster analysis and related to the salinity range and sediment composition by Canonical Correspondence Analysis. Four diatom zones were identified. A mixture of marine, brackish and freshwater diatoms, probably allochthonous, characterized the inlet (zone I). Marine/brackish taxa, represented mainly by Paralia sulcata dominated zone II, characterized by polyhaline conditions and sandy sediments. Zone III was characterized by mesohaline conditions, muddy sediments and the dominance of the estuarine diatom Amphora helenensis. Brackish/freshwater and freshwater diatoms dominated the headwaters (zone IV), where salinity was always below 5‰. The comparison of Quequén Salado diatom assemblages with previous results from the Quequén Grande estuary showed a similar taxonomic composition between both estuaries. However, differences in the salinity ranges of the estuaries (related to differences in the degree of human impact and tidal range) lead to a displacement in their spatial distribution along the longitudinal estuarine axis. This paper contributes to the knowledge of the ecological requirements of South American estuarine diatoms and provides useful data for paleosalinity reconstructions in the region. [ABSTRACT FROM AUTHOR]

The purpose of this paper was to assess the influence of basic limnological variables on bacterioplankton abundance in twenty lagoons in the Upper Paraná River floodplain, Brazil. Twelve abiotic (depth, total nitrogen, total phosphorus, dissolved organic carbon (DOC), water transparency, temperature, dissolved oxygen, alkalinity, pH, turbidity, electrical conductivity and total inorganic carbon), and two biotic (chlorophyll-a and rotifers abundance) limnological factors were used as independent variables to explain bacterial countings. Principal component analysis (PCA) was used to reduce the dimensionality of the abiotic data and chlorophyll-a. Subsequently, multiple regression analysis was used to examine the relationship between bacterial abundance and the principal component axes and rotifer densities. Bacterioplankton abundance fluctuated from 1.8 to 5.9 × 106 cells.ml-1 during the high water phase and from 1.4 to 4.0 × 106 cells.ml-1 during the low water phase and they were significantly higher during the former phase (t=3.06; p=0.007). Bacterial abundance was not significantly correlated with chlorophyll-a and rotifers abundance. On the other hand, it was positively correlated with principal components 1 and 2, indicating that DOC, total nitrogen and phosphorus concentration, temperature and pH are important to explain bacteria abundance. These results indicate that the input of different sources of DOC and inorganic nutrients from the floodplain into the lagoons during the high water phase is more important to explain bacterial abundance than autochthonous phytoplanktonic organic matter or predation by rotifers. [ABSTRACT FROM AUTHOR]

An introduction is presented which discusses various topics within the issue including shallow lakes of the Central Plains of Argentina, reviews of Pampean shallow lakes and effect of the light limitation on the phytoplankton assemblages in a turbid shallow lake.

In this study, we present a new method to compare the distribution of knowledge regarding the biodiversity of a taxa with the distribution of environments inhabited by that taxa, in order to identify the main geographical gaps in biodiversity knowledge. We compared the geographical distribution of studies describing freshwater fish assemblages in Argentina with the distribution of freshwater bodies, delimiting priority areas that require additional sampling effort in order to achieve a more balanced understanding of this taxa's diversity throughout the country. Our results show a substantial geographical bias in the distribution of knowledge regarding freshwater fish assemblages in Argentina, with most of the studies concentrating on a few areas of the country. Many sectors appear as undersampled, and most of the country has been studied only recently, after many sites have undergone severe impact by anthropic activities. The absence of fish assemblages descriptions previous to habitat changes in these sites considerably limits our capacity to understand changes in biodiversity that anthropic habitat modifications might have generated or even the ones they are generating now. We show the areas of Argentina where freshwater fish diversity needs to be studied most urgently based on the distribution of freshwater habitats. [ABSTRACT FROM AUTHOR]

In this study, we tested the significance of environmental influences on non-marine ostracod communities at high taxonomic levels across Patagonia, Argentina. We used a dataset compiled from 243 Patagonian freshwater bodies, representing an area of ~ 900,000 km2 and covering the main local and climatic gradients. We applied generalized linear models to test the effects of hydroperiod, local and climatic variables on the occurrence and abundance of Ostracoda, and the occurrence of the most frequent families (i.e., Cyprididae, Ilyocyprididae and Limnocytheridae). We found ostracods in 77% of the total sampled sites, driven mainly by dissolved oxygen levels and water temperature (local variables) as well as precipitation and air temperature (climatic variables). Specifically, warmer winters and dryer climates favored ostracod occurrences, which covered wide ecological ranges, such as peat bogs or high-altitude wetlands. For the Ilyocyprididae and Limnocytheridae families, precipitation and pH were important predictors of their occurrence. Furthermore, wind speed, nutrient levels and water depth had positive effects on ostracod abundance. This updated Patagonian freshwater dataset contributes towards filling those knowledge gaps related to the habitat suitability of ostracods communities in this vast region. Our findings could help us understand the past and future responses of ostracods to environmental changes. [ABSTRACT FROM AUTHOR]

Pomacea canaliculata is a freshwater snail native to southern South America. The aims of this work are to update its distribution in Argentina and to analyze through niche models whether the environmental conditions from its original distribution can anticipate its recently expanded range. Almost all records of P. canaliculata before 1958 (original records) belong to del Plata or connected basins. A quarter of the new present records are located in basins not connected to del Plata, indicating a recent expansion of the distribution range of P. canaliculata in Argentina. Recently colonized areas are mostly environmentally suitable according to the projection of the original distribution model, thus natural barriers were probably the main limits to its distribution in the past. According to the model, many regions outside its original range, including several not yet colonized, are suitable for the establishment of P. canaliculata. Consequently, it is likely that this species will continue establishing new populations in Argentina, especially if fishermen and aquarists continue to move snails to new locations. Our study revealed that an extensive but overlooked invasion is in process in its native range, where the impacts on diversity and ecosystems functioning may differ from those already described elsewhere. [ABSTRACT FROM AUTHOR]

Lontra longicaudis is a top predator of freshwater systems currently categorized as 'data deficient' by the IUCN. Thus far, no genetic studies have been carried out in Argentina. Using 8 microsatellite loci, we analysed the genetic diversity and individual relatedness in a population located at the Lower Delta of Paraná River, Argentina. Our data, together with available genetic information for other populations and habitat suitability data obtained from species distribution models previously published, were used to test whether genetic diversity, inbreeding, and parentage coefficients are related to habitat suitability both at landscape and at regional scale. Our results show that the levels of heterozygosity found in the Lower Delta are lowest reported to date. A low overall degree of relatedness between otter dyads was found. However, almost 20% showed relatedness values higher than 0.25, suggesting local reproduction. Finally, we found a positive association between habitat quality and levels of heterozygosity, both at landscape and regional scale, suggesting that in highly suitable habitats, the otters are genetically more variable. Our study constitutes the first one on the genetics of L. longicaudis in Argentina and provides baseline information for the development of conservation plans for the Lower Paraná River Delta population. [ABSTRACT FROM AUTHOR]

