South American Basins Research Articles

Isolation and characterization of the first microsatellite markers in the Neotropical freshwater fish piau-verdadeiro, Leporinus obtusidens (Valenciennes, 1837)

Ten microsatellite markers were isolated and characterized for piau-verdadeiro (Leporinus obtusidens), an important Characiform fish species in South American basins. The markers were obtained from an enriched microsatellite genomic library, and polymorphisms were evaluated in a sample of 44 individuals caught in the upper Sao Francisco River. The number of alleles varied from 4 to 39. The observed and expected heterozygosities ranged from 0.614 to 0.977 and from 0.541 to 0.977, respectively. All markers were in Hardy–Weinberg equilibrium and no null alleles were detected, thus making them suitable for assessing the diversity structure of the populations of this species.

Multi-step change of scale approach for deriving coarse-resolution flow directions

When applying a large-scale grid distributed hydrological model, it is necessary to establish the connection of cells through a low-resolution river drainage network (RDN). The state-of-the-art approach to carry out this task starts at determining fine-resolution flow directions and then upscaling this information to the low-resolution grid. Several upscaling algorithms have been proposed recently and very reasonable results have been achieved. This paper aims at testing the following hypothesis: would the low-resolution RDN better resemble the finer-resolution RDN if the flow direction upscaling were applied iteratively with gradual changes in scale, instead of a direct jump from the finer to the coarser resolution? To test this hypothesis, we apply one of the recent upscaling algorithms proposed in the literature in two different ways: 1) following the conventional approach, and 2) through a multi-step procedure. Four large-scale South American basins are taken as study cases (drainage areas ranging from 20,000 to 642,000km²). High-resolution (0.001° or 0.002°) RDN is extracted from SRTM-DEM and used to obtain low-resolution RDN of 0.1–0.5°, according to each basin. The results presented here show that the multi-step upscaling approach outperformed the conventional direct approach regarding the reproduction of high-resolution network patterns. Although this improvement often affects only a small number of cells (5–7% of the total), the impact of the changes can be significant when factors such as river flow paths, junctions and contributing areas are considered. This implies that the proposed approach may be useful in achieving a more physically-sound hydrological model, though it does not eliminate the need to analyze the results carefully and make occasional corrections.

Late Cretaceous palynostratigraphy in the Gongola Basin (Upper Benue Trough, Nigeria)

Palynostratigraphic study that utilizes mainly spores and pollen was done on the Cretaceous sequences of the well Nasara-1 of the Gongola Basin of the Upper Benue Trough, Nigeria. The recovered spores and pollen in the well made the identification of five palynological zones possible: Droseridites senonicus (Coniacian–Santonian), Cretacaeiporites scabratus (Turonian), Triorites africaensis (Upper Cenomanian), Afropollis jardinus (Lower–Middle Cenomanian) and Elateroplicites africaensis– Elaterosporites klaszii– Elaterosporites protensus (Upper Albian). These zones are similar and correlatable to other Cretaceous palynologic zones in the West Africa and northern South American basins.

The Late Palaeozoic of Western Gondwana: New insights from South American records

Late Palaeozoic basins of southern South America offer an excellent opportunity to study sequences deposited in contrasting tectonic regimes, shifting climatic conditions and different sea level positions. From the tectonic point of view, these Late Palaeozoic basins developed in three scenarios (Limarino and Spalletti, 2006): arc-related or Proto-Pacific basins, retroarc or peripheral basins, and large intracratonic or intraplate basins (Fig. 1). Arc-related or Proto-Pacific basins formed along the westernmost and highly mobile area of Gondwana, in which volcanism played a fundamental role. In turn, retroarc or peripheral basins which are characterized by a thick fossiliferous Late Mississippian up to Late Permian record, registered less magmatic activity and minor deformation. Finally, large intracratonic or intraplate basins developed under more stable tectonic regimes and their sedimentary record was mainly controlled by significant climatic and eustatic oscillations (e.g. Parana Basin). The effects of dramatic climatic changes and/or eustatic oscillations on Late Palaeozoic basins have been widely studied. Alternating glacial and interglacial periods as well as processes of progressive aridification towards the Middle and Late Permian times have been recognized (Fig. 2). Moreover, a complete record of sea level changes was defined in the intracratonic basins from the Pennsylvanian to the Latest Permian (Fig. 2).Despite the importance of the aforementioned issues, the stratigraphic and sedimentological information obtained from South American basins has been frequently overlooked in regional or global-scale studies. Although this may be attributed to the lack of knowledge about the Carboniferous and Permian Systems in southern South America, there is abundant literature on the sedimentary and palaeontological record of these basins. This is clearly documented in specific papers and in comprehensive compilations, such as those of Bigarella et al. (1966), Archangelsky (1987, 1996), and Veroslavsky et al. (2006), among others.

