Living planktic foraminifera: tracers of circulation and productivity regimes in the central equatorial Pacific

Planktic foraminifera (shelled protozoans from ∼0.01 to 1 mm in size) respond to equatorial circulation and ecosystem dynamics. In the JGOFS survey I cruise of the equatorial Pacific (9°N-12°S, 140°W, in February–March 1992), responses to upwelling, advection, and biological activity occurred in spite of little upper-ocean temperature contrast. Rather than being abundant within the entire productive equatorial zone, foraminifera concentrated off the equator at convergent fronts. For example, non-spinose, mostly herbivorous species (G. conglomerata, G. tumida, P. obliquiloculata, and N. dutertrei) dominated near 3°N, in the convergence between the South Equatorial Current and the North Equatorial Countercurrent. Juvenile forms outnumbered adults within the convergence, indicating that these foraminifera succeeded and reproduced here (rather than passively accumulating by advection) perhaps by maintaining buoyancy to stay within the convergent, food-rich zone. The South Equatorial Current was favored by spinose, endosymbiont-bearing G. aequilateralis and non-spinose, herbivorous G. glutinata, G. menardii, and P. obliquiloculata, perhaps an advected assemblage. Species hosting dinoflagellate endosymbionts (G. sacculifer, G. ruber, and G. conglobatus) prevailed in food-poor oligotrophic regions, perhaps because they obtain nutrition from their symbionts. Distributions of living foraminifera suggest that paleoceanographic transfer functions to estimate primary productivity in the geological record have merit, but controls of foraminiferal species distributions also include food stocks, light intensity, and advection.