The Pampean region covers a large surface in central Argentina, but despite the extensive agricultural activities and the high nutrient levels recorded in streams of the region, few authors have analysed the influence of land use on water quality. Here, we evaluated the relationships among catchment attributes (size, morphometry and land cover) and water chemistry in 23 Pampean streams in different seasons (autumn, spring and summer) and at three spatial scales: whole catchment and two scales of riparian buffers (200 and 500 m adjacent to both stream margins). Chloride concentration was positively related to catchment area and negatively related to drainage density. Nitrate level was strongly associated to cropland, but soluble phosphorus concentration showed no relationships with any type of land cover. Land cover at the buffer scale seemed to be more influential than land cover at the whole catchment for nitrogen concentration. The main impact of cropland was the increase of nitrate concentration, while cattle breeding was negatively associated to photosynthetically active radiation (PAR) in autumn and summer and to dissolved oxygen concentration in spring. Our results highlighted the importance of local land use and riparian conservation on streamwater quality. [ABSTRACT FROM AUTHOR]

Predatory control of community structure, through consumption of herbivores and subsequent positive effects on the biomass or productivity of primary producers is common in marine ecosystems. We designed an experiment to analyse the effect of large-sized predators on the development of the macrofouling assemblage of a harbour. Predation was manipulated by randomly assigning artificial substrata to three treatments: (1) exclosure cages preventing access of large predators, (2) cage controls, i.e. cages with lateral walls but without roofs, allowing predators free access, (3) uncovered panels. Substrata were submersed horizontally and upper surfaces sampled at quarterly intervals over a 1-year period. No differences in species richness were observed among treatments but diversity was lower within exclosure cages than on cage controls/uncovered panels. In sessile assemblages, structure differed between treatments: cage controls/uncovered panels were dominated by algae and associated mesoherbivores, which were scarce/absent within cages. Exposure to predators also affected the structure of mobile assemblages. The sea urchin Pseudechinus magellanicus (Philippi) occurred within exclosure cages but was never found on exposed panels. We conclude that large predators exert a significant effect on assemblage structure and diversity, indirectly promoting the development of filamentous algae and mesoherbivores by controlling the abundance of browsers. [ABSTRACT FROM AUTHOR]

The Central Plains of Argentina is a heterogeneous environment, but the lakes there share some fundamental features: they are all shallow and polymictic as being well exposed to wind. First, we provide a synthesis of the climate, geology, and hydrological network. We also discussed shallow lakes origin and their limnological and biological salient features. Second, we focus on Pampean shallow lakes from a global perspective, comparing the limnological variables: total phosphorus concentration (TP), total nitrogen concentration (TN), chlorophyll a (Chl a) concentrations, and Secchi disk reading (SD) from a compiled database. No significant differences in the Chl a vs. TP relationship were found between Pampean and other shallow lakes. Otherwise, the chlorophyll yield per unit of phosphorus of Pampean lakes is similar to the world shallow lakes average. Moreover, the relationship SD vs. Chl a differed significantly between Pampean and the remaining world lakes, about 50-60%. When confronted against other lakes worldwide, Pampean shallow lakes depart from most of them as having higher TP, TN, and Chl a concentrations and much lower SD transparency, and therefore they stand as extremes of the trophic-state continuum. Despite their highly turbid state, these lakes provide valuable ecosystem services that are highly appreciated and mobilize important economic resources. [ABSTRACT FROM AUTHOR]

Pampean lakes are characterised by the alternation of flood and drought periods, but little is known about its effects on fish assemblage in an extended temporal scale. This study analyses the temporal variability of the fish assemblage in Chascomús Lake, and discusses the role of temperature and precipitation as potential drivers of fish composition shifts. Data acquisition was based on experimental fishing performed from 1999 to 2013 and from historical fishing records. Two alternative fish assemblage configurations were identified by cluster analysis. Odontesthes bonariensis, Parapimelodus valenciennis and Cyphocharax voga were the dominant species, which accounted for 70-80% of the relative abundance. The species O. bonariensis showed temporal fluctuations in its representativeness, changing from dominant to almost absent, whereas C. voga and P. valenciennis changed their abundance following a similar pattern along time. When historical data were considered, Platanichthys platana appeared as the fourth most relevant species. Precipitation, critical temperatures and fish mortalities were identified as the main drivers of species abundance shifts. This study highlights the importance of long-term assessments to understand the influence of climatic factors and the need to maintain or restore natural ecological processes as the basis to support dynamic sustainable fisheries in Pampean shallow lakes. [ABSTRACT FROM AUTHOR]

We investigated nine Pampean shallow lakes (Argentina) across a moisture gradient (mean annual precipitation 1,000-500 mm), under the framework of a network project of lake monitoring whose main objective was to analyse the responses of ecosystems to different stressors associated with human activities. We postulate that species bloom in different shallow lakes will be determined mainly by local optical features, nutrients and salinity. We analysed the phytoplankton structure during the warm season 2012-2013, identifying those species and functional groups that proliferate in the lakes. Most lakes showed high phytoplankton biomass, except a saline lake located in the driest part of the gradient and an inorganic turbid lake. In some lakes, cyanobacteria blooms occurred in summer, persisting until autumn. Small colonial cyanobacteria and small fast-growing chlorophytes were abundant in most turbid enriched and anthropogenically impacted systems. Lowest species and functional diversity occurred in the saline lake. Multivariate analysis indicated that the phytoplankton structure was mainly defined by conductivity, transparency and variables associated with the trophic state, also reflecting the importance of the geographical location. Our results evidenced the vulnerability of most shallow lakes in the region to water level fluctuations, eutrophication and canalization. [ABSTRACT FROM AUTHOR]

