Stable nitrogen isotope dynamics of a mesoscale iron enrichment experiment in the NE Subarctic Pacific

We report the response in the natural abundance of the stable isotopes of nitrogen (δ15N) during the Fe-enrichment experiment SERIES (Subarctic Ecosystem Response to Iron Enrichment Study) in the NE Pacific. Samples were collected for isotope analysis of nitrate, particulate material (including size fractionated samples), and particles trapped in the water column from beneath the Fe-enriched patch. In all sample types, δ15N changed in response to increased phytoplankton productivity after the Fe enrichment. The nitrate concentration and δ15N of nitrate were inversely related, the result of the opposing effects of isotope fractionation during nitrate assimilation and the addition of new nitrate by periodic mixing of water from outside the Fe patch. During the growth period a decrease in the difference of the δ15N of particulate nitrogen and nitrate occurred that was attributed to physical mixing, shifts in growth from regenerated nitrogen sources to nitrate, and the change in the community assemblage from<5-μm phytoplankton cells to a larger assemblage dominated by diatoms. The surface-tethered sediment trap δ15N samples indicate that the nitrate isotope fractionation signal in surface waters was not transported below the permanent mixed layer until the end of the phytoplankton growth period, and therefore only the highest values associated with the isotope fractionation process were recorded in the sinking material from the patch. An important conclusion from this study is that mesoscale physical mixing effects and nitrogen remineralization can reduce the expression of isotope fractionation during phytoplankton growth, explaining why the high fractionation values measured in laboratory studies are not commonly observed in the natural environment.