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Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon and oxygen isotopes

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Trull, TW and Davies, D and Casciotti, KL (2008) Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon and oxygen isotopes. Deep-Sea Research Part II, 55 (5-7). pp. 820-840. ISSN 0967-0645

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Abstract

The KErguelen Ocean and Plateau compared Study (KEOPS) documented enhanced iron input and phytoplankton biomass over the deep Kerguelen plateau in comparison to surrounding high-nutrient low-chlorophyll (HNLC) waters in late summer 2005. We examined the influence of this iron on nitrogen and carbon metabolism by the microbial food-web, by comparing samples from on-plateau and offplateau. Suspended particulate organic carbon (POC) was 5 times more abundant on-plateau and exhibited greater POC/PON (6.5 vs. 5.5), d13C-POC (21.5 vs. 24.5%) and d15N-PON (+2 vs. 0%) than off-plateau. These differences arose in part from changes in ecosystem structure as demonstrated by size-fractionation (1, 5, 20, 55, 210, and 335-mm filters in series), which revealed large isotopic variations with size (d13C-POC ranged from 28 to 19% and d15N-PON from 3 to +5%) and greater abundances of 13C and 15N-enriched large phytoplankton over the plateau. The 13C enrichment in POC reflected faster growth rates and greater draw-down of dissolved inorganic carbon over the plateau. Quantitative comparison to the d15N of dissolved nitrate indicates that the d15N-PON enrichment derived from increased assimilation of nitrate, corresponding to new production f-ratios of 0.7–0.9 on-plateau vs. 0.4–0.6 offplateau. Results from a sparse set of free-drifting sediment trap samples suggest control of export by zooplankton grazing. The 15N and 18O enrichments in dissolved nitrate exhibited a 1:1 correlation, indicating that phytoplankton assimilation controls nitrate availability and only a relatively small amount of nitrate was regenerated by nitrification. The d15N-NO3 values yield indistinguishable isotopic fractionation factors on and off the plateau (15NO3 of 4–5%). This suggests that variations in iron availability may not bias the interpretation of paleo-environmental 15N records, and leaves intact the view that higher sedimentary d15N-PON values during the last glacial maximum indicate greater fractional nitrate depletion in the Southern Ocean.

Item Type: Article
Keywords: Iron fertilization; Southern Ocean; Nitrate isotopes
Journal or Publication Title: Deep-Sea Research Part II
Page Range: pp. 820-840
ISSN: 0967-0645
Identification Number - DOI: 10.1016/j.dsr2.2007.12.035
Additional Information: The definitive version is available at http://www.sciencedirect.com
Date Deposited: 29 May 2008 00:02
Last Modified: 18 Nov 2014 03:40
URI: http://eprints.utas.edu.au/id/eprint/6375
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