We performed a 30-month field study in a highly turbid, hypertrophic shallow lake from the Pampa Plain (Argentina) in order to investigate the occurrence of steady-state phases of phytoplankton, with an emphasis on the effect of subtle, but measurable, increase in light availability. The results revealed that a steady-state condition prevailed during the first 14 months, during which Aphanocapsa cf. delicatissima represented, on average, 91% of the total phytoplankton biomass. The statistical analyses provide support for the hypothesis that low light availability in the water column regulates the structure of the algal assemblage in this lake. Our results agree with predictions of the light-limited growth theory (i.e. the observed increase in light supply promoted an increase in algal density) and are also consistent with the notion that environmental constraints influence the dominant morphological traits in phytoplankton assemblages, particularly, that small organisms and/or attenuated forms are favoured under very poor light conditions. Nevertheless, further work is needed to get a full understanding of the processes controlling phytoplankton dynamics in this lake, including careful studies of the ecophysiology of the colonies comprising pico-sized cells of Aphanocapsa cf. delicatissima that dominated the assemblage during all the studied period. [ABSTRACT FROM AUTHOR]

We analysed the changes in phytoplankton and in the main limnological features in a shallow lake during its transition from a clear-vegetated regime to a turbid one from 2005 to 2013. As samplings were discontinuous, data were analysed considering three different sampling periods. At the beginning of the first period, the lake was in a clear-vegetated regime, showing low values of chlorophyll a, Kd, total suspended solids and nutrients, and high Secchi depth. Phytoplankton was dominated by nano-phytoplanktonic species. During the second period, some evidences of the shift to a turbid regime were observed (mainly in Kd and total suspended solids). Towards the end of our study, submerged macrophytes sharply declined; in this period Kd and total suspended solids noticeably increased, whereas a significant reduction in Secchi depth occurred. Concomitantly, phytoplankton abundance augmented in two orders of magnitude, changing to a community with a higher proportion of micro-phytoplankton. Although the causes of the regimen shift could not be unequivocally assessed, the drastic reduction in the hydrometric level of the lake probably provoked a declination in macrophytes, with the consequent increase of nutrients in the water column and the increment in phytoplankton densities, carrying the system towards a turbid regime. [ABSTRACT FROM AUTHOR]

Studies based on continuous monitoring of diel changes in dissolved oxygen concentration allow the estimation of ecosystem metabolism and provide a measure of the overall trophic processes of an ecosystem. In this study, net ecosystem production (NEP), community/ecosystem respiration ( R), and gross primary production (GPP) rates were estimated in relation to physicochemical and climatic variables for 18 months in La Salada, a saline shallow lake. Net autotrophic conditions prevailed during the study period (NEP: 64.05 ± 44.22 mmol O m day). GPP and R were positively correlated and were synchronized on a daily timescale, with GPP typically greater than R. Principal component analysis revealed that monthly rates of GPP, R, and NEP responded, as expected, to temperature and light seasonal patterns. Water level and conductivity fluctuations, because of evapoconcentration and water management, were relevant as a driver of the physicochemical and biological characteristics of the lake. In saline lakes as La Salada, an adequate management of water resources will be relevant to maintain the ecosystem equilibrium and the quality of its resources. [ABSTRACT FROM AUTHOR]

Species introductions force sympatry between species that did not coevolve. Introduced salmonids have coexisted with native fish since the early 20th century in Patagonian water bodies, thus generating questions about the mechanisms that facilitate their coexistence. We analyzed the trophic and spatial intra- and inter-specific relationships established among native and salmonid species in a deep oligotrophic lake of Patagonia in order to determine niche partitioning patterns as strategies for their coexistence. Salmonids were more generalist feeders, while native species had narrower trophic niches. Native fish and introduced salmonids partitioned food, mainly through the consumption of the crayfish Samastacus sp. and the native galaxiid Galaxias maculatus, respectively. The diet of most species changed with body size, shifting from insects/amphipods to the larger G. maculatus and crayfish. Trophic interactions varied with season, in association with prey seasonality. In general, fishes feeding on the same prey were captured in the same depth strata, indicating common use of food and space. Our results provide new evidence on the trophic ecology of a mixed fish community (exotic-native), supporting the idea that native and non-native fishes could be avoiding negative interactions (e.g., competition) through trophic and spatial resource partitioning. [ABSTRACT FROM AUTHOR]

The spatial distribution of Percophis brasiliensis at different maturity stages and its relationship to environmental factors was evaluated in the El Rincon area (39°-41°S) between 1994 and 2008 during eight cruises carried out in spring. For this, a canonical correspondence analysis was used. Results indicate that bottom temperature and bottom salinity horizontal gradient (BSHG) were the most important variables affecting the spatial distribution of P. brasiliensis maturity stages. Juveniles were mainly located in shallow waters with low salinity, high temperatures and vertical stratification. Conversely, gravid and running individuals were found in high BSHG and low temperatures. This link suggests that adults spawn in a well-defined area of hydrographic conditions that would tend to retain pelagic eggs and larvae, thus minimizing dispersal, and ensuring their transport from the spawning to nursery areas. On the coastal shelf of El Rincon, the water mass circulation shows an anti-cyclonic gyre whose recirculation cell during spring and summer is more constrained to inshore areas than during winter. Maximum reproductive activity of P. brasiliensis coincides with the highest retention period; therefore, dispersal of early-life stages (eggs) depends on the circulation pattern and transport pathway which would contribute importantly to its recruitment variability. [ABSTRACT FROM AUTHOR]

The study tests whether diurnal microhabitat use by Hatcheria macraei depends upon specific environmental parameters and/or the abundance of other fish. We carried out a 1-year field study in a low-order river of northern Patagonia, Pichileufu River, and used experimental trials to determine substrate preferences. Fishes were captured during daylight and physicochemical environmental variables were recorded. Headwater zones were dominated by rainbow trout ( Oncorhynchus mykiss) and brown trout ( Salmo trutta), while native fishes, H. macraei and the creole perch ( Percichthys trucha) were more abundant downstream. H. macraei inhabited mostly shallow microhabitats with fast water velocity and substrates having significant interstitial spaces, independently of the abundance of other fishes. Experimental trials pointed out that H. macraei preferred mostly coarser substrates (>6 cm), avoiding fine ones. This study highlights the importance of erosional zones with high water velocity, large substrates, and suitable interstitial space in the microhabitat selection of H. macraei. The knowledge of microhabitat use by native fish populations is critical for management and conservation strategies and should be taken into account before any river modification. [ABSTRACT FROM AUTHOR]