Dams and the fish fauna of the Neotropical region: impacts and management related to diversity and fisheries

Reservoirs have been built in almost all of the hydrographic basins of Brazil. Their purposes include water supply for cities, irrigation and mainly, generation of electricity. There are more than 700 large dams and associated reservoirs in the large rivers of the country. These reservoirs favor local and regional economic development, but they also bring serious and irreversible alterations in the natural hydrologic regime of rivers, affecting habitat quality and the dynamics of the biota. In the impounded area, the main impact is the change from lotic to lentic water, which influences aquatic fauna, including fishes. Impacts of reservoirs present relevant spatiotemporal variations. Immediately after reservoir formation, fish species richness usually increases due to incorporation of surrounding habitats, but richness decreases as reservoirs age. However, impacts downstream of dams appear to be similar or stronger than those that occur within the reservoir. Dams promote discharge control, altering the seasonal cycles of floods. These effects are augmented when dams are constructed in cascades. Therefore, dams profoundly influence composition and structure of fish assemblages. Most affected species are the rheophilics and long distance migratory that require distinct habitats to fulfill their life cycles. Populations of migratory species may collapse or even disappear in intensely regulated stretches. Management actions taken to minimize impacts of dams in Brazil historically considered construction of fish passages, fishery control and stocking. The results of these actions are questionable and/or with clear failures. In this paper, we give emphasis to the Paraná River basin, the most affected by dams in Brazil. We describe some patterns in the alteration and decline in fish diversity in areas influenced by dams. We also discuss negative consequences in the fishery and ecosystems functioning. Finally, we argue the relevance and the success of the management actions taken and present some suggestions to improve conservation of the ichthyofauna in South American basins influenced by dams.

Fish fauna destruction after the introduction of a non-native predator (Cichla kelberi) in a Neotropical reservoir

In South America, the introduction of peacock-bass (Cichla), a voracious predator fish, has been an underestimated threat for native fish communities. Although this predator is widespread in many reservoirs, few studies have explored its impact on biodiversity. To investigate the relationship between invasion and fish diversity, the present study followed a natural experiment in the Rosana Reservoir (Parana River basin), where Cichla kelberi were introduced in 2004. We monitored fish assemblages associated with submerged macrophytes between 2003 and 2007, using a 1 m2 throw trap. In the years following the introduction, fish diversity dramatically changed. For example, in March 2007, mean fish density and richness were reduced by ca. 95 and 80%, respectively, and many small-sized species had vanished. One aspect was the gradual change of biodiversity, which unfolded at two times during each year: (1) impacts during summer/autumn periods, which coincided with large shoals of young C. kelberi in the patches; and (2) assemblage recovery during the spring. The sequence of extinction-colonization events, however, might not be able to maintain fish assemblages due to the decrease in recovery intensity each spring; assuming a constant decline rate in the coming years, we predict complete assemblage extinction by the summer of 2010. Results from this natural experiment provided evidence supporting the collapse of fish assemblages soon after the introduction of C. kelberi. Such rapid destruction (2 years) reveals an important homogenizing force behind this predator and stresses the need for control measures that prevent new transferences among South American basins.

