General biological features of the South Atlantic
Biogeography (1)

Early biogeographic schemes of the South Atlantic (and of the World Ocean) were almost exclusively based on qualitative data on the distribution of plant and/or animal species. Hardly any quantitative information was then available, let alone data on primary production, biomass, turnover rates, trophic relationships, and the like. Surprisingly, however, comparison of these early schemes with those accepted today shows that the size and shape of the major, world-wide biogeographic provinces changed very little (Boltovskoy, 1986) (Gb1). Indeed, for the Southwestern Atlantic, for example, the biogeographic zonations outlined as early as in the 1930s and 1940s (e.g., Hentschel, 1938; Bobrinskii et al., 1946) do not differ drastically from the "biogeochemical provinces" proposed recently on the basis of satellite imagery (Longhurst, 1995; Longhurst et al., 1995; see Gb1b).

Several reasons converge to explain this situation. Besides the fact that the early schemes proved good enough as to need only minor adjustments, it seems obvious that the physical environmental setting, as defined by water-mass properties and movements, is the main driving force behind the distribution of planktonic life in the seas. Because the general traits of surface circulation and water-mass patterns have now been more or less well established for over 50 years, the lack of major modifications to the biological zonations defined half a century ago is not surprising. However, due to the tight physical-biological coupling, increasing knowledge about the physical characteristics of the oceans paves the way for more detailed biogeographic analyses. From this point of view, water temperature seems to strongly outweighs all other parameters as far as life types are concerned (but not for the distribution of abundance, biomass and productivity). As a result, biogeographic patterns generally follow the classical 9-belt system (paired polar, subpolar, transitional, and subtropical bands on both sides of a tropical or equatorial one). This 9-belt system is primarily derived from physical data, which raises the long-standing question as to whether the distribution of biogeographic domains reflects physically defined water masses. This question may seem superfluous in the light of the wealth of reports where the distribution of individual species and species assemblages are concluded to match water-mass patterns. However, because extensive interpolations are needed in order to establish large-scale zonations on the basis of the highly spotty biological information, and since these interpolations are usually based on physical trends (chiefly temperature and salinity fields), such comparisons are intrinsically biased. Indeed, when objective data on individual species ranges are analyzed in detail, the water-mass vs. biogeographic range coincidence is often vague (Boltovskoy, 1986), especially in the pelagic realm. Nevertheless, because -as mentioned above- it is the physical environment which primarily drives planktonic life in the oceans, water masses (and their temperature gradients in particular) are intimately linked with biogeographically distinct areas.