A comparison of biogenic iron quotas during a diatom spring bloom using multiple approaches

Biogenic Fe quotas were determined using three distinct techniques on samples collected concurrently in the subtropical Pacific Ocean east of New Zealand. Fe quotas were measured using radioisotope uptake experiments (24 h incubation), bulk filtration and analysis by inductively-coupled plasma mass spectrometer (ICPMS), and single-cell synchrotron x-ray fluorescence (SXRF) analysis over a sixteen-day period (year days 263 to 278 of 2008) during a quasi-Lagrangian drifter experiment that tracked the evolution of the annual spring diatom bloom within a counter-clockwise open-ocean eddy. Overall, radioisotope uptake-determined Fe quotas (washed with oxalate reagent to remove extracellular Fe) were the lowest (0.5–1.0 mmol Fe:mol P; 4–8 μmol Fe:mol C), followed by single-cell Fe quotas (2.3–7.5 mmol Fe:mol P; 17–57 μmol Fe:mol C), and the highest and most variable quotas were from the bulk filtration ICPMS approach that used the oxalate reagent wash, corrected for lithogenic Fe using Al (0.8–21 mmol Fe:mol P; 4–136 μmol Fe:mol C). During the evolution of the spring bloom within the eddy (year days 263 to 272), the surface mixed layer inventories of particulate biogenic elements (C, N, P, Si) and chlorophyll increased while Fe quotas estimated from all three approaches exhibited a general decline. After the onset of the bloom decline, the drogued buoys exited the eddy center (days 273 to 277). Fe quotas returned to pre-bloom values during this part of the study. Our standardized and coordinated sampling protocols reveal the general observed trend in Fe quotas: ICPMS > SXRF > radioisotope uptake. We discuss the inherent differences between the techniques and argue that each technique has its individual merits and uniquely contributes to the characterization of the oceanic particulate Fe pool.