High resolution sedimentary record of dinoflagellate cysts reflects decadal variability and 20th century warming in the Santa Barbara Basin

We present a continuous record of dinoflagellate cysts from a core of laminated sediments collected in the Santa Barbara Basin (SBB), off Southern California. The core spans the last ∼260 years and is analysed at biennial (two-year) resolution. Variations in dinoflagellate cyst assemblages are compared with 20th century historical changes, and are used to examine changes in primary productivity and species composition, which are bound to the variability in upwelling and sea-surface temperature (SST) in the region. Cysts produced by heterotrophic dinoflagellates dominate the assemblages. In particular, Brigantedinium spp. (on average 64.2% of the assemblages) are commonly associated with high levels of primary productivity, typically observed under active upwelling conditions, when nutrient supply is higher. Other heterotrophic taxa such as cysts of Protoperidinium americanum, Protoperidinium fukuyoi, Protoperidinium minutum and Archaeperidinium saanichi, all Echinidinium species, Quinquecuspis concreta and Selenopemphix undulata are more abundant in the early part of the record (∼1750s–1870s). These taxa are generally associated with high primary productivity and are observed predominantly during intervals marked by relatively variable conditions of SST, stratification and nutrient loading. The 20th century is marked by an increase in several species of autotrophic affinity, primarily Lingulodinium machaerophorum and Spiniferites ramosus. In recent surface sediments from the region, these species are more abundant in the Southern California Bight, and they are associated with conditions of relaxed upwelling in the SBB (typically observed during summer and fall), when SST is higher and nutrient supply is moderate. Their increasing concentrations since the early 20th century reflect warmer SST and possibly stronger stratification during the warmest season. Taken together, the changes in cyst assemblages provide further evidence that persistently warmer conditions in the SBB began affecting marine populations by the late 1920s. Decadal-scale variations in primary productivity are encoded in the heterotrophic dinoflagellate cyst record, with higher (lower) concentrations of heterotrophic taxa occurring during “cool” (“warm”) phases of the Pacific Decadal Oscillation (PDO) index. Wavelet analysis of heterotrophic taxa concentrations suggests a weaker influence of the PDO on biota of the region during the 19th century.