Seasonal upwelling links iron recycling with eastern equatorial Pacific nitrate consumption and new primary production

Using iron (Fe) to increase phytoplankton nitrate utilization in nitrate-rich, iron-limited surface waters has been invoked as a potential feedback for global climate (Martin, 1990). Simple calculations show that the average nitrate utilization in the nitrate-rich, iron-limited eastern equatorial Pacific (EEP) can only be explained by the differential cycling of iron in the euphotic zone (Rafter et al. 2017). A time-series of nitrate isotope measurements to quantify the seasonal variability of EEP nitrate utilization and find that stronger upwelling leads to reduced nitrate utilization (and vice versa) (Rafter & Sigman 2016). Here, we argue that this large iron deficit and associated need for differential elemental cycling in the EEP leads to the temporal variability of nitrate utilization in this and other nitrate-rich, iron-limited systems via the residence time of waters in the euphotic zone. Specifically, given a reasonable rate of iron cycling, weaker upwelling and therefore a longer residence time in the euphotic zone leads to more iterations of iron cycling (iron recycling) and more nitrate utilization. This insight to nitrate utilization in iron-limited waters is key to understanding temporal variability of primary production by large phytoplankton (more likely to be exported to the deep sea) on a variety of timescales.