Regulation of primary productivity rate in the equatorial Pacific

Analysis of the Chl-specific rate of primary productivity (PB) as a function of subsurface nutrient concentration at >300 equatorial stations provides an answer to the question: What processes regulate primary productivity rate in the high-nutrient, low-chlorophyll waters of the equatorial Pacific? In the western Pacific where there is a gradient in 60-m [NO3] from 0 to ∼12 µM, the productivity rate is a linear function of nutrient concentration; in the eastern Pacific where the gradient is from 12 to 28 µM, the productivity rate is independent of nutrient concentration and limited to ∼36 mg C(mg d‒1, or a mean euphotic zone C-specific growth rate (µ) of 0.47 d‒1. However, rates downstream of the Galapagos Islands are not limited; they are 46.4 mg C(mg Chl)‒1 d‒1 and µ= 0.57 d‒1, very close to the predicted nutrient-regulated rates in the absence of other limitation. This pattern of rate regulation can be accounted for by a combination of eolian Fe, subsurface nutrients, and sedimentary Fe derived from the Galapagos platform. In the low-nutrient western Pacific the eolian supply of Fe is adequate to allow productivity rate to be set by subsurface nutrient concentration. In the nutrient-rich eastern equatorial region eolian Fe is inadequate to support productivity rates proportional to the higher nutrient concentrations, so in this region eolian Fe is rate limiting. Around the Galapagos Islands productivity rates reach levels consistent with nutrient concentrations; sedimentary Fe from the Galapagos platform seems adequate to support increased nutrient-regulated productivity rates in this region.