General biological features of the South Atlantic
The western South Atlantic

A comprehensive scheme of the major biogeographic divisions in the Southwestern Atlantic was published by E. Boltovskoy in 1970, which was extensively used to define boundaries in the western part of (Gb1). This work was based on the analysis of the planktonic Foraminifera from over 3500 samples (mostly superficial), collected over a ca. 20-year period. Despite some drawbacks, including irregular temporal and spatial coverage, paucity of information in the coastal and neritic areas (where planktonic foraminifers are either very scarce or absent altogether), usage of only one zooplanktonic group (rather than several), this scheme is the first contemporary large-scale attempt at defining a plankton-based zonation of this area. Details of these results were discussed in several subsequent publications, and so far in general terms they seem to hold not only for the foraminifers, but also for many other planktonic taxa. A major contribution of this work was the recognition of the Transition Zone (= Subtropical-Subantarctic Convergence Zone, or Brazil-Malvinas (=Falkland) Confluence), a large area between approx. 30 and 46°S where subtropical waters of the Brazil Current collide, overlap and mix with the subantarctic ones of the Malvinas (= Falkland) Current. Previous and subsequent investigations contributed much information on details of the distribution of various planktonic organisms in this area. Enrique Balech, in particular, on the basis of dinoflagellates and tintinnids, starting in the early 1940's made several major contributions to our understanding of biogeographic traits of the Transition Zone and of the coastal stretch between southern Brazil and Patagonia. However, because E. Boltovskoy's (1970, 1981a) is a time-integrating image, these details of seasonal and interannual variations are not depicted in his map (Gb1), western part).

The domains defined by E. Boltovskoy (1970, 1981a; see (Gb1), western part) include a Tropical zone, stretching roughly from the equator to ca. 20°S, differing little from a Subtropical one (20 to 30-35°S, depending on season and longitude); a vast offshore Transition Zone (30-35°S to 46-48°S in pelagic waters); a Subantarctic Zone; and south of the Polar Front the Antarctic Zone. Of these, the Transition (and its boundaries with the neighboring subtropical and subantarctic zones) is the most dynamic insofar as, as noted above, its limits vary widely seasonally and multiannually. Thus, the northern winter boundary of the Trasition illustrated in (Gb1) (western section) is defined by the northernmost winter records of typically subantarctic planktonic foraminifers throughout a 20 year period. Similarly, the southern winter boundary of the Transition reflects the southernmost winter records of subtropical foraminifers in the same time span. The Transition Zone was defined as an area of mixing of subtropical and subantarctic faunas, as well as large areas occupied by tongues and patches of purely subtropical or purely subantarctic organisms (see (Gb14) below). However, in this time-integrating image these features are necessarily omitted. Interestingly, the southern limit of this Transition agrees well with subsequent information derived from satellite images, while the northern one is noticeably displaced to the north (compare (Gb1) herewith with (Gb7) in Olson et al., 1988). We contend that submersion of cold water plankters, in particular the foraminifer Globoquadrina pachyderma, recorded in subsurface tows by E. Boltovskoy, is responsible for this mismatch.