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 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 13Cand 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 (15 NO3 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.