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General Biological Features of the South Atlantic

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General Biological Features of the South Atlantic General biological features of the South Atlantic Demetrio Boltovskoy, Mark J. Gibbons, Lawrence Hutchìngs and Denis Binet Introduction now been more or less well established for over 50 years, the lack of major modifications to the biologi- The goal of this section is offering an overview of cal zonations defined half a century ago is not surpri- some salient biological traits of the South Atlantic. The sing. However, due to the tight physical-biological scheme outlined below reflects the known or assumed coupling, increasing knowledge about the physical boundaries and gradients between discrete, structurally characteristics of the oceans paves the way for more more or less homogeneous faunal domains, yet traits detailed biogeographic analyses. From this point of other than specific composition (e.g., water masses and view, water temperature seems to strongly outweiglit currents, primary production, biomass, seasonality, all other parameteres as far as life types are concerned specific diversity) are relied upon heavily. Inventorial (but not for the distribution of abundance, biomass analyses alone, often due to the paucity of data, fail to and productivity). As a result, biogeographic patterns identify major ecologically and biogeographically generally follow the classical 9-belt system (paired meaningful features. Furthermore, especially when polar, subpolar, transitional, and subtropical bands on used alone and in a quantitative manner, faunal distri- both sides of a tropical or equatorial one). This 9-belt butional data tend to break areas into too many regions, system is primarily derived from physical data, which many of which have little or no ecological meaning raises the long-standing question as to whether the (Fager and McGowan, 1963; Beklemishev, 1969; distribution of biogeographic domains reflects physi- Dadon and Boltovskoy, 1982). cally defined water masses. This question may seem superfluous in the light of the wealth of reports where Early biogeographic schemes of the South Atlantic the distribution of individual species and species (and of the World Ocean) were almost exclusively assemblages are concluded to match water-mass pat- based on qualitative data on the distribution of plant terns. However, because extensive interpolations are and/or animal species. Hardly any quantitative infor- needed in order to establish large-scale zonations on mation was then available, let alone data on primary the basis of the highly spotty biological information, production, biomass, turnover rates, trophic relation- and since these interpolations are usually based on ships, and the like. Surprisingly, however, comparison physical trends (chiefly temperature and salinity of these early schemes with those accepted today shows fields), such comparisons are intrinsically biased. that the size and shape of the major, world-wide bioge- Indeed, when objective data on individual species ran- ographic provinces changed very little (Boltovskoy, ges are analyzed in detail, the water-mass vs. bioge- 1986, in press). Indeed, for the Southwestem Atlantic, ographic range coincidence is. often vague for example, the biogeographic zonations outlined as (Boltovskoy, 1986), especially in the pelagic realm early as in the 1930’s and 1940’s (e.g., Hentschel, (see below). Nevertheless, because -as mentioned 1938; Bobrinskii et al., 1946) do not differ drastically above- it is the physical environment which primarily fkom the “biogeochemical provinces” proposed recent- drives planktonic life in the oceans, water masses (and ly on the basis of satellite imagery (Longhurst, 1995; their temperature gradients in particular) are intimate- Longhurst et al., 1995; see Fig. 1, inset). ly linked with biogeographically distinct areas. Several reasons converge to explain this situation. A major goal of biogeographic surveys is producing Besides the fact that the early schemes proved good one or a few maps which reflect adequately the distri- enough as to need only minor adjustments, it seems bution of life and life-related processes. Indeed, when obvious that the physical environmental setting, as discussing biogeography in the pelagic realm of the defined by water-mass properties and movements, is World Ocean, the first image that comes to mind is the the main driving force behind the distribution of above-mentioned classical system of belt-like polar to planktonic life in the seas. Because the general traits equatorial zones stretching across the globe. However, of surface circulation and water-mass patterns have this picture changes radically if primary production, or South Atlantic Zoõplänkton - FOP;rd8 ~~~~~~~~~~~ I w.g edited by D.Boltovskoy, pp. 1-42 I I O 1999 Backhuvs Publishers, Leiden, Thq Netherlands Fonda Documentaire IR0 i IL IIIINIlNlllilllNII11IlllllllllhYI~IIMlIMlI 1 2 General biological features of the South Atlantic O0 60' 45O' 3Oo 1so 15' O0 I -.c I -1 Antarctic Biogeochem ¡cal provinces accordi ng to Longhurst (1995) Fig. 