Santa Barbara Basin (SBB) diatom and silicoflagellate assemblages are quantified from a box core record spanning AD 1940-2001 and an Ocean Drilling Program Hole 893A record from nearly equal 220 BC to AD 1880. The combined relative abundance of the diatoms Fragilariopsis doliolus and Nitzschia interrupteseriata from continuous two-year sampling intervals in the box core varies with sea surface temperature (SST), suggesting its utility in SST reconstruction. The assemblage data from the ODP 893A record indicate a broad interval of generally cooler SSTs between nearly equal AD 800 and 1350, which corresponds to the Medieval Climate Anomaly (MCA), a period of generally warmer temperatures across other regions of the northern hemisphere. The assemblages also indicate an interval of generally warmer SSTs between nearly equal AD 1400 and 1800, a period of otherwise global cooling referred to as the Little Ice Age (LIA). The changes in assemblages of diatoms and silicoflagellates support the hypothesis that the widespread droughts of the Medieval Climate Anomaly in the Western US were associated with cooler eastern North Pacific SST. The box core assemblages have higher percentages of tropical and subtropical compared to temperate and subpolar species than the ODP samples, reflecting a response of phytoplankton communities to an unusual 20th century warming. Pseudonitzschia australis, a diatom linked with domoic acid production, begins to become more common (>3% of the diatom assemblage) in the box core only after AD 1985, suggesting a link to anthropogenic activity. Abstract Copyright (2010) Elsevier, B.V.