polykrikoides during blooms in eastern Long Island embayments and elsewhere . However, the potential for B-vitamins to be a limiting or co-limiting nutrient, while observed in other HAB events , has never been explored for this species. This research shows that just as vitamin B12 had the ability to limit the growth of A. anophagefferens during the peak and demise of brown tide blooms , this vitamin restricted the growth of C. polykrikoides in multiple locations across eastern Long Island. This finding is consistent with culture work that measured Ks values that fell within the range of ambient B12 concentrations and the observa-tion that the cell_1 B12 requirements of C. polykrikoides are amongst the highest measured for any harmful alga . 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 , 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 . The occurrence of a high biomass bloom of a vitamin auxotroph along with significant B12 uptake by picoplankton accounts for vitamin B12 turning over several times daily during blooms and limiting the growth of C. polykrikoides nearly as much as nitrogen, usually the limiting nutrient in these systems . C. polykrikoides takes up B12 faster than microplankton outside of the blooms, and its large cellular demand and the more than daily B12 turnover during blooms likely leads to the limitation frequently observed since incubations were 24–48 h. In addition, the ten-fold higher abundance of heterotrophic bacteria and five-fold higher picoeukaryotes likely enhances the planktonic demand for B12 during incubations since this group has been shown to be the main B-vitamin utilizers in all marine systems, surveyed to date . Hence, despite the ability of C. polykrikoides to rapidly and competitively take up B12 during blooms when isolated from some ambient vitamin sources during incubations, bloom communities were likely to have quickly exhausted the B12 supply. In the field, this species undergoes intense diel vertical migration , behavior that has been linked to benthic/deep water nutrient acquisition by other dinoflagellates . Intense microbial activity likely makes sediments a rich source of vitamin B12 and C. polykrikoides may partially fill some of its vitamin quota during migrations to the benthos. Similarly, the low B1 utilization may also be related to a fulfillment of B1 requirement near the sediment water interface.