Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5, 300 km South Pacific Zonal Section (153°E–150°W). Issue 2 (6th February 2018)

• Record Type: Journal Article
• Authors: Ellwood, Michael J.; Bowie, Andrew R.; Baker, Alex; Gault‐Ringold, Melanie; Hassler, Christel; Law, Cliff S.; Maher, William A.; Marriner, Andrew; Nodder, Scott; Sander, Sylvia; Stevens, Craig; Townsend, Ashley; van der Merwe, Pier; Woodward, E. Malcolm S.; Wuttig, Kathrin; Boyd, Philip W.
• Is Part Of: Global biogeochemical cycles. Volume 32:Issue 2(2018:Feb.)
• Page Start: 187
• Page End: 207
• Publication Date: 2018-02-06
• Subjects: iron -- nitrate -- phosphate -- SW Pacific Ocean -- phytoplankton; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2017GB005736
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Iron, phosphate, and nitrate are essential nutrients for phytoplankton growth, and hence, their supply into the surface ocean controls oceanic primary production. Here we present a GEOTRACES zonal section (GP13; 30–33°S, 153°E–150°W) extending eastward from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L −1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L −1 (170°W–150°W). The supply of dissolved iron into the upper ocean (<100 m) from the atmosphere and vertical diffusivity averaged 11 ± 10 nmol m −2 d −1 . In the remote South Pacific Ocean (170°W–150°W), atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface water nitrate concentrations averaged 5 ± 4 nmol L −1 between 170°W and 150°W, while surface water phosphate concentrations averaged 58 ± 30 nmol L −1 . The supply of nitrogen into the upper ocean is primarily from deeper waters (24–1647 μmol m −2 d −1 ) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux along the eastern part of the transect. The deep water N:P ratio averaged 14.5 ± 0.5 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production. Plain Language Summary: Iron, phosphate, and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls phytoplankton growth. We present a 5, 300 km zonal section extending from Australia into the South Pacific Ocean relating to iron, nitrate, and phosphate. We find that the supply of these nutrients into the surface ocean defines that biogeographical distribution of phytoplankton groups. Key Points: Surface dissolved iron concentrations are elevated near continental Australia and decreased eastward in the South Pacific gyre Model calculations suggest that a significant portion of dissolved iron within the remote surface ocean is sourced from the atmosphere The supply stoichiometry for iron and nitrogen relative to phosphate declines eastward leading to two biogeographical provinces