Nitrogen isotopic fractionation during a simulated diatom spring bloom: Importance of N-starvation in controlling fractionation

N isotope fractionation (ε) was first determined during ambient NO₃^- depletion in a simulated diatom spring bloom. After 48 h of N-starvation, NH₄⁺ was resupplied to the diatoms in small pulses to simulate grazer-produced N and then ε was determined. Large variations in ε values were observed: from 2.0-3.6 to 14-0‰ during NO₃^- and NH₄⁺ uptake, respectively. This is the first study reporting an value as low as 0 to 2‰ for NH₄⁺ uptake and we suggest that greater N demand after N-starvation may have drastically reduced NH₃ efflux out of the cells. Thus the N status of the phytoplankton and not the ambient NH₄⁺ concentration may be the important factor controlling ε, because, when N-starvation increased, ε values for NH₄⁺ uptake decreased within 30 h. This study may thus have important implications for interpreting the δ¹⁵N of particulate N in nutrient-depleted regimes in temperate coastal oceans.