Delayed onset of the spring transition and upwelling-favorable winds in the Pacific Northwest during spring-summer 2005 resulted in a positive temperature anomaly and a pronounced negative anomaly in surface phytoplankton biomass (chlorophyll a) and primary productivity. Compared to time periods before and after the warm water event, total biomass was reduced by ca. 50% along a hydrographic line extending seaward from Grays Harbor, WA (47¿ N), with a concomitant decrease of ca. 40% in surface and depth-integrated primary productivity. Associated with these declines in biomass and productivity was a change in mean phytoplankton size, with >50% of the nearshore assemblage less than 5¿m in size during the warm event, compared to <30% during more normal conditions. Unlike higher trophic levels, the phytoplankton rapidly recovered with the onset of upwelling, returning to more typical size structure, biomass, and productivity within one week of the onset of upwelling-favorable winds.