Evaluation of particle sinking velocities near South Georgia using four independent approaches

A critical driver of carbon transport and sequestration to the deep sea is the speed at which particles sink. Unfortunately, it is technically very challenging to measure in situ particle sinking velocities. Here, we synthesise and discuss the results of sinking velocity measurements obtained over >1 month using four independent techniques during the peak and decline of a Southern Ocean phytoplankton bloom. The four approaches used were: in situ imaging on Lagrangian platforms (P-cam), Marine Snow Catchers, optical backscatter sensors on gliders, and radioactive tracer pairs (²¹⁰Po-²¹⁰Pb and ²³⁴Th-²³⁸U). The latter methodology is based on the calculation of the radioactive disequilibrium and the use of inverse modelling.

These measurements were all taken together with an array of other carbon flux and ecosystem measurements as part of the first COMICS (Controls over Ocean Mesopelagic Carbon Storage) Cruise in the naturally iron-fertilized region North of South Georgia. We observed a highly seasonal ecosystem and rapid changes in the community structure, which we captured using repeated observations.

Our data represent a unique opportunity to unravel how sinking velocities relate to export rates and how they vary with community structure, depth, and stage of the bloom. Moreover, our comparison allows evaluation of the performances of these four state-of-the-art methodologies and provides an outlook on possible further improvements in the analysis of results.