This study will examine a novel concept: phytoplankton iron limitation in coastal upwelling regimes. Recent work by Dr. K. Bruland shows that in parts of the central California upwelling region, a narrow continental shelf and low freshwater inputs can result in euphotic zone dissolved Fe levels of <0.1 nM. These concentrations approach the very low levels found in the oceanic High Nutrient Low Chlorophyll (HNLC) regimes. Preliminary biological data demonstrate that as in open ocean HNLC areas, Fe additions along the Big Sur coast of California increase phytoplankton growth rates, promote the dominance of large diatoms and allow nitrate depletion. However, Fe availability has much less effect on silicic acid drawdown. Silicic acid to nitrate (Si:N) drawdown in no-Fe control bottles occurs at molar ratios of >2, but occurs at the normal nutrient-replete diatom ratio of about one in +Fe treatments. These results suggest that Fe-stressed diatoms will deplete Si from the water before N, and Fe limitation thus appears to be a major driving force behind the `silicate pump`. Drs. Ken Bruland and D. Hutchins will return to the California area in 1999 to carry out joint chemical/biological investigations of Fe limitation and it's effects in Si:N utilization ratios. Dr. Bruland will measure Fe and nutrient concentrations throughout the region (accompanying study). Dr. Hutchins' group will use coordinated shipboard incubation experiments to look at the biological and biogeochemical effects of Fe addition. The Peru upwelling has all of the characteristics that are associated with Fe-limited areas off California- a very narrow shelf, low riverine inputs, frequent HNLC conditions and very high Si:N drawdown ratios. However, the potential for Fe limitation in the Peru coastal upwelling system has never been tested using modern clean techniques. Our Peru upwelling cruise in 2000 will emphasize broad regional coverage of the effects of Fe availability on phytoplankton growth and Si:N drawdown ratios. The ultimate impact of this project may be to force a re-evaluation of our entire picture of nutrient biogeochemistry in both the California and Peru coastal upwelling regimes.
