We're studying the impacts of iron on ecosystem processes in the Central California region (Johnson et al., 1999; Johnson et al., 2001). Iron measurements are made every ~20 days at Stations C1, M1 and M2.
The annual cycle of iron at these stations is characterized by a large pulse of iron the occurs during the onset of spring upwelling (Johnson et al., 2001). It is produced by the initial rise of isopycnals that intersect the continental shelf into the euphotic zone. Once isopycnals tilt up, the benthic iron source is dampened and iron concentrations drop 100x during the summer months.
Rates of primary production, biomass (phytoplankton carbon), physiology (Chlorophyll to carbon ratios), and ecosystem structure (picoplankton C/all phytoplankton carbon ratio) are all proportional to dissolved iron concentrations during late June through October. These results are strong indications that iron does, indeed, control this ecosystem.
Iron concentrations are highest at C1, near the shore. The elevated iron concentrations allow the phytoplankton to consume nearly all of the nitrate and produce large chlorophyll concentrations. Slightly further offshore at M1 and M2, iron concentrations are lower and nitrate is not completely depleted. Iron deficiency limits the ecosystem in these offshore waters.
Johnson, K.S., F.P. Chavez, V.A. Elrod, S.E. Fitzwater, J.T. Pennington, K.R. Buck and P.M. Waltz (2001) The annual cycle of iron and the biological response in central California coastal waters. Geophysical Research Letters, 28, 1247-1250.