Atlantic Drainage Basin Research Articles

Phylogeographic pattern of Jenynsia multidentata(Cyprinodontiformes: Anablepidae) in the southern boundary of the Brazilian Subregion, Argentina

The Atlantic drainage basins are located in the southern Pampean area, in the southernmost part of the Brazilian subregion. Tectonic and paleoclimatic phenomena, occurring during the Quaternary, have currently isolated these basins from the present hydrographic system. Their ichthyofaunal composition is similar to that of hydric systems located further northward. Jenynsia multidentata is a species with wide distribution in the Atlantic drainage basins, as well as in those Northern basins. Here we investigated the phylogeographic pattern of Jenynsia multidentataand analyzed its relationships with the paleoclimatic and geologic history of the region. The analysis of the population's assemblage showed moderate genetic diversity, demographic equilibrium and marked genetic structure in the populations that occupy the extremes of the distributional range. The populations in the center of the range did not present genetic diversity, sharing a single haplotype. These results allow inferring that the presence of Jenynsia multidentata in the area results from historical demographic processes that are independent but complementary. In turn, these processes could arise from paleoclimatic changes occurred during the Quaternary.

Orbital, ice sheet, and possible solar controls on Holocene moisture trends in the North Atlantic drainage basin

Holocene patterns of hydroclimatic change in the North Atlantic region are poorly understood despite the importance to climate of the region9s freshwater flux to the ocean. Here we reconstruct the history of moisture balance in the North Atlantic drainage by compiling lake-level records from 95 lakes in Europe and North America. We show that recent millennia were significantly wetter than much of the Holocene in both regions. Only 15% of lakes studied in the North Atlantic basin were low in the past millennium. In contrast, 50% of lakes were low at 10 cal kyr B.P. Low moisture levels in the early Holocene likely resulted from high summer and fall insolation and the influence of the continental ice sheets; water levels rose in many areas as these influences diminished. A principal components analysis of the lake-level data, however, also indicates that Holocene atmospheric circulation changes, possibly driven by solar activity, generated additional moisture variation, including low water levels at 5–2 cal kyr B.P. in eastern North America and Scandinavia.

Population genetics of a cyclostome species pair, river lamprey (Lampetra fluviatilis L.) and brook lamprey (Lampetra planeri Bloch)

Horizontal agarose gel electrophoresis of 24 allozyme loci in four species of Central European lampreys (321 Lampetra planeri, 83 L. fluviatilis, 11 Eudontomyzon mariae and nine Petromyzon marinus) was used to study the ‘paired species’L. fluviatilis and L. planeri. The genetic differentiation of the anadromous river lamprey (L. fluviatilis) from the stationary brook lamprey (L. planeri) was within the range of ingroup differentiation of the latter, but L. fluviatilis exhibited much greater population cohesion over a more extended geographic range: GST = 0.0537 versus GST = 0.3398, Nem = 4.402 versus Nem = 0.4856, mean genetic among-stock distances D = 0.0047 versus D = 0.0257. L. planeri populations coexisting geographically with L. fluviatilis in the Rhine and Elbe river systems were genetically more cohesive than L. planeri stocks from the Danubian basin where L. fluviatilis is absent. Danubian L. planeri populations exhibit a lower degree of heterozygosity than brook lampreys from the Rhine river system, but comprise deeper genetic lineages (GST = 0.4629 versus GST = 0.2434), despite being sampled from a much more restricted area. Isolation-by-distance is observed for L. planeri from the Danubian but not from the Atlantic drainage basins. Transspecific gene flow between L. planeri from Atlantic drainage basins and the long-distant migrating L. fluviatilis is inferred, raising doubt on the validity of two separate biospecies. E. mariae and P. marinus are clearly differentiated from Lampetra spp. at several allozyme loci.

