Abstract
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 (POCphyto) and estimate POC transfer efficiencies at 100 m below the euphotic zone (T_100). While small particles <100 mu M dominated the mixed layer, large particles >100 mu M comprised a significant fraction of POCphyto below the mixed layer in both areas, possibly due to a "shade flora" composed by large diatoms. POCphyto 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 Delta POCphyto was similar to 66 mg m(-2) versus similar to 54 mg m(-2) from large particles. In the Atlantic, Delta POC(phyt)o was similar to 852 mg m(-2) for small particles versus Delta POCphyto similar to 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.