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

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Trull, TW, Davies, D, Casciotti, KL
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2008
• Subjects: 260401 Biological Oceanography; 260402 Chemical Oceanography; Iron fertilization; Southern Ocean; Nitrate isotopes; Kerguelen
• Online Access: https://eprints.utas.edu.au/6375/; https://eprints.utas.edu.au/6375/1/Trull_Davies_Casciotti_KEOPS_nutrient_cycling_published%5B1%5D.pdf; https://doi.org/10.1016/j.dsr2.2007.12.035

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.