1. Sketchofmajorbiogeographic domainsinthe SouthAtlantic. WesternsideischieflybasedonE. Boltovskoy(l970,1981a). ' S General biological features of the South Atlantic 3 40" O" O" O" 60" 2 2 O" O" 1 O" 2 O" 3 4 O" Fig. 2. Areas with distinct biological characteristics, as defined in Table 1. Gi: Guinea Current; Gu: Guiana Current; SSF: Southern Subpolar Front; SST: Southern Tropical Front; see Table 1 for other abbreviations. From Greze, ed. (1984). ' the abundance of life - rather than distinct species functional traits. Thus, much of the ocean surface is assemblages - are considered (compare Fig. 1 ,'with reportedly occupied by transitional areas where dissi- Fig. 3 below). The distribution of endemics, of speci- milar ecosystems intermingle. However, while mixtu- fic diversities, of particle size, of export production, re in the oceanic realm is definitely a major feature, etc. will each yield a different map, all of which are our appreciation of its relative importance is largely relevant to biogeography. Actually, one should proba- derived from the seasonal and interannual fluctuations bly think of biogeography as a multilayered system of of the boundaries of "core communities". In other maps, each layer representing a different property words, at any given point in time ecotones are proba- and/or addressing a different aspect of the biota bly less important than our seasonally and multi- (Boltovskoy, in press). However, such a multilayered annually integrated data in synoptic biogeographic system is not amenable to summarizing in one or even schemes indicate. The rather large Transitional a few simple and straightforward schemes that satis- Oceanic area illustrated in Fig. 1, for example, is chie- factorily represent a significant proportion of the traits fly based on multiseasonal and multiannual data as of pelagic life. The divisions illustrated in Fig. 1 are, defined by the latitudinally extreme locations where therefore, a compromise solution to this maze of con- subantarctic plankters can be carried northwards with flicting maps. For example, specific inventories of the the cold Malvinas (=Falkland) Current (northern Tropical Oceanic domain may not differ greatly from limits), and subtropical ones southwards with warm those of the Central Gyre (Fig. l), yet they do have Brazil Current (southern limits) (cf. Fig. 8 below). The quite dissimilar primary production values, zooplank- area actually dominated by mixed subantarctic-subtro- tonic biomass, phytop1ankton:zooplankton biomass pical assemblages during a restricted period is, howe- relationships, etc. (cf. Table 1). ver, more limited (e.g., Boltovskoy et al., 1996). As opposed to the terrestrial milieu, the dynamics of Table 1 (refer also to Fig. 2) summarizes some rele- . open-ocean systems enhances the importance of areas vant traits. of the various biologically distinct areas of gradients between neighboring communities, or that can be recognized in the South Atlantic. These ecotones, rather than that of clearly circumscribed results are chiefly based on one of the most compre- domains characterized by particular structural and hensive set of surveys carried out so far in the South P Mean annual temperature (OC) 16 .26 22 10 3 22 Greze Primary prod. (1984) 2469 175-344 115 I 120 146 296 I 273 123 I 94 I 201 Longhurst et al. (mg C m-2 day-’) (1995) 880 360-430 830 1300 330-370 21o Koblentz-Mishke (1977) 250- >500 150-250 <I00-1 50 150-500 100-250 <I O0 O6 Phytopl. abundance (cells x 1 per I; 0-50 m) (Greze, 1984) 3218 13 12 7 10 5 9 5 18 5 Phvtod. abundance (cells I-’: O0 ml (Semina. 1977) 03->I o4 03-1o4 02-1o4 103- >I 103- >I o4 <Ioz 0-1 1 1 1 o4 ~~ Phytoplanktonic biomass (mg m-3; 0-50 m) 2785 11 10 12 12 5 16 7 15 11 O-’ Bioluminiscence (1 uW “1, 0-1O0 m 25.2-74.7 3.2-11.8 2.8 2.8 8.0 1.8 3.4 1.4 Zooplanktonic biomass (mn m2) I 70 I 69 I 32 I I 23 I 30 I Biomass of herbivores (% of total zooplankton) 47 27 15 22 8 Biomass of carnivores (?/O of total zooplankton) 0.5 25 15 6 8 General zoopl. diversity (comparative figures, -__I--0-1O0 m) _____3.1 __ __3.91 - _. - __4.39 ____ 3.34 2.52 4.45 Zood. abundance (ind m3: 0-1O0 m: micro + mesozood.1 31816 6492 4565 6549 4858 8945 Zooplanktonic production (mg d-’ at 0-500 m) 3740 1594 1200 846 580 1734 Number of copepod species recorded 176 215 280 161 300 Biomass I Phvtodankton 67.4 5.O 4.7 2.2 8.0 relationships (%I - - . - . -. - - Bacterioplanktonic___ __ biomass (ing m-7 754 120 150 - 372 Table 1. Comparative values for various biological parameters in the major biogeographic domains of the South Atlantic. Where units are not clearly specified (in the original source) values are reproduced for comparative purposes only. See Fig. 2 for location of areas. Data from Greze, ed. (1984),unless otherwise noted. The “South Equatorial” domain is roughly coincident with the “Tropical Oceanic” domain of Fig. 1. Shadedcellsdenote highestvalues. General biological features of the South Atlantic 5 Chlorophyll a Primary production Zooplanktonic biomass (mg 111-3) (mg C m-2d-1) (mg 1000 1-1 ODoUm DOD-<lo0 100-150 150-250 >250 125 25-50 50-100 100-200 >200 Fig.
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