Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Biogeochemical and diatom export fluxes are presented from two bathypelagic sediment trap deployments in the Antarctic Zone of the Southern Ocean. One of the sediment traps was deployed in very
productive, naturally iron-fertilized waters downstream of South Georgia (P3, 2000 m) and compared to
a deployment in moderately productive waters upstream of the island system (P2, 1500 m). At both sites
significant diatom export events occurred in spring (November) and contained mostly empty cells that
were associated with low particulate organic carbon (POC) fluxes. A summer export pulse occurred one
month later at P2 (end February/March) compared to P3 (end January). Diatom fluxes at P3 were one
order of magnitude higher than at P2, a difference mainly attributed to the short and intense export of
resting spores from Chaetoceros Hyalochaete and Thalassiosira antarctica species. Aside from these resting
spores, diatom export assemblages at both sites were dominated by empty Fragilariopsis kerguelensis
frustules. The fraction of diatoms exported as empty frustules was considerably lower at P3 (52%) than P2
(91%). This difference was related to the flux of intact diatom resting spores at P3 and may partially
explain the lower Si:C export stoichiometry observed at P3 (1.1) compared to P2 (1.5). Through the
enumeration of full diatom frustules and subsequent biomass calculations we estimate that diatom
resting spores account for 42% of annual POC flux in the productive waters downstream of South Georgia.
At both sites the contribution of diatom vegetative stages to POC fluxes was considerably lower (o5%).
From these analyses we conclude that resting spore export contributes towards the slightly higher
bathypelagic (POC) flux at P3 (40.6 mmol m2 y1
) compared to P2 (26.4 mmol m2 y1
). We compared
our sediment trap records with previously published diatom assemblage data from the mixed layer and
surface sediments (3760 m) around South Georgia. The relative proportion of diatom resting spores
within diatom assemblages increases as a function of depth and is explained by selective preservation of
their robust frustules. Our study highlights the significance of diatom resting spore export as a carbon
vector out of the mixed layer. Furthermore, the contribution or resting spores to POC flux in the bathypelagic ocean and sediments suggests they play a particularly important role in sequestering biologically fixed CO2 over climatically relevant timescales.