While uptake of B12 by C. polykrikoides constituted the largest percentage of B12 uptake occurring in the >2 mm size fraction of any previous study (Koch et al., 2011, 2012, 2013), picoplankton still accounted for _35% of total vitamin B12 uptake, suggesting there was significant competition with this group for this vitamin. C. polykrikoides blooms occur when rates of primary production and vitamin utilization are already at their annual peak and while inorganic nitrogen concentrations are low