Iron availability, nitrate uptake, and exportable new production in the subarctic Pacific

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Abstract
Rates of nitrate uptake in bioassays from the open Gulf of Alaska by Martin et al. (1989) are reinterpreted. During the exponential phase of phytoplankton growth, iron addition affected the rate of nitrate uptake significantly at only one of the three stations (position of the former ocean weather station “Papa”); results from one station (northern) are not interpretable. Appreciable nitrate uptake and algal growth in all controls during the first few days appears to have consumed a few to several times as much dissolved iron as was found initially; this suggests high solubilization (regeneration) rates or uptake of iron moities other than iron measured as dissolved. Exponential increases of biomass at demonstrated reduced grazing made the iron demand in the controls outrun the supply after the first few days, in contrast to the iron treatments. It is suggested that in the field, grazing normally seems to prevent the phytoplankton from reaching concentrations that reduce the iron (and nitrate) to levels that depress division rates drastically. It is unclear whether the flourishing of medium‐sized diatoms in the assays reflects removal of iron limitation on division rates of these rare species or exclusion of grazers from the containers. Although nitrate uptake may be equated with new production, the export to depth of new organic material (and hence of CO2) is not predictable from assays in small (liter) containers, especially so because the role of the large grazers on the size composition of the phytoplankton and the production of large sinking particles is not evaluated.