Seasonal Associations of Phytoplankton and Planktic Foraminifera in an Upwelling Region and Their Contribution to the Sea-Floor

Seasonal occurrences of both phytoplankton and zooplankton are closely associated with upwelling and stabilization in waters overlying the San Pedro Basin. This region is situated within the Southern California Bight and is affected by variations in the general southward flowing California Current and variable intrusion of oceanic waters. Increased northerly wind stress in spring and early summer results in upwelling of deeper, nutrient-rich waters into the photic zone. A seven month time-series sediment trap experiment revealed clear patterns in seasonal succession of both phytoplankton and planktic foraminifera. Several distinct high and low flux periods occurred over the study period, some of which can be related to rapid changes in the hydrographic environment caused by upwelling and subsequent surface water stratification. Factor analysis of relative abundances of combined phytoplankton and foraminiferal taxa defines seven assemblages reflecting seven stages of production which range in duration from two to five weeks. Factor scores describe phytoplankton and foraminiferal taxa indicative of specific production stages. Periods of high flux of the more abundant diatoms and planktic foraminifera also illustrate preferred production stages and compare well with percentage-based results. A comparison of the sediment trap assemblage with local sediment from an oxygenated portion of the basin suggests that long-term phytoplankton accumulation in this region is dominated by taxa associated with winter bloom and post-upwelling bloom production, implying that brief but massive episodic events overwhelm and dominate the annually averaged diatom species assemblage found in the sediments. Foraminifera, however, are modified by dissolution to the extent that the high production periods of late spring and during upwelling are poorly represented. Rather, species indicative of early spring and post-upwelling conditions dominate the planktic foraminiferal sedimentary assemblage.