Patterns of genetic differentiation in the Gymnogeophagus gymnogenys species complex, a neotropical cichlid from South American basins

The neotropical cichlid genus Gymnogeophagus is distributed in the Rio de la Plata basin and in Dos Patos and Merin coastal lagoons on the border between Uruguay and southern Brazil. A phylogeographic approach based on mitochondrial cytochrome b analysis was performed to assess the patterns and processes of differentiation in this taxon. Gymnogeophagus gymnogenys showed high haplotype diversity (H = 0.992) and corrected mtDNA genetic distances ranged from 0 to 5.3%. Our analyses yielded robust support for the existence of four monophyletic groups within G. gymnogenys from the analyzed basins. No correlation between the aforementioned clades and geographic structure was found, since individuals belonging to different phylogenetic clades inhabit the same locality. The phylogeographic approach presented here showed that these four phylogroups (1, 2, 3 and 4) were sister groups. Our present findings would corroborate that G. gymnogenys could be integrated by different phylogenetic lineages, showing an explosive differentiation pattern and confirming the hypothesis that this taxon constitutes a species complex.

Applicability of LOICZ catchment–coast continuum in a major Caribbean basin: The Magdalena River, Colombia

Within the Land Ocean Interactions in the Coastal Zone (LOICZ)-Basins approach, the Magdalena River Project (MRP) is an interdisciplinary research, which aims to improve the scientific understanding of the linkages between the Magdalena drainage basin and its associated coastal environments. The MRP is an outgrowth of the initial regional planning that resulted from the LOICZ South American Basins (SamBas) and Caribbean Basins (CariBas) studies on land use and hydrological changes during approximately the past century in tropical and temperate benchmark river basins. The results of the MRP presented in this article show that the extent of land-cover change and erosion within the catchment has increased over the last 10–20 yr. The overall increasing trends in sediment load on a regional scale may be attributed to a range of anthropogenic influences including: a 40% decrease in forests over a 20-yr period; a 65% increase in agricultural and pasture; poor practices of land use; mining; and increasing rates of urbanization. These increasing trends in sediment load coincide with the overall decline of live coral cover in a 145-km 2 coral reef complex in the Caribbean Sea. In addition, the impacts of heavy sediment loads and freshwater discharges have greatly contributed not only to the total disappearance of coral formations but also to a considerable reduction in abundance of seagrass beds in Cartagena Bay and neighbouring areas. The synthesis and analysis presented in this article are just first steps toward understanding the natural and human-induced factors that have produced the observed patterns of water discharge and sediment load of the Magdalena River into the Caribbean Sea, and to relating these processes to the impact on coastal ecosystems.

Dispersion and Ecological Impact of the Invasive Freshwater Bivalve Limnoperna fortunei in the Río de la Plata Watershed and Beyond

Limnoperna fortunei is a freshwater bivalve that invaded South America through Rio de la Plata estuary in 1989 and has since become a major macrofouling pest. Along the Parana-Paraguay waterway, which hosts intense boat traffic, L. fortunei has moved upstream at an average rate of of 250 km per year. In contrast, along the Uruguay river, where boat traffic is restricted to the lowermost 200 km section, upstream colonization is almost 10-times slower. This suggests that attachment to vessels is by far the most important dispersion mechanism. It is suggested that the Amazon, Orinoco and Magdalena basins are under high risk of invasion by this mussel, especially through their estuarine gateways. All South American basins host innumerable water bodies with favorable conditions for L. fortunei’s colonization. Known ecological tolerance limits of the mussel also suggest that it may colonize much of the area from Central America to Canada, including waters that due to their low calcium contents, high temperature and pollution levels, and low oxygen are inadequate for the survival of Dreissena polymorpha. Despite it’s remarkable geographic expansion and its extremely high population densities, L. fortunei’s ecological effects have received very little attention so far. It is suggested that the 2.4-fold increase in Argentine landings of freshwater fish between 1992–1993 and 2000–2001 may be associated with the introduction of this prey species.

