Lower Paraguay Research Articles

Contribución al conocimiento limnológico del Río Paraguay en su tramo inferior

The main limnological feactures of the Paraguay river have been studied in two sampling sites: La Herradura and Puerto Bermejo stations. The former is situated above the confluence with the Bermejo river and the latter is located below the confluence.
 The phytoplankton community has been dominated by Diatomophyceae (mainly Melosira granulala) and Chlorophyceae (mainly Monoraphídium, Schroederia and Scenedesmus). Crytophyceae (Cryptonomas) and Dinophyceae (Perldlnium) have always been present in low percentage. Cyanophyceae have usually been less than 4% of the total population density.
 In the low water period of the summer 1978-1979, the maximun phytoplankton density was estimated in 4 000 ind./ml and the production in 750 mg C/m2 d. During the flood period of May through October the lowest phytoplankton density (160 ind./ml) and production (60 mg C/m2 d) were measured.
 Variations in the suspended solid content and salinity of the water are discussed in relation to the hidrologic regime of the river.
 The huge amount of sedimenta carried by the Bermejo river to the lower Paraguay produces a drastic decrease in the transparency, phytoplankton density and production of the waters. On the other hand, conductivity, suspended solids and nutrients are highly increased.

Gaps and challenges in the knowledge of algal biodiversity in Paraguay

ABSTRACT Tropical environments comprise some of the largest megadiverse centres on earth, and yet, even in the face of rapid biodiversity loss, a lack of knowledge of such areas prevails today. Paraguay is a landlocked, tropical country, which still holds enormous forested areas despite intense degradation. The country shows enormous potential algal biodiversity. We searched published records of algal species within Paraguay, checked their taxonomic status, and provided their locations, resulting in a thorough list of the algae known to date in Paraguay. Only 33 articles were published between 1897 and 2020. From the early 20th century and until the late 1970s, there were few publications on algae, and most studies were conducted within the last 7 years. We compiled records of 893 species from 54 orders and 10 phyla. Only freshwater algal records were found; none were terrestrial or from brackish lagoons. Most species belong to Charophyta, Chlorophyta, and Bacillariophyta. The Lower Paraguay basin showed the highest number of records, i.e. 564 species, 309 not found in any other basin in Paraguay. The highest number of algal records are from the Humid Chaco ecoregion, with 610 species, 343 of which did not occur in any other ecoregion of the country. Our results indicate that Paraguay requires extensive research and human resource training programs in phycology. Because of its threatened status, we consider the Dry Chaco to be the most important area where research should be intensified.

A pre-Sturtian depositional age of the lower Paraguay Belt, Western Brazil, and its relationship to western Gondwana magmatism

The Neoproterozoic Paraguay Belt in mid-eastern Mato Grosso, western Brazil, records the geological evolution of the margins of western Gondwana. The lack of absolute geochronology for the Paraguay Belt has hampered both regional and global stratigraphic correlations. Here we determine the depositional age for the Peresopolis black slate that belongs to the base of the Cuiabá Group in the Paraguay Belt using Re-Os geochronometry. We obtain a narrow Re-Os age interval of 784 ± 8 Ma and a juvenile initial 187Os/188Os composition (Osi) of 0.15 ± 0.03. These data indicate that the Peresopolis black slate is Tonian in age and was deposited in a basin strongly influenced by the weathering of juvenile material, and potentially linked to the early rifting of Rodinia on western Gondwana. These slates record a time interval that is poorly represented in Brazil and with few counterparts in other Neoproterozoic basins globally. As such, our work is a foundation for future geochemical correlations and paleoenvironmental reconstructions of this critical transition of Earth history in Brazil and elsewhere.