Global warming affects the hydrological cycle by increasing the frequency and intensity of extreme rainfall events and dry spells. These changes potentially affect the quantity and quality of dissolved organic matter (DOM) input into lakes. In this study, we investigated if changes in precipitation over a 3-year period correspond to changes in DOM and whether these changes affect light attenuation and plankton community composition. We sampled Lake Escondido, a shallow, oligotrophic Andean lake, nine times, analyzing coloured DOM and plankton community composition. During the study period, we observed that variations in the precipitation regime correlated with DOM parameters (water colour and molecular weight), and this, in turn, affected the plankton composition. Chlorophyll a concentrations of both phytoplanktonic fractions (less than and greater than 2 μm) were related to water colour and TDP. We observed in the small fraction (<2 μm) an increase in phycocyanin-rich cells during periods of high water colour. Larger phytoplanktonic cells (>2 μm) presented two biomass peaks corresponding to increases of the cyanophyte Chroococcus planctonicus and of the haptophyte Chrysochromulina parva. As precipitation decreased, the lake became more transparent, favouring C. planctonicus and mixotrophic oligotrich ciliates with endosymbiotic Chlorella. In the context of global climate change, our results highlight the potential impact of changes in precipitation patterns and, consequently, in DOM quality on the plankton community. [ABSTRACT FROM AUTHOR]

Early Mesozoic Argentinian darwinuloids, now re-assigned to the genus Penthesilenula ( incae group), are described and illustrated. These records seem to constitute the oldest known for the genus. The close similarities between the studied species and those described from eastern Asia enlarge the geographical distribution of Penthesilenula ( incae group), which, although restricted to the southern Hemisphere in Recent and Holocene times, may have had a wider distribution in the past. Additional information on the paleoecology, plus a preliminary account on the distribution of darwinulid species from Argentina are also offered. One new species is described: Penthesilenula loana sp. nov. [ABSTRACT FROM AUTHOR]

The objective of this study was to analyze the spatial and temporal dynamics of Chironomidae larvae assemblages in a subtropical mountain river basin of northwestern Argentina. We hypothesized that they would respond to (1) environmental changes along the altitudinal gradient in the Yungas forests; (2) environmental changes caused by spates (e.g., increased discharge or sediment transportation). We sampled five sites along an altitudinal gradient of ca. 1,500 m (from 680 to 2,170 m) during low-water and high-water period for 2 years. The Chironomidae larvae assemblages varied along the altitudinal gradient and between sampling periods based on an indicator species analyses and an ordination analyses (NMDS). The best indicator for high altitude sites was 'Genus X' followed by Onconeura sp.2, 'Genus 9', 'Genus 10', and Rheotanytarsus sp.2; while Rheotanytarsus lamellatus, Oliveiriella sanjavieri, and Thienemanniella sp. were good indicators for low altitude sites. O. sanjavieri and R. lamellatus were the best indicators for high-water period. The most relevant environmental variables influencing their spatial and temporal distribution were altitude, water temperature, conductivity, and pH. This study is the first to analyze and describe in detail the spatial and temporal distribution of Chironomidae larvae assemblages in northwestern Argentina, and most of the taxa reported here are new records for the region. [ABSTRACT FROM AUTHOR]

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell's neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space. [ABSTRACT FROM AUTHOR]

The extreme hydrological fluctuations in the South American Paraná River are unequivocally coupled with El Niño-Southern Oscillation’s (ENSO’s) global climatic phenomenon. In order to identify phytoplankton changes triggered by periods of extraordinary flood (November 1997–1998) and drought (November 1999–2000), the main channel of the Middle stretch was monthly analysed and compared with neutral hydrological conditions (November 1977–1978). These results show that water discharge plays a stirring role on phytoplankton dynamics, and that light availability acts as an important selective force. Phytoplankton transport, density, and biovolume were lower during El Niño and higher during La Niña ( P < 0.01). A great development of small flagellate unicells belonging to morpho-functional groups Y ( Cryptomonas spp.) and X2 ( Chlamydomonas spp., Plagioselmis nannoplanctica and small Cryptophyceae) prevailed during both anomalous periods, accompanied by small centric diatoms and Skeletonema cf. potamos ( D). This scenario significantly differs from those encountered during neutral conditions, where codon P ( Aulacoseira granulata) dominated. The absence of these large filaments during drought and flood peaks could be explained by sedimentation processes in low waters, and by organisms’ retention by obstacles after water displacement from the plain in high waters periods; however, the disruption of the inocula from floodplain areas to the main flow may be the most important factor in both periods. The hydroclimatic anomalies provoked changes in the phytoplankton structure of the Paraná River, favouring X2 and Y functional groups instead of the typical P and C. The interruption of the constant feedback between the main channel and the floodplain water bodies due to low connectivity during droughts and the homogeneity of environment during floods, govern phytoplankton dynamics in this large river. The rapid occurrence of community reorganization after the conclusion of these events and the persistence of certain variations lead to the hypothesis that the magnitude and duration of extreme water discharges will determine the intensity and duration of changes. [ABSTRACT FROM AUTHOR]

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on 14C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent ( Kd 4.5–7.7 m−1), had high DOC concentrations (>45 mg l−1), low phytoplankton Chl a (1.6–2.7 μg l−1) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChl a−1 h−1 W−1 m2) were relatively low. Inorganic-turbid lakes showed highest Kd values (59.8–61.4 m−1), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l−1. Phytoplankton-turbid lakes showed, in general, high Kd (4.9–58.5 m−1) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l−1), and the highest productivities and efficiencies (maximum 0.56 mgC mgChl a−1 h−1 W−1 m2). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 105–41.7 × 105 cells ml−1). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary... [ABSTRACT FROM AUTHOR]

This study was carried out to develop and apply a fish-based biotic integrity index to assess lowland streams in a highly deforested region of the Upper Paraná River basin. Fifty-six first-order segments were randomly selected for environmental and fish evaluation. Because previous analysis had identified the main type of effect on the streams of the region as physical habitat degradation, 22 qualitatively biological attributes were selected and tested over a physical condition gradient between reference and degraded sites. Sensitivity and redundancy of each attribute revealed that five metrics were adequate for discriminating higher quality from degraded sites. Of the fifty-six streams assessed, one (2%) was classified as good, four (7%) as fair, ten (18%) as poor, and forty-one (73%) as very poor, indicating that, on a regional scale, many aspects of biological integrity are altered, indicative of serious degradation. Considering that first-order segments amount to 11,000 km in total, it is noticeable that 10,000 km of the stream segments have no more than half of the expected conditions, indicative of poor or very poor biotic integrity conditions. Possible strategies of mitigating this scenario are discussed. [ABSTRACT FROM AUTHOR]

