General biological features of the South Atlantic
Biogeography (3)

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 (Gb1), 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). Compare (Gb8a) and (Gb8b). The area actually dominated by mixed subantarctic-subtropical assemblages during a restricted period is, however, more limited (e.g., Boltovskoy et al., 1996).