Phytoplankton response to natural and experimental iron addition (original) (raw)

1998, Deep Sea Research Part II: Topical Studies in Oceanography

Primary production, chlorophyll biomass, and the maximum quantum yield of photosynthesis were measured during a survey around the Galapagos Islands and during an open-ocean iron addition experiment. Dissolved iron concentrations exceeded 1 nmol kg\ over the Galapagos shelf, and a plume of chlorophyll was observed to the west of the islands. The quantum yield of photosynthesis was greater than 0.02 mol C mol photons\ at the surface in the base of the plume, and less than 0.01 mol C mol photons\ outside the plume. Primary production was two-fold higher at the base of the plume relative to surrounding waters. The distributions of quantum yield, primary production and chlorophyll are consistent with a direct effect of iron on phytoplankton growth rate. The experimental iron addition was performed 300 km south of the Galapagos and raised surface water dissolved iron concentrations from 0.05 to approximately 3 nmol kg\. Quantum yield increased from less than 0.008 to 0.015 mol C mol photons\ within 24 h of the addition, and remained elevated throughout the experiment. Chlorophyll concentrations were initially near 0.2 mg m\, and reached 0.8 mg m\ in the iron-enriched waters. Primary production increased from 2.0 to 3.1 mmol C m\ d\. These results show that iron directly regulates photosynthesis of the dominant phytoplankton in surface waters of the high-nutrient, low-chlorophyll eastern equatorial Pacific. It appears, however, that the dominant phytoplankton were unable to escape grazing control, limiting the impact of a transient increase in iron availability on surface water nutrient concentrations.