General biological features of the South Atlantic
Patterns of distribution of zooplanktonic species
Results presented in this volume offer a unique opportunity to assess in a comprehensive manner our present understanding of some general aspects of specific richness in the area covered, as well as selected traits of the distribution patterns of the species present. Tentative comparisons are drawn within and across major taxonomic groups. Admittedly, interpretations are very strongly influenced by both uneven intrinsic specific diversities, and by the very variable degrees of knowledge of the various taxa concerned. Indeed, while positive records pose little problem, negative ones (i.e., the absence of a given species) are probably more often the result of lack of information, than of the fact that the species in question does not actually live in the South Atlantic. Sorting out these two influences on the database is clearly impossible with any degree of confidence, yet comparisons between groups can yield suggestive general trends useful for future work.
One of the questions we attempted to address using the information contained in this volume and previous data is how many of the species described for each zooplanktonic group occur in the South Atlantic. The data reported for 24 zooplanktonic taxa (of a total of 30 covered in the volume, some were excluded from the analyses due chiefly to lack of adequate information) indicate that these percentages vary widely, from 100% (Cladocera) to as little as 10-15% (Nemertina, Ctenophora, Mysidacea) (Gb5).
Some of the datapoints illustrated in (Gb5) clearly deviate from the overall trend. For the Nemertines an overall total of 97 marine planktonic species has been described (R. Gibson, pers. comm.), only 11 of which have been found in the South Atlantic. However, most described nemertine species have only been recorded once or twice, which clearly shows that available data are totally insufficient for drawing a world-wide distributional pattern for these scarce and delicate deep-dwelling animals. The Mysidacea are also atypical in that only 96 of the ca. 700 known species have been found in the South Atlantic. As opposed to the nemertines, in this case the actual proportion of geographically restricted species may effectively be higher than for other groups. Most mysid species are closely associated with the sea floor, and therefore their ranges are influenced by bottom topography, rather than by surface water masses and currents. Dispersion by currents is further hindered by their comparatively large size, powerful swimming, and the lack of planktonic larval stages. Thus, even though their coverage in the South Atlantic is poor, and future work will undoubtedly add many species, the proportions of South Atlantic taxa with respect to the overall world total will probably increase further, rather than decrease (M. Murano, pers. comm.).
For most of the groups dealt with, species covered include both those effectively recorded in the South Atlantic, and the ones not yet recorded, but whose presence in this basin is very likely. In a few cases, however, general information available was too scarce or otherwise inadequate for this type of approach, and only actual records were considered. This resulted in some unrealistically low percentages (e.g., Acantharia, Cephalopoda, Radiolaria Phaeodaria, Copepoda). The Radiolaria Polycystina, also with a seemingly relatively low coverage, constitute another exception. In this case, the Figure used as an estimate of the total number of Recent species described is a very rough approximation; most of these taxa are very poorly defined, and in fact they are ignored altogether in most surveys (see “Radiolaria Polycystina” in this volume). The species covered in the corresponding chapter, which include both actual and potential records, are therefore a selection of the most widespread and better defined polycystines.
These considerations indicate that several of the data illustrated in (Gb5) are governed by factors other than actual degrees of endemism within each group. The overall specific richness within each major taxon, in particular, seems to influence the numbers strongly. The species-richest groups, such as the fish larvae and the copepods, have proportionally the lowest numbers of South Atlantic species (<30%); while for the less diversified ones, such as the Cladocera, Pyrosomatida, Heteropoda, Salpida, Appendicularia and Euphausiacea, at least 60-70% of the species described were found to be present in the South Atlantic. Groups which had locally-based (Brazil, Argentina, South Africa) specialists for a number of years, such as the foraminifers, planktonic tunicates, pteropods, euphausiids, chaetognaths, also seem to be better covered than those lacking local expertise.
In view of all these limitations, an accurate estimate of the percentage of planktonic species present in the World Ocean that inhabit the South Atlantic is clearly impossible. Data for the groups which are probably least biased, however, suggest that these proportions vary from 80-100% to below 50%.
