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.