Heavy Rainfall in Paraguay during the 2015/16 Austral Summer: Causes and Subseasonal-to-Seasonal Predictive Skill

During the austral summer 2015/16, severe flooding displaced over 170 000 people on the Paraguay River system in Paraguay, Argentina, and southern Brazil. These floods were driven by repeated heavy rainfall events in the lower Paraguay River basin. Alternating sequences of enhanced moisture inflow from the South American low-level jet and local convergence associated with baroclinic systems were conducive to mesoscale convective activity and enhanced precipitation. These circulation patterns were favored by cross-time-scale interactions of a very strong El Niño event, an unusually persistent Madden–Julian oscillation in phases 4 and 5, and the presence of a dipole SST anomaly in the central southern Atlantic Ocean. The simultaneous use of seasonal and subseasonal heavy rainfall predictions could have provided decision-makers with useful information about the start of these flooding events from two to four weeks in advance. Probabilistic seasonal forecasts available at the beginning of November successfully indicated heightened probability of heavy rainfall (90th percentile) over southern Paraguay and Brazil for December–February. Raw subseasonal forecasts of heavy rainfall exhibited limited skill at lead times beyond the first two predicted weeks, but a model output statistics approach involving principal component regression substantially improved the spatial distribution of skill for week 3 relative to other methods tested, including extended logistic regressions. A continuous monitoring of climate drivers impacting rainfall in the region, and the use of statistically corrected heavy precipitation seasonal and subseasonal forecasts, may help improve flood preparedness in this and other regions.

Molecular and taxonomic characterisation of introduced specimens of Poecilia reticulata in the lower Paraguay River basin (Cyprinodontiformes: Poeciliidae)

ABSTRACT Poeciliids comprise around 300 species inhabiting the fresh and brackish waters of the Americas and Africa. Poecilia reticulata is native to Northeastern South America and Trinidad and Tobago. In this paper, introduced specimens of P. reticulata collected in the lower Paraguay River in Argentina, were characterized by means of molecular and taxonomic approaches. We further explore, by means of DNA Barcoding, the singularity of the genetic identity of these specimens. Ocurrence of P. reticulata in the lower Paraguay River represents the first record of this species in Argentina. Thirteen individuals of P. reticulata were collected. DNA barcoding showed that all five specimens sampled belong to a single mitochondrial lineage, which was also present in 11 countries from five continents. The distance-based tree clearly grouped separetely four different clusters of P. reticulata when including public data. Genetic distance between the most divergent P. reticulata almost paralleled distance between this species and Poecilia mexicana and P. vivipara. Established populations from Paraguay could be one of the plausible sources for the introduced populations recorded in the lower Paraguay River. The presence of P. reticulata in an open waterway with known drainage to a natural stream is of major concern.

A taxonomic revision of the Neotropical electric fish genus Brachyhypopomus (Ostariophysi: Gymnotiformes: Hypopomidae), with descriptions of 15 new species

