Primary productivity and its regulation in the Pacific Sector of the Southern Ocean

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 170W from 54S to 72S 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 20mmol Cm2 d1 and increased to 54mmol Cm2 d1 within 3 weeks. During peak irradiance (December), productivity reached its maximum throughout the study area with values ranging from 33 to 93mmol Cm2 d1 depending on station location. By February, average productivity dropped to 2071mmol Cm2 d1, and individual station values reached a minimum of 13mmol Cm2 d1. In early spring, chlorophyll was less than 0.5mg Chlm3 throughout the study area. In late spring and early summer, chlorophyll values were between 0.15 and 1.5mg Chlm3 depending on station location. By late summer, chlorophyll decreased to less than 0.3mg Chlm3 across the study region. Highest absolute values of productivity and biomass occurred near the southward-propagating Si gradient (DSimax). A spatial gradient in photosynthetic performance correlated with DSimax: photosynthetic performance was elevated in low silicic acid waters (less than 10mM) to th