We investigated phytoplankton biomass, assemblage structure and production along an environmental gradient to evaluate if chlorophyll- a (as proxy for biomass) and primary production peaked under conditions hypothesised to favour phytoplankton growth. During Spring 2003, a wide area from shallow estuarine waters to the shelf slope off the Río de la Plata was sampled and routine measurements included CTD profiles, nutrients, chlorophyll- a, phytoplankton composition and abundance, seston and organic matter loads, downwelling light and, at selected stations, production versus irradiance experiments. Spatial differences in abiotic variables suggested distinct hydrographic zones that differed in phytoplankton biomass and productivity. Chlorophyll- a was highest under estuarine influence and peaked at low salinity when strong stratification developed in the outer estuary, and was minimum at the shelf break and slope. In that area, however, relatively high chlorophyll- a was associated to oceanographic fronts and to the occurrence of Sub Antarctic water within the photic depth range. Productivity was maximum in shallow waters, but biomass-specific productivity peaked at the outer shelf in oceanographic fronts or in upwelled Sub Antarctic waters. Over shelf and slope waters productivity and biomass were not tightly coupled, as indicated by situations of high biomass and low productivity (Station 9), low biomass and high productivity (Station 10), or both high biomass and productivity (Station 22). Ordination analysis of phytoplankton taxa suggested that assemblages changed gradually along the environmental gradient and correlated to abiotic variables defining geographic zones. Overall results were consistent with an interpretation that phytoplankton biomass and growth were modulated by light in estuarine and coastal waters, and by hydrographic processes on the continental shelf and slope. [ABSTRACT FROM AUTHOR]

Biomass assessments of algae in wetlands usually include only the phytoplankton community without considering the contribution of other algal associations to total algal biomass. This omission prevents an accurate evaluation of the phytoplankton community as an integral part of the total ecosystem. In the present work, the biomass contributions (expressed as chlorophyll- a content per m2 of lake) of phytoplankton, epiphyton on both submerged and emergent macrophytes, and epipelon were measured in Lacombe Lake, Argentina, for the purpose of (1) establishing the relative importance of the phytoplankton and (2) evaluating the entire contribution of algal biomass within the context of the Goldsborough & Robinson conceptual model. Our sampling was carried out monthly for a year in sites representative of different conditions with respect to water depth and type of macrophytes. Physicochemical analyses of water were performed following standard methods. Plankton was collected in a five-level profile at deeper stations and in subsurface samples at the shallow one. Samples of sediment obtained with corers were collected for epipelon sampling and segments of plants were cut at different levels, so as to obtain the epiphytes by scraping. Pigment was extracted with aqueous acetone and calculations were made by means of the Lorenzen equation. According to the Goldsborough & Robinson model, a Lake State developed here during the winter (phytoplankton maxima: 150 mg chlorophyll- a per m2). Then, through the subsequent growth of the submerged macrophytes, an Open State was observed, characterized by a maximum epiphyton biomass (at 3,502 mg chlorophyll- a per m2) along with lower levels of phytoplankton biomass. The epiphytic algae on the emergent macrophytes were always present but attained only relatively low biomass values (maximum: 120 mg of chlorophyll- a per m2 in February). The epipelon biomass varied between 50 and 252 mg chlorophyll- a per m2, registering a considerable contribution of settled algae from the water column (phytoplankton). This study contributes to our knowledge of wetland dynamics through its assessment of the rapid changes in the relative contributions of both planktonic and attached algae to the total algal biomass within the context of specific environmental factors. [ABSTRACT FROM AUTHOR]

The Anostracan genus Artemia is composed by several sibling species reproductively isolated, but identical or very similar in outward appearance. The genus shows also an underlying striking variability from the biochemical point of view, regarding especially the fatty acid profile of the cysts and nauplii. In Argentina, Artemia is represented by two bisexual species: A. franciscana and A. persimilis. Former studies have shown that A. franciscana is present in northern of 36º and that A. persimilis is constrained southwards of 37° S. In general, there is good agreement between morphological and cytogenetic comparisons of Argentinean populations with respect to species discrimination. However, new Argentinean Artemia populations are being analyzed morphologically and it becomes necessary to further investigate if the genetic adscription of these populations is congruent with the results obtained from the current morphological analyses. Restriction fragment length polymorphism (RFLP) analysis of a fragment of the 16S rRNA mitochondrial gene was used to investigate the genetic diversity and population structure of 10 new Artemia populations from Argentina. The mitochondrial DNA (mtDNA) results showed a similar pattern to that of previous cytogenetic and morphological analyses with the two Argentinean species appearing as highly divergent. The presence of A. persimilis in southern Argentina and the southernmost Chilean population was confirmed unveiling a novel picture of species distribution in the country. A. franciscana showed a unique haplotype. Populations of A. persimilis appeared highly structured, although their clustering did not follow a clear geographic pattern. The different Argentinean Artemia populations analyzed were characterized by high variability in their fatty acids, showing both marine- and freshwater-type profiles. For the first time, the investigation of the relatedness between the fatty acid composition in Artemia and genetic markers was attempted. The study aimed at the putative association of molecular markers with marine versus freshwater-type populations. A lack of correlation between RFLP patterns at mtDNA and the fatty acid (FA) profiles was found in the A. persimilis populations which was discussed from the point of view of two main genetic hypotheses and/or phenotypic plasticity. [ABSTRACT FROM AUTHOR]

The cheilostome bryozoan Antarctothoa bougainvillei (d’Orbigny) is the most frequent epibiont on the ribbon-like red alga Hymenena laciniata (Hooker f. & Harvey) Kylin in San Sebastián Bay (Tierra del Fuego, Argentina). Twenty-one thalli and 1,484 colonies were examined to analyse the relationship between both species. In most cases, number and area of colonies did not differ significantly at both sides of the thallus. Ancestrulae (i.e., founder zooids originating colonies by asexual budding) were mostly oriented facing the algal growing edge. Colonies were more frequent on central than on marginal zones of the thalli. The population of A. bougainvillei was mainly composed of very small colonies (<10 mm2). Larger colonies predominated and intraspecific competition was more intense near the basal portions of the thalli. Fecundity (number of ovicells) increased at a significantly higher rate in colonies with margins obstructed by conspecific neighbours than in free-growing colonies. Colonies were significantly larger on somatic than on reproductive algal tissues. As total and reproductive surfaces covered by colonies of A. bougainvillei were on average very low (4.43% and 0.53%, respectively), this epibiont is not supposed to produce a negative effect on H. laciniata. [ABSTRACT FROM AUTHOR]