ABSTRACT The bluntnose knifefish genus BrachyhypopomusMago-Leccia, 1994, is diagnosed from other Rhamphichthyoidea (Rhamphichthyidae + Hypopomidae) by the presence of a disk-like ossification in the anterior portion of the palatoquadrate, and by the following external characters: short snout, 18.7-32.6% of head length (vs. 33.3-68.6% in Hypopomus, Gymnorhamphichthys, Iracema, and Rhamphichthys), absence of a paired accessory electric organ in the mental or humeral region (vs. presence in Hypopygus and Steatogenys), presence of 3-4 pectoral proximal radials (vs. 5 in Akawaio), presence of the antorbital + infraorbital, and the preopercular cephalic lateral line canal bones (vs. absence in Racenisia). Brachyhypopomus cannot be diagnosed unambiguously from Microsternarchus or from Procerusternarchus on the basis of external characters alone. Brachyhypopomus comprises 28 species. Here we describe 15 new species, and provide redescriptions of all 13 previously described species, based on meristic, morphometric, and other morphological characters. We include notes on ecology and natural history for each species, and provide regional dichotomous keys and distribution maps, based on the examination of 12,279 specimens from 2,787 museum lots. A lectotype is designated for Brachyhypopomus pinnicaudatus (Hopkins, Comfort, Bastian & Bass, 1990). Brachyhypopomus species are abundant in shallow lentic and slow-flowing freshwater habitats from southern Costa Rica and northern Venezuela to Uruguay and northern Argentina. Species diversity is highest in Greater Amazonia, where 20 species occur: B. alberti, new species, B. arrayae, new species, and B. cunia, new species, in the upper rio Madeira drainage; B. batesi, new species, in the central Amazon and rio Negro; B. beebei, B. brevirostris, B. regani, new species, B. sullivani, new species, and B. walteri, widespread through the Amazon and Orinoco basins and the Guianas; B. belindae, new species, in the central Amazon basin; B. benjamini, new species, and B. verdii, new species, in the upper Amazon basin; B. bennetti, in the upper, central, and lower Amazon, lower Tocantins, and upper Madeira basins; B. bullocki in the Orinoco, Negro and Essequibo drainages; B. diazi in the Orinoco Llanos; B. flavipomus, new species, and B. hamiltoni, new species, in the central and upper Amazon basin; B. hendersoni, new species, in the central Amazon, lower Negro and Essequibo basins; B. pinnicaudatus in the central and lower Amazon, lower, upper Madeira, lower Tocantins and Mearim basins, and coastal French Guiana; and B. provenzanoi, new species, in the upper Orinoco and upper Negro basins. Five species are known from the Paraná-Paraguay-Uruguay basin and adjacent southern Atlantic drainages: B. bombilla in the lower Paraná, upper, central, and lower Paraguay, Uruguay and Patos-Mirim drainages; B. brevirostris in the upper Paraguay basin; B. draco in the lower Paraná, lower Paraguay, Uruguay, Patos-Mirim, and Tramandaí basins; B. gauderio in the lower Paraná, upper, central, and lower Paraguay, Uruguay, Patos-Mirim and Tramandaí basins; and B. walteri in the lower Paraná and upper Paraguay basins. Two species occur in small Atlantic drainages of southern Brazil: B. janeiroensis in the São João, Paraíba and small intervening drainages; and B. jureiae in the Ribeira de Iguape and Una do Prelado. One species occurs in the middle and upper São Francisco basin: B. menezesi, new species. Three species occur in trans-Andean drainages: B. diazi in Caribbean drainages of northern Venezuela; B. occidentalis in Atlantic and Pacific drainages of southern Costa Rica and Panama to Darién, and the Maracaibo, Magdalena, Sinú and Atrato drainages; and B. palenque, new species, in Pacific drainages of Ecuador.

Relaciones biogeográficas de los grandes ríos de la cuenca del Plata basadas en ensambles de serpientes

The large rivers of the Plata flow southward from intertropical to temperate latitudes, extending the distribution of tropical species. We tested biogeographic hypotheses about relationships among sections of the rivers comparing their snake fauna: the High Paraná has greater affinity with the High Uruguay, the Middle Paraná with Lower Paraguay regarding other sections of Paraná itself, and the Upper Delta has greater affinity with the Middle Paraná and the Lower Delta with Lower Uruguay. We compared sections of rivers and these with phytogeographical units, by means of 13,039 records of 104 taxa, using parsimony analysis of endemicity and similarity. Our results support the first two hypotheses, not the third one. Upper Delta-Lower Delta formed a group with high similarity related to the Middle Uruguay-Lower Uruguay, and with PAE it formed a separate clade. Sections were associated with the bioregions that they crossed, although the Middle-Lower Uruguay did it only with the Parana province (parsimony analysis of endemicity) due to Atlantic-Parana species reaching higher latitudes by this river. The Middle Paraná-Lower Paraguay-Upper Paraná are related to Humid Chaco, being discussed previous published criteria of regionalization. Because of the overlap of tropical and temperate biotic components, larger Plata Rivers are biogeographic transition areas that are relevant to be preserved.

