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

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 off-plateau. Suspended particulate organic carbon (POC) was similar to 5 times more abundant on-plateau and exhibited greater POC/PON (similar to 6.5 vs. similar to 5.5), delta C-13-POC (similar to-21.5 vs. similar to-24.5%.) and delta N-15-PON (similar to +2 vs. similar to 0 parts per thousand) 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-mu m filters in series), which revealed large isotopic variations with size (delta C-13-POC ranged from -28 to -19 parts per thousand and delta N-15-PON from -3 to + 5 parts per thousand) and greater abundances of C-13- and N-15-enriched large phytoplankton over the plateau. The C-13 enrichment in POC reflected faster growth rates and greater draw-down of dissolved inorganic carbon over the plateau. Quantitative comparison to the delta N-15 of dissolved nitrate indicates that the delta N-15-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 off-plateau. Results from a sparse set of free-drifting sediment trap samples suggest control of export by zooplankton grazing. The N-15 and O-18 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 delta N-15-NO3 values yield indistinguishable isotopic fractionation factors on and off the plateau ((15)epsilon(NO3) of 4-5 parts per thousand). This suggests that variations in iron availability may not bias the interpretation of paleo-environmental N-15 records, and leaves intact the view that higher sedimentary delta N-15-PON values during the last glacial maximum indicate greater fractional nitrate depletion in the Southern Ocean. (C) 2008 Elsevier Ltd. All rights reserved.