Diatom abundance and assemblage composition determined for 47 surface sediment samples from the Southeast Pacific (50 degrees S-15 degrees N), combined with existing data for the Peru and Chile margins, demonstrate responses to regional temperature, upwelling, and productivity. High diatom abundances (# valves/g) mark the eastern equatorial Pacific upwelling and the coastal upwelling areas, in particular the upwelling centers off Peru. Freshwater diatoms reflect the low-salinity tongue off the Chilean fjords. Diatom species composition distinguishes between coastal and eastern equatorial Pacific upwelling conditions, and records sea-surface temperatures. Q-mode factor analysis defines five floral assemblages. Factors 1 and 4 determined by the genus Chaetoceros (F1) and Thalassioneina (F4) reflect coastal and equatorial upwelling conditions, respectively. Factors 2 and 3 characterized by the genus Thalassiosira and Azpetia nodulifera can be associated with El Nino conditions. A 5th factor, described by Paralia sulcata, records a near-shore upwelling center off Point Concepcion, central Chile. Statistical transfer functions relate diatom species percentages to sea-surface temperature and productivity with error estimates of +/- 0.9 degrees C and +/- 23 gC/m(2) yr, respectively, and provide new tools for estimating past temperature and productivity along the west coast of South America. (c) 2006 Elsevier Ltd. All rights reserved.