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

A L King,D A Hutchins,S A Sañudo-Wilhelmy,M J Ellwood,P W Boyd,C Breene,B S Twining,S W Wilhelm

doi:10.5194/bg-9-667-2012

Abstract

Abstract. 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.

Highlights

Iron (Fe) has a profound effect on phytoplankton growth in open ocean and coastal high nutrient, low chlorophyll (HNLC) regimes where the supply of Fe:mol C radioisotope uptake (Fe) is low (Martin and Fitzwater, 1988; Coale et al, 1996; Hutchins and Bruland, 1998), and Fe helps set global biological production and Published by Copernicus Publications on behalf of the European Geosciences Union.A
Both P and C normalizations were used because all three approaches measured Fe uptake or content, the bulk filtration technique (Fe determined by inductively-coupled plasma mass spectrometry (ICPMS)) included complementary direct measurements of both bulk P and C (P was measured from the >0.2 μm size fraction; POC samples were >0.7 μm), while the radioisotope uptake and SXRFbased techniques only had direct measurements of C uptake and cellular P content, respectively
The Fe:P-based comparison (Fig. 5a) consists of: (1) oxalate-washed total Fe:C radioisotope uptake converted to Fe:P uptake using a C:P ratio of 133:1, (2) oxalate-washed total ICPMS-determined biogenic Fe (BFe):P, and (3) synchrotron x-ray fluorescence (SXRF)-determined Fe:P

Summary

Introduction

Iron (Fe) has a profound effect on phytoplankton growth in open ocean and coastal high nutrient, low chlorophyll (HNLC) regimes where the supply of Fe is low (Martin and Fitzwater, 1988; Coale et al, 1996; Hutchins and Bruland, 1998), and Fe helps set global biological production and Published by Copernicus Publications on behalf of the European Geosciences Union.A. While there has been a considerable effort to characterize the spatial and temporal variability of dissolved Fe (DFe) concentrations and Fe limitation of phytoplankton in the world’s oceans, only recently have researchers begun to address in situ Fe budgets – relating environmental Fe pools to Fe requirements of biogeochemically-significant organisms (Boyd et al, 2005; Sarthou et al, 2008; Bowie et al, 2009; Tovar-Sanchez and Sanudo-Wilhelmy, 2011). Fe:C ratios or Fe quotas are important for constraining phytoplankton Fe requirements, predicting oceanic nutrient limitation scenarios (Moore et al, 2002; Parekh et al, 2005), and quantifying the enhanced downward C export due to Fe addition (de Baar et al, 2005; Boyd et al, 2007). Methods for determining biogenic Fe have included inductively-coupled plasma mass spectrometry (ICPMS) (Ho et al, 2003; Frew et al, 2006), and single-cell synchrotron x-ray fluorescence (SXRF) (Twining et al, 2004b)

Methods

Results

Conclusion