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This paper gives an account of the implementation of hydrochemical and isotopic techniques to identify and explain the processes that govern solute exchange in two groundwater-dependent shallow lakes in the Southeastern Pampa Plain of Argentina. Water samples (lakes, streams, spring water and groundwater) for hydrochemical and stable isotopic determination were collected and the main physical-chemical parameters were measured. The combination of stable isotope data with hydrogeochemical techniques was used for the identification of sources and preferential recharge areas to these aquatic ecosystems which allowed the explanation of the lake water origin. The hydrochemical processes which explain Los Padres Lake water chemistry are evaporation from groundwater, CO input, calcite dissolution, Na release by Ca and Mg exchange, and sulfate reduction. The model that best aligns with La Brava Lake hydrochemical constraints includes: mixing, CO and calcite dissolution, cationic exchange with Na release and Mg adsorption, and to a lesser extent, Ca/Na exchange. This model suggests that the fractured aquifer contribution to this water body is greater than 50 %. An isotopic-specific fingerprint for each lake was identified, finding a higher evaporation rate for La Brava Lake compared to Los Padres Lake. Isotopic data demonstrate the importance of these shallow lakes as recharge areas to the regional aquifer, becoming areas of high groundwater vulnerability. The Tandilia Range System, considered in many hydrogeological studies as the impermeable bedrock of the Pampean aquifer, acts as a fissured aquifer in this area, contributing to low salinity waters and with a fingerprint similar to groundwater isotopic composition. [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.

Mainly through BSi processing, shallow lakes regulate DSi retention, transport, and delivery to the ocean. However, these DSi fluxes in southern lakes are little known. This work evaluated the spatio-temporal variation and the main factors that affect DSi dynamics and retention in a temperate groundwater flow-through shallow lake in Argentina. Water samples from streams, the lake, and groundwater were collected from Los Padres Lake Watershed for DSi determination. DSi retention was calculated through a mass balance approach. The BSi pool from macrophytes was quantified using biomass, BSi production, and satellite images. The phytoplankton community was described by chlorophyll content, species quantification, and alpha diversity metrics. The contribution of Si-enriched water from groundwater (≈ 50 mg L−1) mainly controlled DSi concentrations in the inflow stream (≈ 54 mg L−1). Lake DSi concentration was lower than streams and groundwater, and increased over time (14–34 mg L−1). BSi contribution by macrophytes (≈1.4 Mg Si year−1) has little variation, therefore, the reduction of diatoms due to cyanobacteria increase is the most likely factor controlling DSi dynamics, influencing the role of the lake as a DSi sink/source. These biogeochemical processes regulated the DSi transfer from the lake to the outflow stream, and the delivery to the ocean (≈ 630 Mg Si year−1). Given the importance of small lakes in nutrient cycling and retention along flow paths within a watershed, in-lake Si processes and Si sources to these water bodies are critical to quantifying limnetic contributions to the global Si cycle. [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]

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]

This study gives an account of the implementation of a decision support system as a logical framework for assessing lake pollution hazard. The use of this system is demonstrated with an example from two lake watersheds, each one with different land-use, soil and topographic characteristics and also management regulations for natural resource protection. Lake pollution hazard is assessed as a function of two primary topics: hydrologic and soil conditions. This model shows the state of each evaluated subwatershed with respect to its potential contribution to lake pollution hazard based mainly on easily measurable and commonly used parameters: drainage density, hydraulic gradient, water table depth, soil media, land-cover and topography. Mapped outputs from the logic model allowed the identification of several subwatersheds in each lake system as the main water resource protection areas. La Brava Lake Watershed results reasonably fit the natural conditions of the watershed, identifying those subwatersheds with a moderate to high drainage density, high hydraulic gradient and extensive agricultural activities. Regarding Los Padres Lake Watershed, moderate to high drainage density, a high to a very high hydraulic gradient, shallow water depth, silt-loam soil texture and intensive agricultural and residential lands, contributed to a high lake pollution hazard. Results highlighted differences between both freshwater systems, being Los Padres Lake Watershed a more vulnerable ecosystem, requiring decision maker's intervention based on accurate and up-to-date information. [ABSTRACT FROM AUTHOR]

