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

Abstract

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 0 ) and in the control remained at ~0.1–0.2 nmol L −1 . In the control, chlorophyll (=0.3 mg m −3 ), 14 C 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 14 C 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 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.

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