Seasonal succession of planktonic foraminifera in the subpolar North Pacific

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Abstract
Using an automated time-series sediment trap moored at a depth of 3,858 m at Station PAPA (50.degree.N, 145.degree.W) in the northeast Pacific, consecutive two-week samples were taken over a year-long period in order to monitor the seasonal changes in planktonic foraminiferal shell flux. Station PAPA is located within the Pacific subarctic water mass, near the boundary of the transitional water mass (Bradshaw, 1959). During the summer months, a layer of relatively warm surface water develops in this region, thermally stratifying the upper water column while reducing the influx of nutrients into the surface waters. In contrast, the winter near-surface water is characteristically well mixed with high nutrient concentrations, due to the occurrence of seasonal upwelling within this region of the northeast Pacific. Variations in the shell flux of planktonic foraminifera are considered to represent seasonality in the production of different foraminiferal species within the photic zone, and reflect changes in both phytoplankton productivity and hydrographic conditions in the upper water column. A distinct seasonal succession in the abundance of six planktonic foraminiferal species occurs at Station PAPA. Periods of increased foraminiferal production are due to the association of each species with one of the two seasonal water masses, and/or to periods of high phytoplankton productivity.
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