Influence of zinc and iron enrichments on phytoplankton growth in the northeastern subarctic Pacific

Near-surface seawater from the northeastern subarctic Pacific was incubated on deck for 8 d, supplemented with (1) control, no additions (2) +Zn (3) +Fe (4) +Zn+Fe. Concentrations of total Zn and Fe at time zero (t_o) and in the control remained at ∼0.1-0.2 nmol L^-1. In the control, chlorophyll (<0.3 mg m^-3), ¹⁴C uptake into POC and PIC, and inorganic nutrients all remained relatively constant. Addition of Zn slightly but significantly increased chlorophyll (p < 0.05), decreased phosphate (p < 0.01) and nitrate (p < 0.05), and in P versus E experiments, increased P_m > 10-fold and P_m^chl 2-3-fold. The abundance of small diatoms and coccolithophores was higher in the +Zn treatment compared to the control. The +Fe and +Zn+Fe treatments, compared to the control, both showed >10-fold increases in chlorophyll and ¹⁴C uptake into POC and PIC and complete removal of nitrate (≤0.2 mmol m^-3). However, differences were observed in size-fractionated data; the +Zn+Fe treatment had significantly lower percent chlorophyll in the >20-μm fraction (p < 0.01) and a higher percentage in the 0.2-5-μm fraction (p < 0.01) than the +Fe treatment. In P versus E experiments, both +Fe treatments increased P_m and α around 100-fold and P_m^chl and α^chl by 5-10-fold compared to the control. The +Fe treatment showed a slightly higher α^chl and slightly lower P_m^chl than the +Zn+Fe treatment. Abundance of large diatoms, small diatoms, small flagellates, and coccolithophores all increased substantially (∼7-1,000-fold) in response to Fe addition, whereas dinoflagellate abundance only doubled. The +Zn+Fe treatment had higher abundances of small diatoms and small flagellates than the +Fe treatment. We conclude that Zn additions had limited influence on conventional indices of phytoplankton growth compared to Fe, but that there might be subtle influences of Zn that require further attention.