A feather hydrogen isoscape for Mexico

Developing useful biological isoscapes for areas of the world is a priority. This is the case for Mexico that hosts a large percentage of North America's Neotropical migrant birds. Here we investigated the use of House Sparrow ( Passer domesticus) feathers to create a spatially explicit feather deuterium isoscape for that country using samples ( n = 461) that were collected across Mexico. Considerable and useful spatial hydrogen isotopic structure was observed, suggesting that isotopes may be a potential forensic tool for evaluating origins of Mexican derived fauna and flora. The most positive feather δ D values occurred in the northeast and most negative in the south-central part of the country, roughly matching δ D patterns observed in groundwater. A weak negative isotopic relationship was found with altitude in both the Pacific and Atlantic drainage systems. The most parsimonious model describing isotopic spatial variation in feathers between 300 and 3000 m a.s.l. included groundwater δ D (δ D gw; precipitation proxy), sex, amount of precipitation, and the coefficient of variation in amount of precipitation. Overall, δ D gw was a poor predictor of sparrow δ D f values for all of Mexico. However, this relationship was considerably strengthened when we considered sex separately, removed the Baja peninsula from our sample, and considered the Atlantic and Pacific drainage basins separately. The strongest relationship between δ D gw and δ D f was found for female sparrows in the Atlantic drainage basin ( r 2 = 0.464). We recommend that researchers interested in inferring origins of migratory birds and other animals in Mexico create species specific isotopic basemaps that may be guided by the isotopic patterns we have observed for House Sparrows and groundwater.

Changes in the hydrological cycle, ocean circulation, and carbon/nutrient cycling during the last interglacial and glacial transition

A complex Earth system model has been forced by insolation changes for the last interglacial (LIG) for which geological records evidence a climate cooling that culminated in the last glacial inception. During an early warm period (127–125 ka B.P.) the Arctic/North Atlantic drainage basin north of 30°N receives ∼27,000 m3/s more fresh water mainly imported across the watershed of North America compared to a cooler period centered around 115 ka B.P. Together with a strong surface warming by up to 2 K, this results in a weaker North Atlantic overturning (reduced by ∼20%). Sea ice cover is reduced by 3 × 106 km2 in the Northern Hemisphere, stimulating a stronger productivity in the Barents and Kara seas. In the North Pacific a lowered salinity and a surface warming of more than 3 K increase the stratification. The eastern Pacific becomes nutrient poorer, which leads to a reduction in the export production in areas with vigorous upwelling. The climate cooling in the course of the LIG induces a carbon release by the terrestrial biosphere of 308 Gt, which is counteracted by an oceanic/sedimentary uptake of 290 Gt. The rest remains in the atmosphere increasing the pCO2 by 9 ppm. The large oceanic CO2 uptake corresponds to a rise of the calcite lysocline of about 300 m in the midlatitude Atlantic. The chemically interactive carbonate sediment pool in the Atlantic is reduced by 10%. For nearly the entire North Atlantic, our results are contradictory to the common interpretation of carbonate preservation proxies as a reliable tool for monitoring changes in deep water circulation.

The Impact of Tropical Atlantic Freshwater Fluxes on the North Atlantic Meridional Overturning Circulation

Abstract The influence of ENSO-related changes in the Atlantic-to-Pacific freshwater budget on the North Atlantic meridional overturning is examined using the University of Victoria (UVic) Earth System Climate Model. The initial analysis of freshwater fluxes in the 50-yr NCEP–NCAR (NCEP50) reanalysis product and Global Precipitation Climatology Project (GPCP) dataset reveals that the transport of water vapor out of the tropical Atlantic drainage basin is enhanced during El Niño phases and reduced during La Niña phases; a one standard deviation in the Southern Oscillation index alters the tropical Atlantic freshwater balance by about 0.09 Sv (Sv ≡ 106 m3 s−1). A weaker link with ENSO is found in the 40-yr ECMWF Re-Analysis (ERA-40), although its usefulness is severely limited by a strong and spurious trend in tropical precipitation. Model results suggest that tropical Atlantic salinity anomalies generated with the frequency and amplitude of ENSO tend not to impact deep-water formation as they are diluted en route to the North Atlantic. Lower frequency, decadal time-scale anomalies, however, do have an impact, albeit weak, on the rate of North Atlantic Deep Water formation. In addition, and contrary to earlier results, it is found that even a shift of the tropical Atlantic freshwater balance toward permanent El Niño conditions only slightly mitigates the transient reduction of North Atlantic Deep Water formation associated with the increase of anthropogenic greenhouse gases. Taken together, the results suggest that the poleward propagation of salinity anomalies from the tropical Atlantic, associated with changes in ENSO, should not be considered a significant mechanism for the variability of the North Atlantic meridional overturning in the present and foreseeable future climate.