Cytogenetics and biogeography: considerations about the natural origin of Hoplias malabaricus (Characiformes, Erythrinidae) on the Iguaçu river

Hoplias malabaricus (trahyra) is a widespread fish species over the Neotropical region with diversified inter-populational karyotypes (cytotypes), which may correspond to a species complex. Despite the wide distribution in the South American basins, some authors have questioned its natural origin in the Iguaçu river, an important Brazilian river basin which is characterized by several endemic fish species. We have analyzed the karyotype of H. malabaricus from different collection sites of this river, by conventional and banding methods. Our results, in addition to our previous data concerning geographic distribution of the cytotypes, contribute to better understand the origin of H. malabaricus on the Iguaçu river, reinforcing the proposition that it is a natural fish species in this river basin.

Variability in the discharge of South American rivers and in climate / Variabilité des débits de rivières d'Amérique du Sud et du climat

Changes in trend and quasi-periodicities are sought in the time series of river discharges in all major South American basins. The relationship between trends and quasi-periodicities found and climate variations on interannual and longer time scales are discussed. Consideration of multiple rivers gives insight into the geographical extent of hydrological signals and climate impacts. It is found that the streamflow of all major rivers of South America has experienced an increased trend since the early 1970s. It is suggested that this simultaneity may reflect the impact of a large-scale climate change. All the time series of river streamflows that were analysed show El Niño-like periodicities. Only for La Plata Basin do these explain a larger part of the total variance than the other quasi-periodicities. There are two other quasi-oscillations in the time series analysed: one of them with a longer period—around 17 years—and the other of about 9 years. Previous work has related these oscillations to sea-surface temperature anomalies in the Atlantic Ocean.

An uncommon condition for a sex chromosome system in Characidae fish. Distribution and differentiation of the ZZ/ZW system in Triportheus.

Triportheus is a neotropical freshwater Characidae fish that has a well-differentiated ZZ/ZW sex chromosome system. The W chromosome of this genus contains a large amount of heterochromatin and is smaller than the Z chromosome. This contrasts with other ZW fish systems where the W chromosome is larger in size due to increased heterochromatin. All species of Triportheus that have been studied cytologically (about 50% of the known species for this genus, from some of the major South American hydrographic basins) share this sex chromosome system, indicating a probable synapomorphic condition not present in other genera of the large Characidae family. However, while the Z chromosome appears to be largely conserved, the W chromosome shows a differential evolution with morphological differentiations not only among species, but also among populations from the same hydrographic basin, and with some species presenting a greater homology between the W and the Z chromosomes than others.

Coherent deep flow variation in the Iceland and American basins during the last interglacial

The first direct evidence that the deep flow speed of North Atlantic Deep Water (NADW) in the South Iceland and North American Basins declined dramatically over a period of a few hundred years at about 120,000 years ago is presented. The flow rate inferred from silt grain-size then rose into the subsequent cool period (stage 5d). These changes are clear and synchronous in records from both locations. These circulation events are recorded less markedly in chemical and isotopic proxies for the nutrient status of water masses. The congruence of flow variation and hydrography at the two separated sites suggests they are primarily controlled by variation in the deep thermohaline circulation.

Oil- and gas-resource assessment in certain South American Basins—An application of ARDS (Ver. 5.0) to complex exploration and discovery histories