Mediative adjustment of river dynamics: The role of chute channels in tropical sand-bed meandering rivers

This paper examines processes of chute channel formation in four tropical sand-bed meandering rivers; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics-based modelling work. GIS analyses indicate that bend widening may be driven by a variety of mechanisms, including scour and cutbank bench formation at sharply-curving bends, point bar erosion due to cutbank impingement against cohesive terrace material, rapid cutbank erosion at rapidly extending bends, and spontaneous mid-channel bar formation. Chute channel initiation is observed to be predominantly associated with two of these widening mechanisms; i) an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension, and ii) bank erosion forced by spontaneous mid-channel bar development. The work extends previous empirical analyses, which highlighted the role of bend extension (elongation) in driving chute initiation, with the observation that the frequency of chute initiation increases once bend extension rates and/or widening ratios exceed a reach-scale threshold. A temporal pattern of increased chute initiation frequency on the Ok Tedi, in response to channel steepening and mid-channel bar development following the addition of mine tailings, mirrors the inter- and intra-reach spatial patterns of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power and sediment load are associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy, reminiscent of slope–ratio thresholds in river avulsion. Furthermore, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation may also be rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy (and sediment load), predominantly affecting sites of rapid channel elongation, and thereby mediating the river response.

Chute channel dynamics in large, sand‐bed meandering rivers

ABSTRACTMeander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley & Sons, Ltd.

A panbiogeographical model to prioritize areas for conservation along large rivers

AbstractAim The large rivers of the Neotropics are considered areas of high diversity and endemism, which play an important role in the distribution patterns and evolution of Neotropical biota. Several methods have been proposed for prioritizing terrestrial conservation areas, but there has been little effort to develop models for river systems. We propose a panbiogeographical approach to identify priority areas for conservation along rivers.Location The Plata Basin rivers.Methods We compared the individual tracks (IT) of 96 snake taxa and identified the species associated with rivers using the concept of preferential direction of distribution. For each taxon, we measured the angular deviations between the line of its IT and the course of the rivers on a 100 × 100 km scaled grid. Average angular values < 45° indicated a positive association with rivers. We detected 35 taxa associated with rivers, and their IT were used to determine the generalized tracks (GT) and nodes. We applied a complementarity algorithm to identify the minimum set of nodes required to represent all species.Results Six nodes were found. The region where the High and Upper Paraná Rivers converge (Node 1) is of first priority, with 60 of 96 species. The second priority is the Lower Paraguay River and northern section of Middle Paraná River (Node 2). The third is the High Paraná, which together with Nodes 1 and 2, comprises 94% of the total species. The fourth and fifth are the High and Middle Uruguay, and the western section of the Upper Paraná and Iberá Marsh system, respectively. These five nodes include all species.Main conclusions Our results highlight the areas of particular interest for the conservation of rivers and provide a biogeographical algorithm for detecting priority conservation areas. The nodes are a biogeographical approach that allows evolutionary and ecological traits to be included in conservation assessment.

Influence of large South American rivers of the Plata Basin on distributional patterns of tropical snakes: a panbiogeographical analysis

AbstractAim The main drainages of the Plata Basin – the Paraná, Paraguay and Uruguay rivers – begin in tropical latitudes and run in a north–south direction into subtropical–temperate latitudes. Consequently, the biota of these rivers has tropical elements that contrast with temperate biomes through which the rivers run. We apply a panbiogeographical approach, to test whether the large rivers of the Plata Basin have a differential influence on distributional patterns of tropical snakes in subtropical and temperate latitudes of South America.Location Subtropical and temperate sections of the major Plata Basin rivers, South America.Methods We compared the individual tracks of 94 snake taxa. The track analysis consisted of: (1) plotting the localities of each taxon on maps, (2) connecting the localities of each taxon using a minimal geographical proximity determinant of the ‘individual tracks’, and (3) superimposing the individual tracks to determine generalized tracks. To detect tropical snakes that reach higher latitudes through the rivers we used the preferential direction of distribution concept. For each taxon we measured the angular deviations between the line of its individual track and the course of the rivers in a 100 × 100 km scaled grid. Average angular values < 45° indicated a positive association with the rivers.Results Thirty‐five of 94 taxa showed distributions associated with the major rivers of the Plata Basin, including fauna from distinct biogeographical lineages, supported by the occurrence of five generalized tracks as follows: (1) the Paraguay–Middle Paraná, (2) the Paraguay–Paraná fluvial axis, Upper Paraná and Middle Paraná to Upper Delta, (3) the Lower Paraguay, Paraná and Uruguay rivers, excluding the sectors High Paraná and High Uruguay, (4) the Uruguay River and Upper Paraná, and (5) the High Paraná. The Atlantic species occurred with significantly higher frequency in the Uruguay River and High Paraná river sections, the Amazon species were found with significantly higher frequency in the Paraguay and Middle Paraná sections, and the species with a Pantanal distribution were found in all sections.Main conclusions The observed distributional patterns may be explained by the interaction of ecological, geographical and historical factors. Previous authors have developed ecological (hydrological or environmental similarity) or dispersalist (effect of rivers as migration routes) explanations. The coincidence between generalized tracks and past geomorphological events that caused displacements and changed relationships between the Paraguay, Paraná and Uruguay river sections supports hypotheses involving the strong influence of historical factors in the present configuration of tropical snake distribution in temperate latitudes.

