SEASONAL PRODUCTION OF ORGANIC-WALLED DINOFLAGELLATE CYSTS IN AN UPWELLING SYSTEM: A SEDIMENT TRAP STUDY FROM THE SANTA BARBARA BASIN, CALIFORNIA

Seasonal variations in dinoflagellate cyst fluxes and assemblage composition were investigated for the first time on the west coast of the United States of America. We analyzed the palynological content of an ~two year-long (May 1995 to March 1997) fortnightly sediment trap time series from the Santa Barbara Basin (SBB, off Southern California), a region characterized by seasonal upwelling and high levels of primary productivity. A total of 47 dinoflagellate cyst taxa were identified in the trap samples, with assemblages dominated by cysts produced by heterotrophic taxa. Multivariate analyses support that dinoflagellate cyst fluxes and assemblages are reliable indicators of primary productivity, and reflect sea surface temperature (SST) variations associated with upwelling in the SBB. In particular, Brigantedinium spp. are associated with active upwelling intervals (fluxes up to 127,430 cysts m⁻² day⁻¹ and up to 86.6% of the assemblage), when SST is lower, stratification is weaker and diatom production is maximal. Conversely, Lingulodinium machaerophorum indicates relaxed upwelling conditions (up to 9640 cysts m⁻² day⁻¹ and 29.9% of the assemblage) characterized by higher SST, stronger stratification and reduced primary productivity. Selenopemphix undulata is associated with colder SST in the region, whereas cyst type A abundances increase with higher SST. Thecae of potentially toxic dinoflagellates are also documented, such as Lingulodinium polyedrum and Prorocentrum micans, which are mainly recorded under conditions of higher SST and strong stratification, and Dinophysis spp. with higher fluxes between June and September of both 1995 and 1996.