General biological features of the South Atlantic.

BOLTOVSKOY DEMETRIO; GIBBONS MARK; HUTCHINGS LARRY; BINET DAVID

South Atlantic Zooplankton

Backhuys Publishers

Lugar: Leiden; Año: 1999; p. 1 - 42

The goal of this section is offering an overview of some salient biological traits of the South Atlantic. The scheme outlined below reflects the known or assumed boundaries and gradients between discrete, structurally more or less homogeneous faunal domains, yet traits other than specific composition (e.g., water masses and currents, primary production, biomass, seasonality, specific diversity) are relied upon heavily. Inventorial analyses alone, often due to the paucity of data, fail to identify major ecologically and biogeographically meaningful features. Furthermore, especially when used alone and in a quantitative manner, faunal distributional data tend to break areas into too many regions, many of which have little or no ecological meaning (Fager and McGowan, 1963; Beklemishev, 1969; Dadon and Boltovskoy, 1982). 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, in press). Indeed, for the Southwestern Atlantic, for example, the biogeographic zonations outlined as early as in the 1930´s and 1940´s (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 Fig. 1, inset). 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 outweight all other parameteres 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 (see below). 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. A major goal of biogeographic surveys is producing one or a few maps which reflect adequately the distribution of life and life-related processes. Indeed, when discussing biogeography in the pelagic realm of the World Ocean, the first image that comes to mind is the above-mentioned classical system of belt-like polar to equatorial zones stretching across the globe. However, this picture changes radically if primary production, or the abundance of life - rather than distinct species assemblages - are considered (compare Fig. 1 with Fig. 3 below). The distribution of endemics, of specific diversities, of particle size, of export production, etc. will each yield a different map, all of which are relevant to biogeography. Actually, one should probably think of biogeography as a multilayered system of maps, each layer representing a different property and/or addressing a different aspect of the biota (Boltovskoy, in press). However, such a multilayered system is not amenable to summarizing in one or even a few simple and straightforward schemes that satisfactorily represent a significant proportion of the traits of pelagic life. The divisions illustrated in Fig. 1 are, therefore, a compromise solution to this maze of conflicting maps. For example, specific inventories of the Tropical Oceanic domain may not differ greatly from those of the Central Gyre (Fig. 1), yet they do have quite dissimilar primary production values, zooplanktonic biomass, phytoplankton:zooplankton biomass relationships, etc. (cf. Table 1). As opposed to the terrestrial milieu, the dynamics of open-ocean systems enhances the importance of areas of gradients between neighboring communities, or ecotones, rather than that of clearly circumscribed domains characterized by particular structural and functional traits. Thus, much of the ocean surface is reportedly occupied by transitional areas where dissimilar ecosystems intermingle. However, while mixture in the oceanic realm is definitely a major feature, our appreciation of its relative importance is largely derived from the seasonal and interannual fluctuations of the boundaries of "core communities". In other words, at any given point in time ecotones are probably less important than our seasonally and multiannually integrated data in synoptic biogeographic schemes indicate. The rather large Transitional Oceanic area illustrated in Fig. 1, for example, is chiefly based on multiseasonal and multiannual data as defined by the latitudinally extreme locations where subantarctic plankters can be carried northwards with the cold Malvinas (=Falkland) Current (northern limits), and subtropical ones southwards with warm Brazil Current (southern limits) (cf. Fig. 8 below). The area actually dominated by mixed subantarctic-subtropical assemblages during a restricted period is, however, more limited (e.g., Boltovskoy et al., 1996). Table 1 (refer also to Fig. 2) summarizes some relevant traits of the various biologically distinct areas that can be recognized in the South Atlantic. These results are chiefly based on one of the most comprehensive set of surveys carried out so far in the South Atlantic after the German "Meteor" expeditions of 1925-1927, yet it should be noticed that some of them are based on few observations and therefore probably do not represent adequately average values for the corresponding area. For example, differences in primary production levels between the Benguela Current and other areas seem too high when compared with other estimates. Nevertheless, because they constitute the results of a major interdisciplinary effort and for some parameters (e.g., bioluminiscence, biomass proportions of the various trophic levels, zooplanktonic production, bacterioplankton biomass) are the only large-scale estimates available for the area, their usefulness for a general characterization of the South Atlantic is beyond doubt.