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings. [ABSTRACT FROM AUTHOR]

Natural dams are caused by the blockage of streams as a result of the sudden arrival of detritus material during a glacial surge advance or fast slope collapse threatening downstream populations. Nevertheless, origin, morphological characteristics and stability of dammed lakes are frequently ignored. Six impounded palaeo-lakes were geomorphologically studied in the Benjamin Matienzo gully (32°14′ S-70°02′ W), denominated, from north to south: Goyete, Negro, Casa de Piedra, Lagunita, Susanita and Matienzo. We determined the origin, morphometric parameters, stability index and rate of streamflow rupture for these dammed ancient lakes in order to shed light on the potential outburst flood hazard in this Andean mountain region. According to our findings, we concluded that palaeo-lakes would have had a short life as they turned out to be unstable, except for the Goyete Lake, matching with the lack of fine lacustrine sediments in all river blockages. We also estimated the maximum peak discharge of a probable outburst flow generated during a drastic collapse of these dams. Obtained values range between 22 and 151 m/s being notably higher than the average annual streamflow of Las Cuevas River (6.6 m/s), but similar to the instantaneous streamflow of this river (157 m/s, 2-year recurrence time). [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]

The present study focuses on the geographic distribution and biodiversity of the Chlorococcales in Patagonian lakes, covering a latitudinal transect. Two different approaches are considered (a) a review of the historical records of Chlorococcales in Patagonia and (b) the analysis of the morphology-based species diversity, ecological remarks, and geographic distribution of the chlorococcalean species recorded in 33 aquatic environments during surveys 2007 and 2008. A total of 308 chlorococcalean species were recorded throughout a 60-year period in Patagonian freshwater systems, encountering the highest chlorococcalean richness in lakes and shallow lakes located in the more intensively sampled areas. In our surveys 2007-2008, 72 chlorococcalean taxa were registered, among which 80% are worldwide distributed, about 19% are restricted to cold temperate areas in both hemispheres and one species ( Pediastrum patagonicum) is one of the taxa probably endemic for Patagonia. The chlorococcalean richness in relation to the environmental factors, revealed that it is directly correlated with the trophic status of the water bodies (chl a: r = 0.52 and DIN: r = 0.47; P < 0.05). The latitude was inversely correlated with the species richness ( r = −0.40; P < 0.05), and the chlorococcalean biodiversity of the Andean lakes (mean value: 3.64) was significantly lower ( P = 0.0022) than that of the lakes from the Patagonian Plateau (mean value: 9.62). In this study we observed a decreasing trend in the biodiversity of Chlorococcales with increasing latitude, which fits in the existing ecological pattern described for different groups of organisms. [ABSTRACT FROM AUTHOR]

As a result of a low pH, the inorganic carbon of acidic lakes is present as CO2 at air-equilibrium concentration and is substantially lower than the inorganic carbon concentration in higher-pH waters with bicarbonate. This situation is quite common in artificially acidified lakes and where inorganic carbon is considered the limiting factor in phytoplankton growth. Apart from low inorganic carbon content, Lake Caviahue in Argentina has low nitrogen and high phosphorus content. The aim of this work was to assess the importance of inorganic carbon, phosphorus, and nitrogen, relating data on lake nutrients to phytoplankton species requirements. Lake samples taken in the 2004–2006 period did not show any particular trend in the vertical distribution of the water column of ammonium, inorganic carbon, and phosphorus with reference to either seasonality or depth. A decrease of some 15% in the lake’s phosphorus concentration was observed over the same period. Although the total phytoplankton biomass in Lake Caviahue was similar throughout the period, a seasonal variation was observed. Lab bioassays were carried out with solutions of bicarbonates, ammonium, nitrates, and phosphate. We worked with three species separately, namely, two chlorophytes, Keratococcus rhaphidioides and Watanabea sp.; and one euglenophyte, Euglena mutabilis. Answers to specific nutrient requirements differed for each algal species: both chlorophytes prefer ammonium or nitrates added on their own, whereas the euglenophyte registered a higher growth rate with the joint addition of ammonium and phosphorus. Even when the limiting nutrient(s) for phytoplankton yield and rate varied between species, we observed a tendency for nitrogen limitation in Lake Caviahue. [ABSTRACT FROM AUTHOR]

