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 ∼5 times more abundant on-plateau and exhibited greater POC/PON (∼6.5 vs. ∼5.5), δ13C-POC (∼−21.5 vs. ∼−24.5‰) and δ15N-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-μm filters in series), which revealed large isotopic variations with size (δ13C-POC ranged from −28 to −19‰ and δ15N-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 δ15N of dissolved nitrate indicates that the δ15N-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 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 δ15N-NO3 values yield indistinguishable isotopic fractionation factors on and off the plateau (15εNO315εNO3 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 δ15N-PON values during the last glacial maximum indicate greater fractional nitrate depletion in the Southern Ocean.