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Stable isotopic analyses (δ18O and δ13Crpar; on four species of planktonic foraminifera collected from high‐resolution time series sediment trap sampling indicate that rapid and significant seasonal variability occurred in response to rapid changes in both the hydrographic environment and the isotopic composition of ambient seawater. Specimens were collected at a depth of 500 m for 28 consecutive 1‐week periods in the San Pedro Basin, within the Southern California Bight, a region seasonally influenced by upwelling. Hydrographic monitoring of the study area revealed a 6‐week period (late April to mid‐June) during which pulses of upwelling occurred, causing significant increases in the oxygen isotopic composition of surrounding waters, while decreasing the carbon isotopic composition. Analyses of Globigerina bulloides, Orbulina universa, Neogloboquadrina dutertrei, and Neogloboquadrina pachyderma (right‐coiling) indicate that during the upwelling period these species live, respectively, at the surface, in the surface mixed layer, within the thermocline, and below the thermocline. Globigerina bulloides ascends from well below the thermocline to the surface at the onset of upwelling and increases production to its maximum simultaneously as its morphology changes to less‐calcified, open forms. Both N. dutertrei and N. pachyderma follow isothermal ranges, migrating to shallower waters during upwelling and subsequently descending after upwelling ceases. Neogloboquadrina dutertrei appears to inhabit the region associated with both the thermocline base and chlorophyll maximum when it is most abundant, following upwelling. Orbulina universa increases its depth habitat as solar irradiance increases, but this species decreases in abundance during upwelling. The δ18O signal of O. universa appears to be a useful indicator of seasonal temperature variations above the thermocline, whereas the δ18O of G. bulloides documents near‐surface temperatures during upwelling. The δ13C signal of both N. dutertrei and N. pachyderma appears to record the δ13C of equilibrium calcite for the regions of the thermocline and just below the thermocline, respectively. Due to the relative preservation potentials of the four species analyzed, only N. dutertrei and N. pachyderma are likely to be preserved in the sedimentary record in regions of oxygenated bottom waters. Thus an actual record of hydrographic conditions during upwelling, as would be found in tests of G. bulloides, is not preserved; instead, conditions before (N. pachyderma) and just after (N. dutertrei) are more likely recorded.

A planktonic foraminiferal faunal and stable isotopic study has been carried out on sediment trap samples collected in 1988 from the San Pedro Basin (Southern California Bight) in order to examine the response of this group of plankton to coastal upwelling. Hydrographic monitoring indicates that a period of upwelling occurred in the basin from late April to early June, with a brief spring bloom of planktonic foraminifera occurring prior to upwelling. The onset of upwelling resulted in a significant increase in total foraminiferal shell flux; the upwelling assemblage was initially dominated by Globigerina quinqueloba , with Globigerina bulloides dominating the latter half of upwelling. Following upwelling, the upper water column became thermally stratified and Neogloboquadrina dutertrei dominated the fauna. The observed pattern of faunal succession may be partly related to changes in food availability. Stable isotope analyses (δ 18 O and δ 13 C) of Neogloboquadrina pachyderma, N. dutertrei and G. bulloides indicate that all three species adjust their depth habitats in response to upwelling. Globigerina bulloides migrates from below the thermocline to the surface at the onset of upwelling. Both N. pachyderma and N. dutertrei appear to adjust their depth habitats in order to remain within specific temperature ranges.

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