Biogeography of Glandulocaudinae (Teleostei: Characiformes: Characidae) revisited: phylogenetic patterns, historical geology and genetic connectivity

The biogeography of the Glandulocaudinae (former Glandulocaudini) is reviewed. The major pattern of diversification presented by this group of freshwater fishes can be clearly associated to the main aspects of the tectonic evolution of the southern portion of the Cis-Andean South American Platform. The phylogenetic relationships within the group suggest that the clade represented by Lophiobrycon is the sister-group of the more derived clade represented by the genus Glandulocauda and Mimagoniates. Lophiobrycon and Glandulocauda occur in areas of the ancient crystalline shield of southeastern Brazil and their present allopatric distribution is probably due to relict survival and tectonic vicariant events. Populations of Glandulocauda melanogenys are found in contiguous drainages in presently isolated upper parts of the Tietê, Guaratuba, Itatinga, and Ribeira de Iguape basins and this pattern of distribution is probably the result of river capture caused by tectonic processes that affected a large area in eastern and southeastern Brazil. The species of Mimagoniates are predominantly distributed along the eastern and southeastern coastal areas, but M. microlepis is additionally found in the rio Iguaçu and Tibagi basins. Mimagoniates barberi occurs in both SW margin of the upper rio Paraná basin and the lower Paraguay and Mimagoniates sp. occurs in the upper Paraguay river basin. Tectonic activations of the Continental Rift of Southeastern Brazil along the eastern margin of the Upper Paraná basin promoted population fragmentation responsible of the present day distribution presented by Glandulocauda melanogenys. We hypothesize that occurrence of Mimagoniates along the lowland area around the Paraná basin was due to a single or a multiple fragmentation of populations along the W-SW border of the upper Paraná Basin, probably due to the major tectonic origin of the Chaco-Pantanal wetland foreland basins since the Miocene as well as Cenozoic tectonic activity along the borders of the upper Paraná basin, such as in the eastern Paraguay, in the Asunción Rift. Distributional pattern of Mimagoniates suggests that its initial diversification may be related to the tectonic evolution of the Chaco-Pantanal foreland basin system and a minimum age of 2.5 M.Y are proposed for this monophyletic group. Previous hypotheses on sea level fluctuations of the late Quaternary as being the main causal mechanism promoting cladogenesis and speciation of the group are critically reviewed. Phylogeographic studies based on molecular data indicate significant differences among the isolated populations of M. microlepis. These findings suggest that a much longer period of time and a paleogeographic landscape configuration of the Brazilian southeastern coastal region explain the present observed phylogenetic and biogeographic patterns.

The Lower Paraguay river-floodplain habitats in the context of the Fluvial Hydrosystem Approach

We report herein the first description of the physical structure of the aquatic habitats of the Lower Paraguay River along 390km from Asunción city (Paraguay) to the confluence with the Paraná River. The hierarchical ordination of the Fluvial Hydrosystem Approach (FHA) allowed us to classify the Lower Paraguay as a meandering functional sector where five functional sets were identified: (a) main channel, (b) floodplain channel, (c) floodplain lentic water body, (d) tributary, and (f) aquaticterrestrial transition zone. These functional sets encompassed twenty one functional units and fifty eight major mesohabitats. We attribute the riverine habitat diversity to the changes in the channel-floodplain morphology and in the strength, duration and frequency of their hydrological connectivity. The variable river-floodplain-tributary complex developed several types of fluvial-lacustrine boundaries and riverine ecotones.

