Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

Abstract

Abstract Ship-board iron enrichment bottle experiments were carried out with samples collected at the mesoscale iron fertilization experimental site (SERIES) in the subarctic NE Pacific in the summer of 2002. Samples were collected on Day 14 of the experiment outside the patch that was in a typical high nitrate and low chlorophyll (HNLC) condition. The iron concentration in the incubation bottles ranged from 0.1 to 2.0 nM by adding FeCl3 solution. The increase in chlorophyll-a (chl-a) in the micro (>10 μm) and nano-sized (2–10 μm) fraction was observed as a function of the added iron. Chl-a in the pico-sized fraction (0.7–2 μm) showed no increase with time. Nitrate and silicate were exhausted in the Fe-amended bottles, while those in the control bottle remained at the end of incubation. The relative consumption ratio of silicate to nitrate for the control bottles was significantly higher than that for the Fe-amended bottles. As a hyperbolic relation was found between iron concentration and the rate of increase in Chl-a (specific growth rate) for the micro and nano-sized fraction, the Monod equation was fit to obtain a maximum growth rate (μmax) and a half-saturation constant for iron (KFe). The μmax values were 0.72 and 0.48 d−1 for the micro and nano-sized fraction, respectively. The KFe values were 0.10 and 0.08 nM for the micro and nano-sized fraction, respectively. The μmax agreed with the rate of increase in Chl-a observed in situ for the mesoscale iron fertilization experiment. The μmax value for micro-sized fraction at 12 °C was half of that in the western subarctic Pacific Ocean (SEEDS experiment in 2001), indicating the Chl-a increase rate (potential growth rate) after iron enrichment was much higher in SEEDS than that in SERIES. The KFe values were much lower than that in SEEDS, suggesting that the phytoplankton community in the NE subarctic Pacific Ocean acclimates to a lower ambient Fe concentration. This difference in KFe between SERIES (NE) and SEEDS (NW) may reflect the previously suggested gradient in Fe flux to the subarctic Pacific Ocean. A temperature gradient was also applied to investigate the effect of temperature on the growth response of the phytoplankton community. No obvious effect of temperature increase to 16 °C was found in SERIES, while μmax and KFe changed significantly with temperature in SEEDS.