Seasonal succession of planktonic foraminifera; results from a four-year time-series sediment trap experiment in the Northeast Pacific

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Abstract
A time-series sediment trap experiment was conducted from September 1982 to August 1986 at Ocean Station P in the northeast Pacific (50.degree.N, 145.degree.W, depth: 3,800 m). A Mark V automated sediment trap collected 82 samples of particulate flux, each for a duration of approximately two weeks. Shell fluxes (no. shells/m2/day) of planktonic foraminifera were calculated for the five species present. Upper water column hydrographic data collected monthly from 1956 to 1981 were used to represent average hydrographic conditions for the region, and monthly sea surface temperature data measured throughout the trap experiment were also used. The first three years of sample collection yielded similar seasonal shell flux distributions. Sub-polar species Globigerina quinqueloba and Neogloboquadrina pachyderma dominated the assemblage throughout the year, with peak shell fluxes occurring in spring when the upper water column was isothermal and cold. Left-coiling N. pachyderma predominated over the right-coiling form during spring, whereas the reverse was true in fall. Globigerinita glutinata was also present throughout the year, but had production peaks during both spring and fall. Transitional species Globigerina bulloides and Orbulina universa considered to prefer warmer regimes were present during summer months when a strong thermocline existed. Records from the fourth year of sampling did not fit this pattern and often contradicted expected occurrences of several species. Both O. universa and G. bulloides were present in abundance in fall/winter, and there was a reversal in the predominance of left- and right-coiling N. pachyderma in spring and fall. Anomalously warm sea surface temperatures that occurred during the last sampling year can only partially explain the unusual species distribution. Annual assemblages from the trap are significantly different from surface sediment assemblages from this region which are 90% left-coiling N. pachyderma and 10% G. bulloides.

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