The modified Arps-Roberts Discovery Process Modeling System [ARDS (Ver. 4.01)] has recently been upgraded [ARDS (Ver. 5.0)] and applied to a wide variety of field discovery and wildcat drilling data with differing characteristics. ARDS is designed to forecast the number and sizes of undiscovered fields in an exploration play or basin by using historical drilling and discovery data. Fields used as input may be grown or ungrown. Two models for field growth—one offshore and the other onshore—have been implemented (Schuenemeyer and Drew, 1996). Uncertainty attributable to field growth is estimated via simulation. This upgrade of ARDS has been designed to handle situations when the data cannot be partitioned into homogeneous regions, but where estimation of the number of remaining oil and gas fields is still meaningful. In this upgrade of ARDS, many restrictions, which include those on the number of fields and wildcat wells required to forecast the size distribution of the oil and gas fields that remain to be discovered in an exploration play, a basin, or other target area, have been removed. In addition, flexibility has been gained by reforming the criteria for convergence of the model. In all, 32 basins and subbasins in South America were examined, 18 of which had sufficient data to be amenable to forecasting the field-size distribution of undiscovered oil and gas resources directly by using the Petroconsultants Inc. (1993) field discovery and wildcat drilling data. Overall, ARDS (Ver. 5.0) performed well in estimating the field-size distribution of undiscovered oil and gas resources in the 18 basins and subbasins. The aggregate volume of undiscovered petroleum resources was characterized by using histograms of the distribution of resources and the following five statistics: the mean, the 80% trimmed mean, and the 10,50 (median), and 90 quantiles. More than 38 billion barrels of oil equivalent (BOE) in fields that contain more than one million BOE individually were forecast as remaining to be discovered. The largest basin, the Campos (Brazil), is forecast to contain nearly 10 billion BOE undiscovered resources. The East Venezuela Basin (excluding the Furrial Trend) is forecast to contain about 8 billion BOE; the Austral-Magallanes Basin (Argentina and Chile), about 7 billion BOE; and the Napo (Colombia and Ecuador) and the Neuquen (Argentina) Basins, between 3 billion and 4 billion BOE. A subset of these basins that illustrate the increased flexibility of ARDS are discussed.

The geological evolution of the Falkland Islands continental shelf

The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.

Undiscovered Petroleum of the Brazilian Interior Sag Basins

This paper, the second in a series dealing with undiscovered petroleum in South American basins (exclusive of Venezuela and Colombia), focuses on the four large intracratonic (or interior) sag basins of Brazil: the Solimoes, Amazonas, Parnaiba, and Parana. The smaller Chaco basin also is discussed, although in less detail. Preliminary estimates of undiscovered hydrocarbons in the five basins are 0.228 billion barrels of oil (BBO) and 11.870 trillion cubic feet of gas (TCFG). Because the mean values originally estimated by the World Energy Resources Program at the U.S. Geological Survey appear to be low when viewed in light of the recent (1983-1993) history of petroleum reserves in South America, the mean values were revised upward based on the original identified reserves. The recalculated mean values are 10.18 BBO and 20.52 TCFG. In general, exploration in these basins is in an immature stage. The Solimoes basin has the greatest initial estimated undiscovered reserves of oil (0.04 BBO), and the Parana basin has the greatest initial estimated undiscovered reserves of gas (5.08 TCFG). The most important plays in the Solimoes and structurally similar Amazonas basins are the fold and fault closures associated with the wrench systems found in the basins. The plays in the Parana basin are limited to possible structural traps linked to faultassociated closures, and possibly to some wrench-related features.

Controls on the Distribution of Cretaceous Source Rocks in South America

More than thirty South American basins, exhibiting a variety of structural styles, contain petroleum source rocks of Cretaceous age. However, the presence of truly [open quote]world-class[close quote] source rocks, capable of supplying multi-billion barrel oil provinces, is restricted to relatively few basins and appears to be primarily a function of large scale Cretaceous tectonic setting. In Early Cretaceous times the best source rocks were preserved in both a southern ocean and in the rift between South America and Africa. By the Late Cretaceous, these southern and eastern continental limits had become narrow passive margins. In contrast, on the northern continental margin a wide shelf to a restricted tropical sea was developing at this time. Periodic upwelling enhanced surface productivity on this shelf, which led to development of some of the world's richest source rocks. On the tectonically active western margin moderate quality source rocks were forming in a series of back-arc basins, whilst further west, in the Pacific fore-arc, organic-rich intervals were rarely deposited. This article documents what is known about each of the explored basins (including the volume and character of discovered petroleums), it investigates the geologic factors which governed the richness and quality of petroleum source rocks andmore » it assesses how continued tectonic activity has modified or even destroyed primary source quality. Finally it predicts which of the as yet underexplored basins should contain good quality source rocks and could become prolific petroleum provinces of the future.« less