We measured primary productivity in the Pacific Sector of the Southern Ocean as part of the Joint Global Ocean Flux Study. We collected data along 170 degree W from 54 degree S to 72 degree S on four cruises during the austral growing season of 1997- 1998. The cruises crossed the Subantarctic Front, the Antarctic Polar Front (APF), the Southern Antarctic Circumpolar Current (ACC) Front, and the Southern Boundary of the ACC. Primary productivity and chlorophyll a increased rapidly in spring, peaked in summer, and decreased rapidly in fall, following the seasonal pattern of irradiance. In early spring (October), primary productivity was 20 mmol C m super(-2) d super(-1) and increased to 54 mmol C m super(-2) d super(-1) within 3 weeks. During peak irradiance (December), productivity reached its maximum throughout the study area with values ranging from 33 to 93 mmol C m super(-2) d super(-1) depending on station location. By February, average productivity dropped to 20+/-1 mmol C m super(-2) d super(-1), and individual station values reached a minimum of 13 mmol C m super(-2) d super(-1). In early spring, chlorophyll was less than 0.5 mg Chl m super(-3) throughout the study area. In late spring and early summer, chlorophyll values were between 0.15 and 1.5 mg Chl m super(-3) depending on station location. By late summer, chlorophyll decreased to less than 0.3 mg Chl m super(-3) across the study region. Highest absolute values of productivity and biomass occurred near the southward-propagating Si gradient ([Delta]Si sub(max)). A spatial gradient in photosynthetic performance correlated with [Delta]Si sub(max): photosynthetic performance was elevated in low silicic acid waters (less than 10 mu M) to the north of [Delta]Si sub(max) and was depressed in high silicic acid waters (greater than 30 mu M) to the south of [Delta]Si sub(max). Photosynthetic performance also was correlated with iron- enrichment response: when photosynthetic performance was low, iron-enrichment response was high, and when photosynthetic performance was high, iron-enrichment response was low. These results suggest that phytoplankton were iron sufficient north of [Delta]Si sub(max) and iron limited south of [Delta]Si sub(max). We argue that the southward-traveling [Delta]Si sub(max), the APF, and the location of upwelling, iron-rich Upper Circumpolar Deep Water (UCDW) define three regions with differing iron sufficiency. Furthermore, we suggest that a winter recharge of upwelled, iron-rich UCDW within the Antarctic and Southern ACC Zones provides enough iron to support a diatom bloom that annually propagates poleward across the Antarctic and Southern ACC Zones to the Southern Boundary of the ACC, where the absence of UCDW prevents the bloom's progression into the Subpolar Regime.