Longitudinal and seasonal distributions of Plecoptera species were examined along the Chubut River in the Patagonian Precordillera and Plateau, Argentina. Chubut River (>1000 km) is the largest river in the area and the hydrological regime is modified in the lower section by an artificial reservoir (Florentino Ameghino Dam). Quantitative samples were collected in 13 sites in the higher, middle, and lower sections of the river basin. Sites were visited four times during 2004, and a total of nine species and 5772 individuals were collected in the study. Plecoptera richness decreased dramatically from the headwaters to the mouth of the river system. Two species, Antarctoperla michaelseni and Potamoperla myrmidon, were able to live below the impoundment but they were not abundant. Notoperlopsis femina and A. michaelseni abundances were higher in summer than in the other seasons, while Limnoperla jaffueli peaked in spring. Species–environmental relationships were examined using Canonical Correspondence Analysis and six independent variables were identified as the major factors structuring stoneflies assemblages. First axis was highly related to environmental variables reflecting the hydro-geological and land use gradients in the basin (conductivity, total suspended solids, periphyton Chlorophyll a). Second axis was more related to variables that changed seasonally (wet width, water temperature and soluble reactive phosphate). [ABSTRACT FROM AUTHOR]

In situ experiments were conducted at various depths in the water column to determine the effects of solar ultraviolet radiation (UVR, 280–400 nm) on photosynthesis of natural phytoplankton assemblages from the subtropical Lake La Angostura (Argentina, 26°45′ S; 65°37° W, 1980 m asl.). Water samples were taken daily and incubated under three radiation treatments: (a) Samples exposed to UVR + Photosynthetic Available Radiation (PAR) – PAB treatment (280–700 nm); (b) Samples exposed to ultraviolet-A radiation (UV-A) + PAR – PA treatment (320–700 nm), and, (c) Samples exposed to PAR only – P treatment (400–700 nm). Additionally, depth profiles were done to determine different physical (i.e., temperature and underwater radiation field) and biological characteristics of the water column – photosynthetic pigments, UV-absorbing compounds, cell concentration, deoxyribonucleic acid (DNA) and cyclobutane pyrimidine dimers (CPDs). The effects of UVR on natural phytoplankton assemblages were significant only in the first 50 cm of the water column, causing a decrease in photosynthetic rates of 36 and 20% due to UV-A and ultraviolet-B radiation (UV-B), respectively; below this depth, however, there were no significant differences between radiation treatments. Concentration of CPDs per mega base of DNA in natural phytoplankton was low, <27 CPDs MB−1 between 0 and 4 m. Data on net DNA damage, together with that on mixing conditions of the water column, suggest that mixing can favour phytoplankton by allowing cells to be transported to depths where active repair can take place. This mechanism to reduce UVR-induced DNA damage would be of great advantage for these assemblages dominated by small cyanobacteria and chlorophytes where UV-absorbing compounds that could act as sunscreens are virtually absent. [ABSTRACT FROM AUTHOR]

The distribution and abundance of tardigrades in the city of General Pico (Argentina) are analyzed from samples collected during autumn and spring 2001. Sample sites included paved urban locations with different traffic intensities, non-paved periurban places, and places with peculiar conditions such as the city industrial area and the bus station. Trees of the same species were selected in each area and from each of them nine subsamples of lichens and/or moss, 11 mm in diameter, were taken with steel corers. The diversity, density and relative abundance of tardigrades was recorded and analyzed. Sampling sites were ordinated and classified with PCA and clustering. The number total of species collected in the research was 5. The maximum number of species per site and per tree was 4. The species recorded were: Echiniscus rufoviridis, Milnesium cf. tardigradum, Ramazzottius oberhaeuseri, Macrobiotus areolatus and an undescribed species of the genus Macrobiotus. The average density was approximately 10 specimens per cm2 and the maximum values reach over 50 specimens per cm2. R. oberhaeuseri and M. cf. tardigradum were the most frequent species. R. oberhaeuseri dominates in periurban areas with high suspension dust and very exposed to the sun. M. cf. tardigradum dominates on paved streets with intense vehicle traffic. Results supported the hypothesis of the relationship between air quality and tardigrade diversity. [ABSTRACT FROM AUTHOR]

Climatic and hydrological variability is usually high in the Pampa Plain (Argentina). However it has not studied yet how this variability may affect the phytoplankton and zooplankton biomass and community structure in aquatic systems of this region. The main purpose of this study was to assess flushing effects on nutrient and plankton dynamics in two interconnected very shallow lakes of the Pampa Plain. In order to study the impact of hydrology on the plankton biomass and community structure, we compared the summer plankton community among three consecutive years with contrasting hydrological characteristics. Water residence time varied an order of magnitude among years and this variability was correlated to strong changes in physicochemical and biological lake characteristics. Depending on the water discharge level, the hydrological regime within the lakes ranged from lentic to more lotic conditions. Nutrient and phytoplankton biomass were positively related to water discharges. During high flushing periods, nutrients import from intensive agriculture lands leads to a dramatic increase in trophic conditions. On the other hand, macrozooplankton biomass was positively related to water residence time and showed a dramatic decrease during high flushing years. Rotifers biomass was not affected by interannual water discharge variability during the study period. Our results support that in case of lakes with high flushing rates, zooplankton development is dependent on water residence time and that hydrology may have stronger effects on macrozooplankton biomass than top-down control by planktivores. [ABSTRACT FROM AUTHOR]

Specimens of flounder Paralichthys orbignyanus (Jenyns, 1842) were collected in Bahía Blanca estuary between February 1997 and January 1998, and their feeding habits were examined in relation to season and size class. The stomach contents of 823 specimens, ranging from 70 to 875 mm total length, were analysed. Their diet included organisms from 17 taxa. The highest vacuity index values were found during autumn and winter. The stomach fullness index indicated that flounders increased their feeding activity between October and March, reaching a highest point in February and decreasing after February. Fish were the primary prey item in frequency, number and weight, followed by crustaceans, such as shrimps and crabs. A seasonal and size class variation was detected in the diet. During summer all-size flounders consumed mainly fish. In autumn, for all-size classes the main food were fish and crustaceans even though fish were dominant in terms of biomass. In spring, crustaceans (mysids Arthromysis magellanica (Cunningham), shrimps Artemesia longinaris Bate, and prawns Pleoticus muelleri (Bate)) were the dominant prey in terms of number and biomass for flounders ≤ 450 mm TL. Size classes larger than 451 mm TL fed on crustaceans and fish though the most important prey item in terms of biomass was fish. In Bahía Blanca estuary, P. orbignyanus evidenced mainly an ichthyophagou – carcinophagous diet. The results collected from the present study lead to conclude that P. orbignyanus shows, in this region, a clear preference for fish. [ABSTRACT FROM AUTHOR]