Channel-floodplain geomorphology and connectivity of the Lower Paraguay hydrosystem

The Paraguay River is the main tributary of the Paraná River hydrosystem. Despite several studies resulting from the proposed navigation project known as the Hidrovía Paraguay-Paraná, little is known about the physical structure of their aquatic environments and its biota. The main purpose of this work is the knowledge of the primary factors which form the structure of the physical habitats in this fluvial segment. In this way, the hydrological and hydraulic regime, channel shape, substrate, hydrological connectivity and the floodplain ontogeny and its evolution were analyzed. Synthesizing we stressed herein that the dynamics of the river-floodplain morphology dependent on the large-scale longitudinal and lateral hydrological connections, and the type and degree of these connections between lotic and lentic environments drive the changes of this seasonally inundated floodplain and its water bodies. In a second paper in this volume, is presented a classification of the Lower Paraguay physical habitats and their relationships with the main physical factors.

Benthos of a large neotropical river: spatial patterns and species assemblages in the Lower Paraguay and its floodplains

We studied the composition and spatial structure of benthic invertebrate assemblages and their physical and chemical environment on a 331 km long section of the Lower Paraguay River between Asuncion (Paraguay) and the river's confluence with the Parana River near Corrientes (Argentina). Applying the same methods used in former studies in the Upper Paraguay and Parana Rivers, we sampled bottom fauna, bed sediments and water at ten riverine stations, two tributaries, and six floodplain lake sites. Seventy-six taxa, mostly oligochaetes (18 species) and chironomids (17 species), were identified. There was a clear distinction between central sites, river banks, and floodplain habitats. Highly significant regressions between sediment composition and biotic parameters were obtained (clay/diversity, R = 0.60, p < 0.0001; silt/diversity, R = 0.61, p < 0.0001; organic matter/diversity, R = 0.63, p <0.0001; coarse-middle sand/density, R = 0.75, p = 0.001). Central channel sites were dominated by the psammophilous oligochaete Narapa bonettoi, which attained densities of 10-3,500 ind. m -2 . The river banks had higher diversity indices, but lower average densities than the main channel center. Some sites with solid substrates were densely colonized by an invading mollusk species, Limnoperna fortunei, which had maximum densities of 13,500 ind. m -2 . Diversity was highest in the floodplain lakes (Shannon's H = 2.4 ± 0.67) compared with banks (H = 1.79 ± 0.77) and central sites above the confluence of the Bermejo River (H = 0.89 ± 0.35). This tributary carries large amounts of fine sediments and organic matter into the Paraguay River, thereby reducing biodiversity and abundance of Narapa bonettoi. The faunal composition of the Lower Paraguay was more similar to that of the Middle Parana than to that of the Upper Paraguay. From this and former studies, two general patterns of river ecology become apparent: 1) naturally flowing sand-bed rivers can have continuous species assemblages over very long distances, characterized by few, well-adapted species that occur in high densities, but contribute relatively little biomass to the riverine foodweb; and 2) the biodiversity in the floodplain water bodies is higher than in the river channel as a result of the greater habitat diversity.

Fish populations in the Paraguay river waters of the Formosa area, Argentina

Fish populations of water bodies of the lower Paraguay River at Formosa area (26° 10´S ‐ 58° 10´W) sampled during 1971 are studied. Qualitative and quantitative structure of fish communities, as well as dominance and occurrence, and size of fishes are analysed. Some limnological parameters, such as surface area, depth, and plant cover of the lagoons are included. Comparisons are made between the fish fauna present in these water bodies and those caught in other ones of similar characteristics located in the middle Paraná River.