Influence of zinc and iron enrichments on phytoplankton growth in the northeastern subarctic Pacific
D. W. Crawford, M. S. Lipsen, D. A. Purdie, M. C. Lohan, P. J. Statham, F. A. Whitney, J. N. Putland, W. K. Johnson, N. Sutherland, T. D. Peterson, P. J. Harrison, C. S. Wong
Limnol. Oceanogr., 48(4), 2003, 1583–1600

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 (t₀) 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 around 100-fold and P_m^chl and ^chl by 5–10-fold compared to the control. The +Fe treatment showed a slightly higher ^chl and slightly lower P_m^chl than the +Zn+Fe treatment. Abundance of large diatoms, small diatoms, small flagellates, and coccol- chl m ithophores 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.