The fish fauna and its relationships with physiography and climate were studied in northwestern Argentina from 21°30′ S to 26°30′ S and 63°30′ W to 65°50′ W, an area about 10,276 km2. Along a southeast–northwest gradient, the Chaco forest at low altitudes gives way to the Yungas cloud forest in highlands, and then, to an increasing desertic landscape, with xerophytic vegetation and scarce rainfall along the Grande River. Finally, extreme desert conditions prevail in the most northern part at the Puna plateau. Water chemistry was sampled from sites from 400 to over 3800 m a.s.l. In all 3278 fish specimens of 52 species were collected. Previous lists included 84 species. Only 19 were shared, meaning that 40 species are new for the area and/or particular localities, including 7 reports from Aguas Calientes. These results increase by one third the number of species in northwestern Argentina. The fish fauna was represented by eurytopic species of Paranensean genera as Astyanax, Bryconamericus and other characoids, mixed with locally distributed siluriforms. Under extreme climatic conditions, species of Trichomycterus predominate. Species assemblages show a combination of a large number of species typical of, sometimes endemic to or rather abundant in, the area, combined with a few species of Paranensean character. Fish assemblages were clearly defined by faunistic composition and distribution related with physiography and climate traits. A significant negative correlation is observed between both species number and abundance, and increasing altitude, and positive relationships exist with mean annual temperature and other climate traits. Diversity values (Shannon index) agree with the described pattern of increasing impoverishment of the fish fauna of northwestern Argentina, along gradients of increasing altitude and dryness and decreasing temperature. [ABSTRACT FROM AUTHOR]

The Suquía River (Córdoba, Argentina) has become an important issue because it flows into Mar Chiquita Lake, one of the largest saline lakes in the world. This water body, together with the expansive swamps of the Dulce River on the northern shore and the mouth of Suquía and Xanaes River, is considered one of the most important wetlands in Argentina in terms of biodiversity in a range of freshwater to very saline environments. Nevertheless, the presence of densely populated urban settlements and the increasing environmental impact due to anthropogenic activities characterize the central and lower sections of Suquía River Basin. Fishes are particularly affected and change their distribution and abundance as a consequence of the environmental deterioration. We collected information on fish fauna to develop and validate a Biotic Index to assess degradation of the Suquía River Basin. We classified fish species according to their sensitiveness or tolerance to environmental degradation, based on their distribution and abundance variations along a water quality gradient in order to design a Biotic Index for Suquía River Basin. The set of metrics used in the Biotic Index calculation was conformed by: the abundance of Astyanax eigenmanniorum, Rineloricaria catamarcensis, Gambusia affinis and Cnesterodon decemmacultus, the proportion of sensitive species richness, and the proportion of tolerant species richness. They clearly distinguished between the impaired and referenced sites. We demonstrated that it is possible to use fish as indicators of water quality in Córdoba Province (central part of Argentina) in order to carry out rapid and relatively inexpensive monitoring and conservation programs. The application of this Biotic Index showed that fish assemblages reflect the watershed conditions and are sensitive to changes in water quality across the environmental gradient. [ABSTRACT FROM AUTHOR]

Amphicypris argentinensissp. nov., is described and illustrated from Laguna Caliba, an ephemeral fresh water lake from the southern coast of Buenos Aires Province, Argentina. Specimens were raised from dried sediment recovered from a bank of ostracod shells found at the edges of the lake. Deposition of progressively smaller ostracod valves stacked one into another (cup-in-cup structure) is documented. The geographical distribution and ecology of the genus is briefly discussed. [ABSTRACT FROM AUTHOR]

The aim of this study was to analyze the feeding habits of the juvenile striped weakfish,Cynoscion guatucupaCuvier, from Bahía Blanca estuary, Argentina.C. guatucupais one of the most important regional fishing resources although in the last 10 years landings have greatly decreased. The year class strength of fish as well as stock size and yield are determined during the early life stages. Knowledge about the diet of youngC. guatucupamay therefore contribute to our better understanding of the influence that the changes occurring in the biota of the estuary exert on the abundance fluctuations of this species. To this end, we investigated the ontogenetic and seasonal dietary changes ofC. guatucupabetween 1.00 and 12.99 cm total length (age 0+), and we related them to changes in habitat use and prey selection. Two dietary shifts were found during ontogeny. The first shift, at approximately 4 cm total length, involved a change from demersal-pelagic prey (mysid shrimps) to demersal-benthic prey (Peisos petrunkevitchiBurkenroad). The second shift, at approximately 8 cm total length, involved a progressive increase in ichthyophagy (mainly an increase consumption of the Argentine anchovyEngraulis anchoitaHubbs&Marini). Seasonal dietary changes were also found. They evidenced the changes in the availability of organisms. In autumn, the chaetognathSagitta fridericiRitter-Zahoni,P. petrunkevitchi,andE. anchoitawere the dominant prey items; in winter, the copepodLabidocera fluviatilisDahl along withS. friderici,mysids, andP. petrunkevitchiconstituted the majority of the diet; and, in summer, mysids were the most important prey item consumed. Selectivity data showed that whereas some prey items, such as the copepodsAcartia tonsaDana andParacalanus parvus(Claus) andS. friderici,were consistently negatively selected by all size classes throughout the year, other prey items, such as fish larvae, mysids, andP. petrunkevitchi,were selected in one season and avoided in another. Additional dietary seasonal differences observed in our research indicate that the limited food supply and the low water temperatures are limiting factors for winter juveniles in BahÕa Blanca estuary. These factors may also directly influence the more-than-10 cm total length seaward migration of large numbers ofC. guatucupathat is registered at the end of every autumn. [ABSTRACT FROM AUTHOR]

In the early nineties, Undaria pinnatifida has been accidentally introduced to Nuevo Gulf (Patagonia, Argentina) where the environmental conditions would have favored its expansion. The effect of the secondary treated sewage discharge from Puerto Madryn city into Nueva Bay (located in the western extreme of Nuevo Gulf) is one of the probable factors to be taken into account. Laboratory cultures of this macroalgae were conducted in seawater enriched with the effluent. The nutrients (ammonium, nitrate and phosphate) uptake kinetics was studied at constant temperature and radiation (16 °C and 50 μE m-2 s-1 respectively). Uptake kinetics of both inorganic forms of nitrogen were described by the Michaelis–Menten model during the surge phase (ammonium: Vmaxsur: 218.1 μmol h-1 g-1, Kssur: 476.5 μM and nitrate Vmaxsur: 10.7 μmol h-1 g-1, Kssur: 6.1 μM) and during the assimilation phase (ammonium: Vmaxass: 135.6 μmol h-1 g-1, Ksass: 407.2 μM and nitrate Vmaxass: 1.9 μmol h-1 g-1, Ksass: 2.2 μM), with ammonium rates always higher than those of nitrate. Even though a net phosphate disappearance was observed in all treatments, uptake kinetics of this ion could not be properly estimated by the employed methodology. [ABSTRACT FROM AUTHOR]

