Evolution of phytoplankton pigments in an in-situ iron enrichment experiment in the subarctic NE Pacific

Phytoplankton pigments were analysed using high performance liquid chromatography (HPLC) analysis following an in-situ iron (Fe) enrichment in the subarctic NE Pacific. Profiles of pigment concentrations in the upper 100 m were measured both inside and outside the Fe-enriched patch over the first 14 days of the experiment. In the upper 50 m, all phytoplankton pigments measured, with the exception of zeaxanthin, were significantly higher inside the Fe-enriched patch than in surrounding waters. Chlorophyll a increased strongly in the Fe patch, with concentration in the upper 20 m reaching 1–1.3 mg m−3 between 7 and 14 days after enrichment, an order of magnitude higher than outside (0.1–0.15 mg m−3). Depth integrated chlorophyll a (mg m−2) in the upper 50 m inside the patch revealed two major peaks at around 7–9 and 12–14 days. During the first peak, 19′-hexanoyloxyfucoxanthin was the other major pigment, suggesting that prymnesiophytes were abundant at this time, and diagnostic pigment indices confirmed that nanoplankton dominated the phytoplankton community. Degradation pigments phaeophytin a1 and a2, and phaeophorbide a2 also peaked during this nanoplankton bloom. During the second peak in chlorophyll a (12–14 days), the other major pigment was fucoxanthin suggesting that diatoms were abundant, and diagnostic pigment indices confirmed that this later bloom was dominated by microplankton. Phaeophytin a1 and a2 and phaeophorbide a1 also strongly increased in the Fe patch during this second peak. The only phytoplankton pigment not significantly higher in the upper 50 m of the Fe-enriched patch was zeaxanthin, and diagnostic pigment indices confirmed that picoplankton were relatively insensitive to Fe enrichment. The potential for pigments as markers for other biogeochemical consequences of the Fe fertilisation is briefly discussed.