The annual cycle of iron and the biological response in central Californiacoastal waters

Iron has been measured for 16 months with similar to 21 day resolution at t hree stations in the upwelling ecosystem of central California, providing t he 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, a lthough upwelling continues. Excess surface nitrate and low iron are the re sult during the summer and fall at the two stations furthest offshore (20 a nd 45 km), while nitrate is depleted and iron high nearshore (5 km). Phytop lankton biomass, primary production and community structure appear to be co ntrolled by iron concentrations in offshore waters during this period.