Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon and oxygen isotopes
Trull, T and Davies, DM and Casciotti, K, Insights into nutrient assimilation and export in naturally iron-fertilized waters of the Southern Ocean from nitrogen, carbon and oxygen isotopes, Deep Sea Research Part II: Topical Studies in Oceanography, 55, (5-7) pp. 820-840. ISSN 0967-0645 (2008) [Refereed Article]
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.
Iron fertilization; Southern Ocean; Nitrate isotopes