Iron has been measured for 16 months with ~21 day resolution at three stations in the upwelling ecosystem of central California, providing the first detailed assessment of the annual iron cycle in the coastal zone. A large pulse of iron occurs during the first spring upwelling event of the year. Iron concentrations then decay up to 100-fold over several months, although upwelling continues. Excess surface nitrate and low iron are the result during the summer and fall at the two stations furthest offshore (20 and 45 km), while nitrate is depleted and iron high nearshore (5 km). Phytoplankton biomass, primary production and community structure appear to be controlled by iron concentrations in offshore waters during this period. ¿ 2001 American Geophysical Union