Water Environments: Anthropogenic Pressures and Ecosystem Changes in the Atlantic Drainage Basins of Brazil

Densely occupied drainage basins and coastal zones in developing countries that are facing economic growth are likely to suffer from moderate to severe environmental impacts regarding different issues. The catchment basins draining towards the Atlantic coast from northeastern to southern Brazil include a wide range of climatic zones and diverse ecosystems. Within its borders lies the Atlantic rain forest, significant extensions of semiarid thorn forests (caatinga), vast tree and scrub woodlands (cerrado) and most of the 6670 km of the Brazilian coast and its marine ecosystems. In recent decades, human activities have increasingly advanced over these natural resources. Littoralization has imposed a burden on coastal habitats and communities. Most of the native vegetation of the cerrado and caatinga was removed and only 7% of the original Atlantic rainforest still exists. Estuaries, bays and coastal lagoons have been irreversibly damaged. Land uses, damming and water diversion have become the major driving forces for habitat loss and aquatic ecosystem modification. Regardless of the contrast between the drought-affected northeastern Brazil and the much more prosperous and industrialized southeastern/southern Brazil, the impacts on habitat and communities were found equally severe in both cases. Attempts to halt environmental degradation have not been effective. Instead of focusing on natural resources separately, it is suggested that more integrated environmental policies that focus on aquatic ecosystems integrity are introduced.

Water Environments: Anthropogenic Pressures and Ecosystem Changes in the Atlantic Drainage Basins of Brazil

Densely occupied drainage basins and coastal zones in developing countries that are facing economic growth are likely to suffer from moderate to severe environmental impacts regarding different issues. The catchment basins draining towards the Atlantic coast from northeastern to southern Brazil include a wide range of climatic zones and diverse ecosystems. Within its borders lies the Atlantic rain forest, significant extensions of semiarid thorn forests (caatinga), vast tree and scrub woodlands (cerrado) and most of the 6670 km of the Brazilian coast and its marine ecosystems. In recent decades, human activities have increasingly advanced over these natural resources. Littoralization has imposed a burden on coastal habitats and communities. Most of the native vegetation of the cerrado and caatinga was removed and only 7% of the original Atlantic rainforest still exists. Estuaries, bays and coastal lagoons have been irreversibly damaged. Land uses, damming and water diversion have become the major driving forces for habitat loss and aquatic ecosystem modification. Regardless of the contrast between the drought-affected northeastern Brazil and the much more prosperous and industrialized southeastern/southern Brazil, the impacts on habitat and communities were found equally severe in both cases. Attempts to halt environmental degradation have not been effective. Instead of focusing on natural resources separately, it is suggested that more integrated environmental policies that focus on aquatic ecosystems integrity are introduced.

