Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part I—Biomass and assemblage

We report results from the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) experiment in waters of the NE subarctic Pacific in which a large scale iron (Fe) enrichment lead to a shift in the phytoplankton assemblage from pico- and nanophytoplankton to one dominated by large diatoms. The phytoplankton response to the added Fe was monitored for 26 days following two infusions into a 77 km 2 patch of seawater. During the course of the experiment, the resulting algal bloom was constrained within the upper 30 m and spread to a region measuring over 1000 km 2. Phytoplankton chlorophyll a (chl a) increased from 0.3 mg m −3 to a peak of 6.3 mg m −3 18 days after the initial addition of Fe. Water-column integrated chl a was enhanced 8-fold, reaching a maximum of 114 mg m −2 on day 17. The resulting bloom is described in two ecological phases based on dominant phytoplankton groups. In Phase I, which encompassed the initial infusion up to day 10, all size-fractions (0.2–2, 2–20 and >20 μm) increased in biomass as indicated by chl a, contributing to a surface standing stock of 2 mg m −3. In Phase II, from days 10 to 18, the bloom was dominated by microphytoplankton (>20 μm), with a concomitant decrease in phytoplankton