Boris Cisewski

Deep carbon export from a Southern Ocean iron-fertilized diatom bloom

Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current

Significance Silica-shelled diatoms dominate marine phytoplankton blooms and play a key role in ocean ecology and the global carbon cycle. We show how differences in ecological traits of dominant Southern Ocean diatom species, observed during the in situ European Iron Fertilization Experiment (EIFEX), can influence ocean carbon and silicon cycles. We argue that the ecology of thick-shelled diatom species, selected for by heavy copepod grazing, sequesters silicon relative to other nutrients in the deep Southern Ocean and underlying sediments to the detriment of diatom growth elsewhere. This evolutionary arms race provides a framework to link ecology with biogeochemistry of the ocean. Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.

Mixing in cyclonic eddies in the Antarctic Circumpolar Current

Observations over a period of 39 days of the increasing minimum core temperature of Winter Water trapped within a mature cyclonic eddy in the Antarctic Circumpolar Current encouraged us to assess the mixing processes which might lead to this temperature rise. Using diffusive heat budgets for alternatively diapycnic (∼vertical) and then isopycnic (∼horizontal) mixing combined with the observed rate of warming allowed upper limits for diapycnic and isopycnic diffusivities to be inferred. This gave values of (3.3 ± 0.8) × 10−4m2s−1 and 87 ± 20m2s−1 for the diapycnic and isopycnic diffusivities respectively. These were then in turn applied to the isopycnic temperature distributions of a juvenile eddy observed earlier in the cruise and integrated forward in time. While both forms of diffusion undoubtedly play a role in modifying the temperature, it was the horizontal diffusivity which was better able to reproduce the tighter θS relationship and the horizontal spread of the temperature minimum observed in the mature eddy