Planktonic foraminiferal fluxes in the Santa Barbara Basin: response to seasonal and interannual hydrographic changes

Abstract

Results of a three-year time-series sediment trap experiment in the Santa Barbara Basin indicate linkages between water-column dynamics and variations in plankton fluxes on both seasonal and interannual time scales. The two-week collection interval utilized for this study allows evaluation of high-frequency changes in the production and flux of planktonic foraminifera, which are compared to those of siliceous microorganisms (diatoms, radiolaria and silicoflagellates). Annual hydrographic conditions in Santa Barbara Basin are influenced by seasonal changes in insolation, the wind field and source waters. This study includes periods of weak to moderate El Niño conditions followed by a transitional period to a non-El Niño state. Planktonic foraminiferal fluxes and species turnover are related to seasonal and interannual changes in source waters and water column conditions. Globigerina bulloides, G. quinqueloba and Neogloboquadrina pachyderma (dextral) dominate the planktonic foraminiferal population in this location and are present throughout the year. Globigerina bulloides and G. quinqueloba have similar flux patterns, and highest fluxes occur in association with diatom blooms. The flux patterns of G. bulloides and G. quinqueloba in general are inversely related to that of N. pachyderma (dextral). Globigerina quinqueloba seems to respond earlier to the onset of upwelling than G. bulloides. Three temperature-limited species, Neogloboquadrina dutertrei, Orbulina universa and Globigerinoides ruber, are present in lower numbers and show more restricted seasonal ranges. In the thermal regime of the Santa Barbara Basin, N. dutertrei, like O. universa, disappears during upwelling. Neogloboquadrina dutertrei occurs in highest abundance when the water column is thermally stratified and there is a well developed deep chlorophyll maximum. Increased fluxes of Globigerinoi ruber the most thermally restricted of the dominant species in the basin, are associated with the presence of warm oceanic diatoms and warm-water radiolarians. Total planktonic foraminiferal fluxes were lower in 1995 than during the previous two years, which were characterized by El Niño conditions.