Diversidad y biogeografía del zooplancton del Atlántico Sur.

BOLTOVSKOY DEMETRIO

Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales de Buenos Aires

ACADEMIA NACIONAL DE CIENCIAS EXACTAS FÝSICAS Y NATURALES

Lugar: Buenos Aires; Año: 1999 vol. 51 p. 111 - 136

0365-1185

The total number of marine zooplanktonic species (including subspecific categories) described to date is close to 7000. This is almost two times more than the number of known phytoplankters (ca. 4000), but several times lower than the number of benthic marine animals (over 180,000 species). In marine pelagic zooplankton diversity is unassociated with higher taxonomic categories (phylum, class): most phyla include highly diverse, as well as species-poor zooplanktonic groups. Copepods, the ecologically most important taxon, have almost 3 times more species than the second most diverse marine zooplanktonic group, but this relationship between ecological importance and specific richness is not a general rule. The South Atlantic presumably hosts at least 60% of the zooplanktonic species of the World Ocean, but only 35-40% of these have been effectively recorded. Highest degrees of endemism characterize groups linked with the bottom and coastal areas. The Indo-Pacific is richer than the Atlantic in marine benthic and coastal species, but not in zooplanktonic pelagic animals. Analysis of the history of the erection of new species suggests that we presently know less than half of the extant zooplanktonic animals. The unknown fraction, however, is chiefly among the most criptic taxa, the rarest, the most delicate, and the least accessible. The tropical South Atlantic hosts 72% of all the species recorded in the Atlantic between the equator and 60°S, in the subtropics 78% of the species dwell, 57% in the transitional waters, 39% in the subantarctic, and 20% in the antarctic. Similar numbers for the species restricted to each of these areas are: 8, 2, 3, 2 and 5%, respectively. Cosmopolitan organisms, present between the pole and the equator, comprise only 10% of the total. Very few zooplankters are restricted to just one of the biogeographic areas (between 2 and 8%); most are present in at least two adjoining biogeographic areas, and somewhat fewer dwell in three contiguous areas. The specific richness of the tropical-subtropical zone is 3-4 times higher than that of polar and subpolar waters; however, the number of species restricted to the warm waters (as opposed to those merely present there) is almost 10 times higher than that in the cold waters. These are average values; the behavior of the different groups in this respect is dissimilar, and can differ strongly from these general trends. Appendicularian species have the widest latitudinal distribution ranges, their cosmopolitanism is almost two times as high as that of the less cosmopolitan group - the tintinnids. The zooplankters surveyed also differ noticeably in their affinity for high or low latitudes: “warmest” taxa have only 10-20% of antarctic and subantarctic representatives, whereas for the “coldest” ones antarctic and subantarctic species account for over 50% of the total. In general terms, groups with higher proportions of wide-ranging species have more cold-tolerant species. Moving southwards, in the transition is where zooplanktonic diversity values drop most abruptly. The limits of the transition have ca. twice as many distributional breaks as the tropical-subtropical and the subantarctic-antarctic boundaries. This transitional barrier not only affects a large number of species, it also is much wider geographically, covering about 12-14° in latitude. Deeper-dwelling species extend over broader geographic areas: in the epiplankton wide-ranging species only represent 11-12% of all the organisms, whereas below 500-1000 m this proportion is up to 38-42%. Although the numbers of practically all terrestrial, freshwater and marine plant and animal species increase towards the equator, in the marine pelagic zooplankton highest numbers of species are recorded at middle latitudes.