In general terms, highest zooplanktonic specific richness occurs in the subtropical waters, roughly between 10 and 35°S, followed by the equatorial belt ( (Gb6). Numbers of species that can effectively be recorded in the mixed subtropical-subantarctic area are probably underestimated by data illustrated in (Gb6) (compare with (Gb4), since they emphasize “core” areas of occurrence, rather than expatriation ranges. As discussed above, expatriation of both warm water and cold water organisms is extensive in the Transition Zone (see (Gb4). Diversity drops sharply in the subantarctic, and further decreases south of the Polar Front, where on average it is less than 50% that in the tropics and subtropics (Gb6).
North-south decrease rates in species numbers, however, are uneven between groups. For the “warmest” taxa, such as the Pteropoda and Copepoda, subantarctic and Antarctic representatives are only about 10-20% of the totals present in the entire South Atlantic; on the other extreme, for the Appendicularia and Phaeodaria Antarctic waters host almost 50% of the species.
Data available for some of the groups treated also allow a very rough estimate of the latitudinal span of the zooplanktonic species covered. (Gb7) shows that few taxa are restricted to ranges below 10-20°, or occupy areas in excess of 50°; instead, most zooplankters occur over moderately large areas spanning around 40° in latitude. In terms of biogeographic divisions, these estimates confirm some previous data (Boltovskoy, 1986) concluding that both widely cosmopolitan (i.e., tropical to polar), and strongly restricted zooplanktonic species are rare (Gb7). In other words, most species are not restricted to any one water mass; their ranges usually cover at least one entirely, and at least parts of one or two others.
Definition of faunistically discrete biogeographic units is chiefly based on estimates of the distributional boundaries of the species. In an attempt to validate the scheme presented in (Gb1), on the basis of the data available we calculated how many of the 687 taxa considered appear or disappear at each degree latitude between 3 and 58°S. Since data used for this calculation are very approximate, and because most specific ranges are asymmetric on the two sides of the ocean (see (Gb1), these results are clearly very rough. Nevertheless, we contend that they still do reveal a meaningful trend (Gb8a, Gb8b).
By far highest numbers of distributional boundaries occur within the Transition Zone. This confirms the widely accepted notion, already noticed in the first large scale biogeographic schemes proposed (e.g., Meisenheimer, 1905a), that the two major structural units in the area are the cold water and the warm water assemblages. Quantification of this particular limit as compared with other boundaries, however, has seldom been attempted. Our results indicate that, in these terms, the cold water vs. warm water boundary is about twice as important as the second strongest one, the Polar Front, and about 3 times stronger than the one separating tropical from subtropical domains (Gb8a, Gb8b). These latter two also appear as significant breaks in the geographic ranges of the zooplankters considered. The one centering on 17-20°S represents the boundary between the equatorial/tropical and the subtropical domains on the west, and the Angola/Benguela Frontal Zone on the east (cf. (Gb1), 27). In open-ocean waters this limit is probably less pronounced and occurs farther north, around 10°S (Gb1).
The bottom panel of (Gb8b) confirms the results of the graph insofar as the numbers of species shared between areas are generally inversely proportional to the numbers of specific distributional boundaries between them. Thus, 84% of the 629 tropical and/or subtropical species considered are present in both these areas, whereas subantarctic and Antarctic waters share 60% of the 294 taxa present in either. As opposed to the above, the Transition Zone is actually an ecotone, a diffuse boundary in itself, rather than a “community”. Almost all its inhabitants are expatriates from neighboring areas, and recirculating physical mechanisms allowing for the maintenance of permanent, exclusively transitional populations are unknown (Reid et al., 1978; Boltovskoy, 1986). The numbers illustrated in (Gb8b) support this notion insofar as the corresponding percentages of shared species are the second highest (62%). Interestingly, penetration of subantarctic organisms northwards seems much stronger than expatriation of subtropical organisms southwards. Indeed, only 63% of the 609 species present in the subtropics are also recorded in the Transition, whereas for the 285 subantarctic plankters the Figure is 90%. It is probable, however, that this difference is at least partly due to submersion and northward expatriation of cold water organisms at subsurface and deep layers, rather than at the surface (see Gb.14 “Argentinian coastal and offshore waters” ).