Chemical studies on 19 lakes of the Argentine sector of the Patagonian Tierra del Fuego Island were performed. These lakes are placed in two geological provinces, Extra Andine (northern) and Fueguian Cordillera (southern). Range of the results obtained, pH (5.80–8.60), TDS (29.5–10,232 mg/l), ionic composition (from calcium bicarbonate to sodium chloride type), Secchi disc depth (0.05–12 m), total inorganic nitrogen (20–237 μg N/l), orthophosphate (5–498 μg P/l), total phosphorus (5–3,800 μg P/l) and chlorophyll a (0.1–63.0 μg/l), indicated the presence of a wide variety of aquatic ecosystems. According to Gibbs model, the mechanisms that control the water chemistry are mainly rock dominance and evaporation–crystallization processes. [ABSTRACT FROM AUTHOR]

Abstract  Diatom assemblages preserved in continental sedimentary successions of the southern Pampas (Argentina) can provide useful indicators of past environmental changes. Ecological data, however, are scarce. In order to provide modern data for diatom-based paleoenvironmental reconstructions, sediments from shallow lakes and streams of the region were analyzed. A total of 131 diatom species were identified. Canonical correspondence analysis was used to identify the physical and chemical environmental parameters that best explained the distribution of taxa. Cocconeis placentula, Hippodonta hungarica, Navicula veneta, Cyclotella meneghiniana and Nitzschia amphibia were the most widely extended species. The highest percentage variance in diatom data was explained by conductivity and nutrient concentrations. Diatom assemblage composition was more variable in shallow lakes than in streams, which is in agreement with the different hydrological and geomorphological characteristics of these environments. The results obtained in the present work provide useful analogues for paleoenvironmental reconstructions of water bodies in southern Pampas. [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]

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 large Argentine marine littoral zone is characterized by great number of wetlands and includes only one coastal lagoon, Mar Chiquita, which has been declared as a Biosphere Reserve by the Man and Biosphere Reserve Program from UNESCO. Its margins present populations of Chasmagnathus granulatus, a semiterrestrial crab distributed along wide salinity gradients that plays an important role as a key species within the corresponding trophic web. Dissolved cadmium (Cd) and zinc (Zn) concentrations present in this ecosystem were determined. Cadmium concentrations ranged between n.d. and 0.82 μg/L and zinc levels ranged between n.d. and 1224.38 μg/L within the mentioned coastal lagoon. Cd and Zn acute semistatic toxicity bioassays were carried out for 96 h on juvenile crabs of C. granulatus. LC50 96-h values were 2.24 mg Cd2+/L and 7.07 mg Zn2+/L at 5 psu, and 15.42 mg Cd2+/L and 11.41 mg Zn2+/L at 25 psu. Higher salinities resulted in lower metal toxicity. This effect was stronger for Cd than for Zn. C. granulatus juveniles LC50 96-h values determined for Cd were three to four orders of magnitude higher than the corresponding dissolved metal concentrations in the Mar Chiquita coastal lagoon; nevertheless, those Zn values determined were similar to several ones corresponding to natural water samples. [ABSTRACT FROM AUTHOR]

Volcanic ashes erupted by recent magmatic events were collected from one sedimentary sequence of lake Nahuel Huapi (Patagonia, Argentina) and then analyzed by Instrumental Neutron Activation Analysis. Primary volcanic components, including white and brownish pumice fragments, scoriae and glass shards, as well as bulk sediments from four tephra layers identified in the sedimentary sequence, were characterized by their major and trace element contents. The two tephra layers being in the upper section of the sequence can be linked to the eruptions of Cordon Caulle fissure (Puyehue-Cordon Caulle region) occurred in 1921-1922 and 1960, according to core dating. The third tephra layer, which is clearly associated to an event occurred in the 1890-1900 decade, has different compositional patterns showing a different source from those upper tephras. Whereas, the fourth, which could be associated to an event occurred at the end of the 18 th century, shows the same composition patterns as those registered for the Cordon Caulle fissure. [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]