Forcing of the deep ocean circulation in simulations of the Last Glacial Maximum

From the interpretation of different proxy data it is widely believed that the North Atlantic thermohaline circulation during the maximum of the last ice age ∼21,000 years ago was considerably weaker than today. Recent equilibrium simulations with a coupled ocean‐atmosphere‐sea ice model successfully simulated a reduction in North Atlantic Deep Water (NADW) formation consistent with reconstructions. Here we examine the influence of different air‐sea fluxes on simulated changes in the deep ocean circulation between the Last Glacial Maximum and present day. We find that changes in the oceanic surface freshwater fluxes are the dominant forcing mechanism for the reduced Atlantic overturning. Diminished export of freshwater out of the Atlantic drainage basin through the atmosphere decreases surface salinities in the North Atlantic, leading to less NADW formation in the colder climate. Changes in heat fluxes, which lead to increased sea surface densities in the North Atlantic and therefore to an enhanced overturning, are of secondary importance. Wind stress variations seem to play a negligible role. The degree to which the Atlantic freshwater export and hence the NADW formation are reduced depends on the formulation of the atmospheric hydrological cycle and on the strength of the overturning in the present‐day simulation. Simulated changes in sea surface properties for a large variety of overturning strengths are compared with different reconstruction data sets. The results depend strongly on the data set used. Sea surface temperature reconstructions from Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) and earlier salinity reconstructions based on planktonic foraminifera are most consistent with a significant reduction of the circulation, while recent reconstructions using dinocyst assemblages allow no unequivocal conclusion.

The role of the individual air-sea flux components in CO 2 -induced changes of the ocean's circulation and climate

In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO2 concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO2 concentrations have been performed. In a set of sensitivity experiments either freshwater (precipitation, evaporation and runoff from the continents) and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%, each. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the Atlantic overturning circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem in almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes. Important effects of the momentum fluxes are a poleward shift of the front between Northern Hemisphere subtropical and subpolar gyres and a southward shift in the position of the Antarctic circumpolar current, with a clear signal in sea level.

Enhanced Atlantic freshwater export during El Niño

Using two reanalysis data sets, the influence of El Niño/Southern Oscillation (ENSO) on the surface freshwater balance of the Atlantic ocean is examined. We present evidence that the transport of water vapour out of the tropical Atlantic is enhanced during warm ENSO phases, while it is reduced during cold phases. These tropical changes alter the freshwater budget of the entire Atlantic drainage basin. The difference in freshwater export between La Niña and El Niño years is on the order of 0.1 Sv (1Sv=106 m³s−1) for the entire Atlantic. Results from an idealized coupled ocean‐atmosphere model suggest that this amplitude is of significance for the strength of the North Atlantic thermohaline ocean circulation, if the persistence of the anomaly is on the order of decades.

Length‐scale dependence of relief along the south eastern border of Massif Central (France)

Relief has been calculated over four orders of length‐scales (75 m to 800 km) for several drainage basins along the south‐eastern border of Massif Central, an asymmetric plateau edge feature. Relief is not uniform over the length‐scale domain, but rather appears as smooth at small and large scales, and rough at meso‐scale. This heterogeneous behaviour is interpreted as a consequence of the different processes that shape topography at such ranges of scales. The threshold between small and meso scales is rather well constrained for all basins (400–500 m), and may represent the limit where channel networks appear, while the threshold between meso‐scale and large scale varies with geological context: where relief is fault controlled (Mediterranean drainage basins), the threshold value is of the order of 15–20 km. Where topography is flexurally controlled (Atlantic drainage basins), the threshold value is different (40–80 km).

The influence of atmospheric moisture transport on the fresh water balance of the Atlantic drainage basin: General circulation model simulations and observations

An attempt has been made to evaluate the reliability of water vapor transport estimates from atmospheric general circulation models (GCMs) by comparing the vapor export from the Atlantic basin obtained using the GISS 4° × 5° grid GCM with that obtained from observations of wind velocity and humidity [Oort, 1983]. We find substantial differences. The model gives an export of 0.13 Sverdrups (Sv) from the Atlantic basin compared to 0.32 Sv from the Oort data set. We note that the GCM produces far stronger easterly winds and correspondingly larger tropical vapor transports. Further, because of the model's low orography, steering of the winds and drying of air masses by mountain chains are not adequately represented. This leads to a dramatic overestimation of water vapor transport across mountain ranges. In an additional model experiment, we calculated the water vapor transports in a double‐CO2 scenario. We find that atmospheric freshwater loss from the Atlantic basin is increased to 0.30 Sv compared to 0.13 Sv in the control run. Because of the deficiencies of the model mentioned above, however, we conclude that estimates of vapor transport for future greenhouse conditions and past glacial conditions made using GCMs have to be taken with great caution until models with more realistic flow patterns, especially around mountain ranges, are available.

