Spatial and temporal variability in phytoplankton iron limitation along the California coast and consequences for Si, N, and C biogeochemistry

Iron limitation was investigated along the Northern California coast in the summer of 1999. Small-volume (1 l) shipboard iron addition bottle experiments were performed at 44 stations to gain the greatest possible temporal and spatial coverage of the area. Parameters measured in these 4-day incubations included size-fractionated chlorophyll a and particulate nutrients (C, N, and Si). Degrees of community iron limitation were quantified and compared using various iron limitation indexes, calculated as the ratio of chlorophyll a, particulate organic nitrogen, or particulate organic carbon produced in Fe-amended bottles to the amounts produced in controls. Iron limitation occurred most frequently and was most severe on stations off the continental shelf, away from sedimentary sources of iron, as well as during relaxation events in aged upwelled water. Size-fractionated chlorophyll a data did not suggest large Fe-mediated changes in phytoplankton community composition. Fe limitation reduced phytoplankton production of particulate organic nitrogen and carbon, but had much less effect on biogenic silica production. The result is an increase in particulate Si: N and Si: C ratios of control samples, which were frequently double those of Fe-amended samples. Particulate C: N ratios also decreased under Fe limitation, indicating that iron availability exerts a strong control on C, N, and Si elemental composition of phytoplankton, and thus on the biogeochemical cycling of these nutrients in the California upwelling region.