Opheliid polychaetes belonging to the genera Armandia, Ophelia, Ophelina, and Travisia, collected along the Southwestern Atlantic coast off Brazil and Argentina, are recorded or given extended descriptions. Travisia amadoi sp. n., Ophelina gaucha sp. n. and Ophelina alata sp. n. are described. [ABSTRACT FROM AUTHOR]

Phytoplankton, as well as physical and chemical variables of six lotic systems of Buenos Aires province, tributaries of Río de la Plata river (Rodríguez, El Gato, El Pescado, Buñirrigo and J. Blanco streams, Samborombón river) were analysed and results were related to the degree of anthropic pollution. Surveys were carried out seasonally from May 1997 to June 1998. Sample sites were stablished at 2 or 3 stations depending on the length of the systems, from the headwaters to the mouths. Specific composition, species richness, dominance, diversity and eveness were assessed. Diatoms and clorophytes presented the highest number of species. Cyanobacteria were dominant in 60% of the samples and diatoms were subdominant in 42% of them. Samborombón river showed minimun and maximun phytoplankton densities among the six systems (50 cell.ml-1 and 188 475 cell.ml-1, respectively). Indexes values showed a non-defined pattern, increasing or decreasing in the most polluted sites. The first axis of the PCA which included the physical and chemical variables is characterized by the nutrient concentrations and it is related to the pollution of these systems, separating samples of Rodríguez and El Gato streams, which presented high positive scores, from the rest of them. The second axis is represented by variables which determine the water conductivity and separates samples from the mouths of Samborombón river and Buñirrigo stream, which presented high positive scores, from the rest of them. The species PCA indicate that its distribution is related to the first two components of the chemical data set PCA. The first axis separates species found in high-conductivity sites, such as the mouths of Samborombón and Buñirrigo systems (i.e. Cocconeis placentula, Aulacoseira granulata, Melosira varians, Actinocyclus normanii, Dictyosphaerium ehrenbergianum) from those registered in low-conductivity sites, such as J. Blanco stream (i.e. Pseudanabaena catenata, Nitzschia gracilis, Mallomonas sp.). The second axis separates α and β mesosaprobic species from polluted sites -Rodríguez and El Gato streams- (i.e. Gomphonema parvulum, Navicula cryptocephala, Nitzschia palea, N. umbonata, Oscillatoria amphibia, Lyngbya limnetica, Euglena acus) from those found in oligosaprobic non-polluted sites – J. Blanco stream – (Nitzschia gracilis, Pseudanabaena catenata, Gomphonema clavatum). [ABSTRACT FROM AUTHOR]

Feeding habits of Acetes paraguayensis (Hansen) were studied by examining the stomach content of specimens from two lakes. A. paraguayensis is omnivorous, feeding mainly on members of littoral-benthic and lotic communities. Algae, rotifers and microcrustaceans (copepods and cladocerans) are the more frequent items in the bulk of the stomach content. Oligochaeta and Diptera larvae were an alternative food source when available. Shrimp feeding habits difference between the two lakes may be due to the resource availability. A. paraguayensis are trophic linkers between limnetic and lotic habitats with an important role in the transportation of energy from the floodplain to the principal channel in the Paraná Rivers System. [ABSTRACT FROM AUTHOR]

The brood cycle in anostracans results from the overlapping of two successive reproductive cycles in the genital tract of females. While a clutch of oocytes begins to mature at the ovaries (ovarian period), the nauplii, cysts or unfertilised products of a previous clutch are developing in the ovisac or being released to the medium (oviduct-ovisac period). Models are proposed for the sequence of stages and their duration in a complete brood cycle under two treatments, i.e., females with males (1:1 ratio), and isolated virginal females of Artemia persimilis in laboratory conditions. The whole brood cycle in both treatments took 6 days, however, they differed in the duration of some stages. The differences are discussed and observations on amplexus are added. [ABSTRACT FROM AUTHOR]

At present, the American genus Thamnocephalus has three species, recorded from several states of U.S.A., México and Venezuela: Thamnocephalus platyurus Packard, T. mexicanus Linder and T. venezuelensis Belk & Pereira. The new species Thamnocephalus salinarum, from a turbid saline temporary pond inside the Salinas Grandes of Córdoba, Argentina is described here. The new species differs from all other congeners by the presence in the male, of an unbranched frontal appendage and a pair of soft cordiform parapenial lobes; also, by the sexually dimorphic caudal fin-like lobe. This new species may represent a new genus, however since only five specimens were collected it was not possible to observe if the penial ornamentation conforms to the Thamnocephalus type. Therefore, a conservative position is taken until new material allows proper analysis, by keeping the new species in the genus Thamnocephalus under the new subgenus Simplicephalus. [ABSTRACT FROM AUTHOR]

Investigates the changes in phytoplankton community along a transversal section of the floodplains in Argentina. Physical and chemical properties of the valleys; Detection of mixotrophic algae in the phytoplankton of the swamp; Importance of nutrients for phytoplankton growth.

Examines the Diacylops species found in northwestern Argentina. Demonstration of a subequal terminal spines of the distal endopod segment of leg 4; Presence of smooth furcal rami; Illustration of the pattern of the spine formula of swimming.

Describes the new species of crustacea, Boeckella antiqua collected in an ephemeral pond on the Patagonian plateau in Argentina. Relation of B. antiqua with B. poppei; Distinction of male fifth pair of legs with female specie; Features of the genus Pseudoboeckella.

Focuses on various species of Cervonema and Laimella found at the Strait of Magellan and the Beagle Channel in Chile and Argentina. Characteristic feature of Cervonema chilensis; Presence of cephalic seta in Cervonema shiae; Features of Laimella sandrae n. sp that separate it from Laimella annae.

Describes the post-embryonic development of Artemia persimilis Piccinelli & Prosdocini in Argentina. Development of genital differentiation; Enlargement of the hairy gnathobase; Reduction of the structures of antennae.