Abstract
More than 30% of Buccella peruviana (D'Orbigny), Globocassidulina crassa porrecta (Earland & Heron-Allen), Cibicides mackannai (Galloway & Wissler) and C. refulgens (Montfort) indicate the presence of cold Sub Antarctic Shelf Water in winter, from 33.5 to 38.3º S, deeper than 100 m, in the southern part of the study area. In summer, the abundance of this association decreases to less than 15% around 37.5-38.9º S where two species (Globocassidulina subglobosa (Brady), Uvigerina peregrina (Cushman) take over. G. subglobosa, U. peregrina, and Hanzawaia boueana (D'Orbigny) are found at 27-33º S in both seasons in less than 55 m deep in the northern part, and are linked with warm Subtropical Shelf Water and Tropical Water. Freshwater influence was signalized by high silicate concentration and by the presence of Pseudononion atlanticum (Cushman), Bolivina striatula (Cushman), Buliminella elegantissima (D'Orbigny), Bulimina elongata (D'Orbigny), Elphidium excavatum (Terquem), E. poeyanum (D'Orbigny), Ammobaculites exiguus (Cushman & Brönnimann), Arenoparrella mexicana (Kornfeld), Gaudryina exillis (Cushman & Brönnimann), Textularia earlandi (Parker) and thecamoebians in four sectors of the shelf. The presence of Bulimina marginata (D'Orbigny) between 34.1-32.8º S in the winter and 34.2-32.7º S in the summer indicates that the influence of the Subtropical Shelf Front on the sediment does not change seasonally, otherwise, the presence of Angulogerina angulosa (Williamson) in the winter, only in Mar del Plata (38.9º S), show that Malvinas currents are not influencing the sediment in the summer.
Highlights
The factors that control the latitude-diversity of benthic Foraminifera are still unclear
The seasonality and reach of these water masses are responsible for the environment formed in the sediment, that in turn, propitiates the establishment of benthic species of Foraminifera in 8 groups: Continental Water (CW) species, organisms of continental origin, estuaries and salt marshes, and species related to the plume of the Plata River and the Patos Lagoon, and other estuarine systems like Laguna and Itajaí-Açu as well (PPW), Subantarctic Shelf Water (SASW), Malvinas currents (MC), Subtropical Shelf Water (STSW), Tropical Water (TW), South Atlantic Central Water (SACW) and Subtropical Shelf Front (STSF)
In Callao, Peru, Buccella peruviana was described as being carried to the Atlantic via subantarctic waters (Boltovskoy, 1950) and our results showed that this species is characteristic species of the Subantarctic Shelf Water (SASW) (T < 15 °C and 33.7 < S < 34.15) dominant mainly below Punta Del Diablo (33° S) in the two cruises
Summary
The factors that control the latitude-diversity of benthic Foraminifera are still unclear. Some researchers have pointed out the importance of historical climatic stability, productivity, and amount of energy of oceanic environments (Gaston and Blackburn, 2000). The most acceptable cause for latitudinal gradient of regional species richness is considered to be variation in the rates of extinction and speciation, and the history of secular dispersal (Huston, 1994); all of these variations reflect differences in biogeographically or evolutionary processes along latitude and can be detected by Foraminifera analysis. Besides the macro-scale distribution, the influence of freshwater runoff and oceanic fronts can disrupt these patterns providing interesting phenomena to be explored. We will draw on sediment and water samples taken on the western south Atlantic continental shelf between 27-39.9° S, a region strongly influenced by the discharges of the Plata River and Patos Lagoon
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