Abstract. Natural iron fertilization downstream of Southern Ocean island plateaus
supports large phytoplankton blooms and promotes carbon export from the mixed
layer. In addition to sequestering atmospheric CO2, the biological
carbon pump also supplies organic matter (OM) to deep-ocean ecosystems.
Although the total flux of OM arriving at the seafloor sets the energy input
to the system, the chemical nature of OM is also of significance. However, a
quantitative framework linking ecological flux vectors to OM composition is
currently lacking. In the present study we report the lipid composition of
export fluxes collected by five moored sediment traps deployed in contrasting
productivity regimes of Southern Ocean island systems (Kerguelen, Crozet and
South Georgia) and compile them with quantitative data on diatom and faecal
pellet fluxes. At the three naturally iron-fertilized sites, the relative
contribution of labile lipids (mono- and polyunsaturated fatty acids,
unsaturated fatty alcohols) is 2–4 times higher than at low productivity
sites. There is a strong attenuation of labile components as a function of
depth, irrespective of productivity. The three island systems also display
regional characteristics in lipid export. An enrichment of zooplankton
dietary sterols, such as C27Δ5, at South Georgia is consistent
with high zooplankton and krill biomass in the region and the importance of
faecal pellets to particulate
organic carbon (POC) flux. There is a strong association of diatom resting
spore fluxes that dominate productive flux regimes with energy-rich
unsaturated fatty acids. At the Kerguelen Plateau we provide a statistical
framework to link seasonal variation in ecological flux vectors and lipid
composition over a complete annual cycle. Our analyses demonstrate that
ecological processes in the upper ocean, e.g. resting spore formation and
grazing, not only impact the magnitude and stoichiometry of the Southern
Ocean biological pump, but also regulate the composition of exported OM and
the nature of pelagic–benthic coupling.