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

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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 420mm) increased in biomass as indicated by chl a, contributing to a surface standing stock of 2 mg m � 3 .I n Phase II, from days 10 to 18, the bloom was dominated by microphytoplankton (420mm), with a concomitant decrease in phytoplankton o20mm. Microphytoplankton, which initially accounted for 25% of the phytoplankton biomass and increased by a factor of 50, consisted primarily of the pennate diatom genera, Pseudo-nitzschia, Neodenticula and Thalassiothrix and the centric diatom genera, Chaetoceros, Rhizosolenia, and Proboscia. Particulate carbon-to-chl a (PC: chl a) ratios for large cells (X5mm) decreased 5-fold by day 18, indicative of enhanced cellular chl a content and increased phytoplankton contributions to PC. Pennate diatoms were most abundant in the patch, although when converted to biovolume, centric diatoms contributed larger amounts of algal carbon (C) to the bloom. A rapid decline in chl a on day 19 marked the onset of bloom decline. The magnitude, duration and composition of the phytoplankton response to the Fe enrichment clearly depicted a major shift in the structure of the algal assemblage and increased C export potential. r 2006 Elsevier Ltd. All rights reserved.