Fo. 3 Distribution

Foraminifera
Distribution

Planktic Foraminifera occur in all oceans, although they are scarce in shelf areas and closed marine basins like the Red Sea. Densities depend greatly on food availability and hydrographic characteristics, varying from less than 1 to over 100 specimens per mö3 (e.g., Caron and Swanberg, 1990; see planktic biogeographic zones 2), with vertical flux rates calculated between 0.1 and 100 g CaCO3 mö-2 yö-1.
(Changes in abundances)

Published planktic foraminiferal distributional patterns are based mainly on ocean floor sediment samples (planktic distribution).

Species-specific geographic distributional data are moderately abundant in the literature, but scarce and fragmentary for most of the rare forms. It is widely accepted that, in general terms, species occurrences are controlled chiefly by climatic belts (mostly temperature-dependent), by current systems, and by food availability. All three boundary conditions vary seasonally. Additionally, factors such as reproductive behavior (e.g. semi-lunar, lunar, or yearly cycles), and local water properties (e.g., river discharge, such as the Zaire River, cf. Oberhänsli et al., 1992; Ufkes et al., 1998) may also be of major importance. Thus, the actual distribution of a species may be rather patchy both in space and time. Despite these difficulties, planktic Foraminifera may be used to trace surface water hydrographic conditions.

Foraminiferal diversity decreases drastically from approximately 15-20 species in a tropical assemblage, where 6-8 species dominate, to 5-6 species in the antarctic realm where only one species dominates. Mixed assemblages occur where cold water currents enter more temperate belts, in upwelling areas, or where river outflow creates a surface layer of low salinity. Mixing of water masses either vertically or horizontally usually leads to expatriation of species with concomitant abundance drops, but it also may enhance a population if nutrient-rich water is introduced.

planktic biogeographic zones 1
planktic biogeographic zones 2

Although at global scales the influence of temperature is dominant, its role decreases at more restricted scales, as most species tolerate rather wide temperature ranges. At these scales foraminiferal distribution is strongly dependent on food availability. This may partly explain observations of warm and cold water species coexisting in the same water mass (e.g., Boltovskoy et al., 1996).

The species Globigerinoides ruber, Globigerinoides sacculifer, Globoturborotalita rubescens, Globigerinita glutinata and Turborotalita quinqueloba dominate South Atlantic tropical to temperate assemblages, followed by Globorotalia menardii and Neogloboquadrina dutertrei. Most individuals are found in the upper 50 m of the water column where temperature, food, and light (for symbiont photosynthesis) are appropriate. Below this depth foraminiferal concentrations usually decrease exponentially. Thus, most of the shells originate from the upper 50 to 80 m, which should be kept in mind when using planktic foraminiferal abundances for paleoceanographic purposes.

Changes in abundances illustrate the occurrence and relative abundances of the most common foraminiferal species in the Southwestern Atlantic (pooled data for 0-50m). Notice that, while the distribution of some species exhibits distinct geographic patterns, others, such as Globigerinita glutinata and Turborotalita quinqueloba (= Globigerina quinqueloba in Boltovskoy et al., 1996), are present at almost all locations, albeit in variable proportions. For the eastern South Atlantic, species distributions in the plankton (0-50 m) are illustrated with a series of maps based on the works of Wefer et al. (1988, 1992), Oberhänsli et al. (1991), and Kemle-von Mücke (1994).
(Globigerinoides ruber 3), abundance of this species in the eastern South Atlantic.

In the speciesmodule more detailed pictures of foraminiferal species-specific distributional patterns are offered, from the temperate to tropical South Atlantic based on sedimentary materials (from Pflaumann et al., 1996).