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Abstract Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle processes modulate oceanic dissolved Fe (dFe) signatures (δ56Fediss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ56Fediss. Using the North Pacific as a natural laboratory, the response of dFe, δ56Fediss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ56Fedissdoes trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources.
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Probing the Bioavailability of Dissolved Iron to Marine Eukaryotic Phytoplankton Using In Situ Single Cell Iron Quotas
Abstract We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (kin‐app) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high‐affinity Fe uptake system, so data were screened, yielding 560 single cellkin‐appvalues from 31 low‐Fe stations. We normalizedkin‐appto cell surface area (S.A.) to account for cell‐size differences. The resulting bioavailability proxy (kin‐app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe‐siderophore complexes and often approaches that of highly available inorganic Fe′. Station averagedkin‐app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency ofkin‐app/S.A. among stations (ca. five‐fold variation), we computed a grand‐averaged dFe availability, which upon normalization to cell carbon (C) yieldskin‐app/C of 42,200 ± 11,000 L mol C−1 d−1. We utilizekin‐app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability ofkin‐app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe‐limited Equatorial Pacific.
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- PAR ID:
- 10365960
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 35
- Issue:
- 8
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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