Salinity history of the northern Atlantic during the last deglaciation

The claim has been made (see Broecker et al., 1988) that production of North Atlantic Deep Water terminated during Younger Dryas time and that the onset of this termination occurred about 11,000 years ago when the flow of meltwater from a large segment of the southern margin of the Laurentide ice sheet was diverted from the Mississippi to the St. Lawrence drainage. Fairbanks [1989] points out a serious weakness in this argument. Based on a sea level curve derived from radiocarbon dates on coral obtained from borings made off the Barbados coast, he suggests that a lull in the melting of the ice caps during Younger Dryas time may have more than compensated for the impact of the diversion. The purpose of this paper is to reassess the situation regarding the origin of the Younger Dryas in light of this new evidence. Currently the salinity of surface waters in the northern Atlantic is influenced by three fluxes. Water vapor transport from the Atlantic drainage basin to the Pacific‐Indian basin tends to raise the salinity of the entire Atlantic. The excess over evaporation of precipitation and runoff poleward of 40°N tends to reduce the salinity of waters in this region relative to the Atlantic average. The conveyor circulation of the Atlantic trades more salty waters of the Atlantic with less salty waters outside the Atlantic tending to drive down the Atlantic's salinity. The conveyor circulation also flushes the northern Atlantic, pushing its salinity toward the mean for the Atlantic. During the period of deglaciation meltwater emanating from the Laurentide and Scandinavian ice sheets was also important. This flux tended to lower not only the salinity of the entire Atlantic but also the salinity of surface waters in the northern Atlantic relative to the Atlantic's mean. As deepwater formation in the northern Atlantic depends critically on the salinity of surface waters, the interactions among these fluxes can change the strength of the conveyor.

Notropis tropicus, a New Cyprinid Fish from Eastern Mexico

A distinctive new dwarfed species from Tamesi-Pdnuco stream complex of Atlantic drainage basin of Mdxico is belatedly described. It is one of southernmost members of speciose family Cyprinidae. Among from fresh waters of eastern M6xico discovered by senior author and late Myron Gordon in 1930's and early 1940's, and scheduled for publication in a never consummated regional monograph, is a distinctive little minnow that we are now at long last describing, as still another member of speciose genus Notropis. It is one of southernmost species of family in Atlantic drainage basin, sharing with Notropis lutrensis forlonensis Meek (1904: 70-71, Fig. 30) waters of Tamesi-Panuco stream complex (Darnell, 1962: 325-326). Species referred to Dionda occur in same stream system, and also even farther south, in tierra caliente of eastern drainage of Mdxico (Hubbs and Miller, 1974). Distribution and Habitat.-This species proves to be widely distributed at relatively low elevations (probably not above 800 ft) through drainage basins of terminally lagoon-connected rios Tamesi and PTnuco, but appears to avoid coastwise strip near junction of these two rivers. Darnell (1962: 325) erroneously thought to be limited to Rio Tamesi drainage system, where he took in the middle and lower stretches of Rio Sabinas and Rio Guayalejo at Adjuntas, and added that it was not found in Rios Frio, Boquilla, or Mante. He regarded as apparently attributable to this species his observations on feeding and swimming habits of small cyprinid fishes in Rio Sabinas. His collections led him to suggest that whereas this fish does inhabit larger rivers, is also at home in shallow upstream backwaters such as mouth of Arroyo Encino. The distribution of species in Rio Panuco system appears similar for we have no records toward mouth of that river and none are in-