Seasonal variations of sinking velocities in Austral diatom blooms: Lessons learned from COMICS

The sinking velocity (SV) of organic particles is a critical driver of carbon transport to the deep sea. Accurate determination of marine particle SV and their influencing factors is therefore a key to better understanding of biological carbon storage in the ocean. We used two different approaches to estimate average SVs of particles during a Southern Ocean spring bloom (North of South Georgia): optical backscatter sensors on gliders (“large”, >50 μm diameter), and radioactive pairs (²³⁴Th–²³⁸U and ²¹⁰Po-²¹⁰Pb). Our results were complemented with time-of flight estimations of bulk SVs from deep sediment traps deployed at 1950 m.

Bulk SVs increased consistently with depth from 15 ± 1 m d⁻¹ at 10 m to 50 ± 10 m d⁻¹ at the depth of export (Z_p = 95 m) and from 96 ± 35 m d⁻¹ at 150 m to 119 ± m d⁻¹ at 450 m. Only the fastest particles, mainly comprised by faecal pellets (FPs) and diatom aggregates, survived remineralization and dominated carbon fluxes at deep depth.

The SV variability at the base of the Euphotic Zone was studied in relation to the stage of the bloom by analysing three different moments of the spring diatom bloom in the region during the years 2012, 2013 and 2017. The export efficiency (ExpEff), defined as the ratio POC flux exported below the Euphotic Zone to the satellite derived surface NPP, was also evaluated. It was found from the temporal series that ExpEff and SV vary throughout the diatom bloom as the community structure progresses. A good correlation between both variables was observed (ExpEff = (0.023 ± 0.006) SV, r = 0.82, p = 0.04). Showing that the variability in how efficiently the carbon flux is exported out of the Euphotic Zone can be explained by the SV at which the particles sink. Further investigations are required to analyse if this is a specific model of the functioning of the BCP during the diatom bloom in North South Georgia or if it can be extrapolated to other scenarios.