PLANKTONIC FORAMINIFERAL RESPONSE TO UPWELLING AND SEASONAL HYDROGRAPHIC CONDITIONS - SEDIMENT TRAP RESULTS FROM SAN-PEDRO-BASIN, SOUTHERN CALIFORNIA BIGHT

A sediment trap study was conducted between January and July 1988 in the San Pedro Basin, off the coast of Los Angeles, California. During this seven-month period, 28 consecutive week-long samples were collected from a depth of 500 m. The study site, located within the Southern California Bight, was influenced by upwelling events during a six-week period. Four hydrographic periods were identified: isothermal conditions during winter (January-March), weak stratification during early spring (April), upwelling of cold, nutrient-rich waters (late April to early June), and the development of strong stratification after early June (post-upwelling/early summer). Distinct planktonic foraminiferal assemblages are associated with each hydrographic period, including separate post-upwelling and early summer assemblages, and a strong seasonal succession of eleven species occurs throughout the seven months, with maximum flux of each species lasting approximately two to four weeks. Seasonal variation in species dominance also occurs in this region, changing from Globigerina bulloides in the winter, to Neogloboquadrina pachyderma and Globigerina quinqueloba in the spring (April), to G. bulloides again during upwelling, and to Neogloboquadrina dutertrei just after upwelling. A summer assemblage that includes Globigerinoides ruber (white) appears as surface waters become strongly stratified. Utilizing information from the San Pedro Basin sediment trap material and from our previous trap studies along the eastern Pacific margin, certain characteristics of the ecologies of nine species of planktonic foraminifera were determined. Surface sediments from an oxygenated region of the basin reveal that species highly resistant to dissolution (N. pachyderma and N. dutertrei) dominate the fossil assemblage, while the important "upwelling indicator," G. bulloides, is nearly absent.