Seasonality modulates particulate organic carbon dynamics in mid-latitudes of South Pacific and South Atlantic Oceans.

Here we used data from six BGC-floats deployed in the southeast Pacific and southwest Atlantic Oceans, within the Southern Ocean's Subtropical Zone, to assess the seasonality of particulate organic carbon production from phytoplankton (POC phyto) and estimate POC transfer efficiencies at 100 m below the euphotic zone (T_100). While small particles <100 μM dominated the mixed layer, large particles >100 μM comprised a significant fraction of POC phyto below the mixed layer in both areas, possibly due to a "shade flora" composed by large diatoms. POC phyto was highly seasonal with highest biomass accumulation in the Atlantic side for both small and large particles. In the Pacific, the seasonal change in small particle production ΔPOC phyto was ∼66 mg m−2 versus ∼54 mg m−2 from large particles. In the Atlantic, ΔPOC phyto was ∼852 mg m−2 for small particles versus ΔPOC phyto ∼ 262 mg m−2 for large particles. Monthly T_100s in the Pacific ranged from 76% to 92% with maximum efficiencies during the deepening of the mixed layer depth. In the Atlantic, T_100s ranged from 43% to 76% with two periods of high T_100s: the first coinciding with the decline of large particles from the "shade flora", and the second coinciding with the deepening of the mixed layer during elevated small particle production. • Large phytoplankton particles accumulated below the mixed layer at mid-latitudes • The Atlantic Subtropical Zone produced more particulate organic carbon than the Pacific • Particle transfer efficiency in the Pacific was 85%, with a monthly amplitude of 76–92% • Particle transfer efficiency in the Atlantic was 60%, with a monthly amplitude of 43–76% [ABSTRACT FROM AUTHOR]