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, T, Davies, DM, Casciotti, K
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon 2008
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Southern Ocean; Kerguelen
• Online Access: http://www.sciencedirect.com; https://doi.org/10.1016/j.dsr2.2007.12.035; http://ecite.utas.edu.au/54191

The KErguelen Ocean and Plateau compared Study (KEOPS) documented enhanced iron input and phytoplankton biomass over thedeep Kerguelen plateau in comparison to surrounding high-nutrient low-chlorophyll (HNLC) waters in late summer 2005. We examinedthe 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 ( 6.5vs. 5.5), d13C-POC ( 21.5 vs. 24.5%) and d15N-PON ( +2 vs. 0%) than off-plateau. These differences arose in part fromchanges in ecosystem structure as demonstrated by size-fractionation (1, 5, 20, 55, 210, and 335-mm filters in series), which revealed largeisotopic variations with size (d13C-POC ranged from 28 to 19% and d15N-PON from 3 to +5%) and greater abundances of 13Cand15N-enriched large phytoplankton over the plateau. The 13C enrichment in POC reflected faster growth rates and greater draw-downof dissolved inorganic carbon over the plateau. Quantitative comparison to the d15N of dissolved nitrate indicates that the d15N-PONenrichment derived from increased assimilation of nitrate, corresponding to new production f-ratios of 0.70.9 on-plateau vs. 0.40.6 offplateau.Results from a sparse set of free-drifting sediment trap samples suggest control of export by zooplankton grazing. The 15N and18O enrichments in dissolved nitrate exhibited a 1:1 correlation, indicating that phytoplankton assimilation controls nitrate availabilityand only a relatively small amount of nitrate was regenerated by nitrification. The d15N-NO3 values yield indistinguishable isotopicfractionation factors on and off the plateau (15 NO3 of 45%). This suggests that variations in iron availability may not bias theinterpretation of paleo-environmental 15N records, and leaves intact the view that higher sedimentary d15N-PON values during the lastglacial maximum indicate greater fractional nitrate depletion in the Southern Ocean.