Primary productivity and nitrogenous nutrient assimilation dynamics during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study

Abstract

Primary productivity and uptake rates of nitrate and ammonium were measured during an iron enrichment-experiment in the western subarctic Pacific Ocean using stable 13C and 15N isotope methods with simulated, on-board, for 24-h incubations. Nitrate, phosphate and silicate before the iron enrichment in the surface mixed layer (0–10 m) were abundant at 17, 1.5 and 34 μM, respectively. Primary productivity in the upper 20 m ranged from 10 to 30 mg C m −3 d −1 on Day 0 (the day of iron fertilization) and did not change outside the iron patch during 13 days of the observation. Primary productivity in the iron patch increased more than 10-fold after Day 7 and the maximum of 310 mg C m −3 d −1 was observed on Day 9. The elevated primary productivity continued until Day 13 (end of the observation). The integrated primary productivity in the photic zone was about 2 g C m −2 d −1 after Day 7. During 13 days of the observation, 16.5 g C m −2 was fixed in the patch, while C fixation was 4.5 g C m −2 outside. Absolute nitrate uptake at 5 m depth was 0.2 μM d −1 on Day 0, almost same as the ammonium uptake rate. Nitrate uptake sharply increased 20 times after Day 7. The maximum rate of 4.2 μM d −1 was observed on Day 9. Total nitrate uptake at 5 m between Days 4 and 9 was 10.8 μM, which was comparable to the net decrease of nitrate at 8.5 μM at this depth. Uptake rate of ammonium was 0.2 μM d −1 on Day 0 and had not changed by Day 13. The f-ratio (a ratio of nitrate uptake to total nitrogen (nitrate and ammonium) uptake) was 0.4–0.6 before Day 4, but increased to 0.95 on Day 7 and remained high until Day 13. These results suggest that the nitrate assimilation pathway was induced by the iron enrichment to meet the high nitrogen demand from rapid growth of the dominant diatom Chaetoceros dedilis. Relative uptake rate among nutrients was estimated from their apparent concentration change at 5 m in the patch. Nitrate and phosphate decreased at a molar ratio of 15.5, close to Redfield’s stoichiometry. Silicate to nitrate uptake ratio, however, changed sharply on Day 9 from 1.6 to 3.0, indicating physiological stress on growing diatoms. Photosynthetically active radiation (PAR) at 5 m decreased from 400 to <100 μmol m −2 s −1 after Day 8 due to the high biomass of phytoplankton within the patch. Dissolved iron concentration also decreased to less than 0.3 nM after Day 9. This concentration was below the half saturation level for iron ( K s) obtained from on-board iron-addition incubations. These results suggest that low light and low iron concentration caused the physiological stress on diatoms after Day 8.