Influence of iron and temperature on growth, nutrient utilization ratios and phytoplankton species composition in the western subarctic Pacific Ocean during the SEEDS experiment

On-board bottle incubation experiments were carried out by adding different amounts of iron as part of the SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) experiment to elucidate the phytoplankton community response to various iron concentrations. A temperature gradient from 5 to 18 °C was also applied to the incubation experiment because temperature also affects growth rate and metabolic functions such as enzyme reactions. Chlorophyll-a increased with increasing iron concentration, while the increase in chlorophyll-a in the control bottles (no iron addition) was nominal. Size-fractionated chlorophyll-a indicated that micro-phytoplankton (>10 μm) became dominant in the iron-enriched bottles below 13 °C, which was consistent with the results from in situ observations in the simultaneous mesoscale iron-enrichment experiment and similar experiments for the other high nitrate and low chlorophyll (HNLC) waters. Macronutrients (silicate, nitrate and phosphate) were consumed in the bottles below 13 °C. The consumption ratio of silicate to nitrate was around 1:1 for iron-enriched bottles, but this ratio was slightly higher for control bottles, indicating a physiological stress due to low iron concentration on the diatoms. In contrast, nanophytoplankton (<10 μm) were dominant in the iron-enriched bottles at 18 °C. There was no silicate utilization, but nitrate and phosphate were consumed in these bottles. Bacillariophyceae were dominant in the bottles below 13 °C, but Prymnesiophyceae became dominant at 18 °C. This is the first report demonstrating that temperature influences phytoplankton composition after iron addition in HNLC waters. lorophyll-a specific increase rate for micro-plankton (>10 μm) doubled from 5 to 8 °C and was the highest between 8 and 13 °C at the same iron concentration. The surface mixed layer temperature was 8 °C at the beginning of iron fertilization, but it had been 5 °C just 2 weeks before the experiment. This dramatic increase in growth rate with temperature is hypothesized to be the reason for SEEDS 2001 biomass changes being the highest reported response to a mesoscale Fe-enrichment experiment in HNLC waters.