Most phytoplankton species have a requirement for one of the B-vitamins (B1 and B12). Prior studies have found low, picomolar, B-vitamin concentrations in the marine environment and have demonstrated that vitamin B12 can limit or co-limit phytoplankton growth in coastal and open ocean environments. The extent, to which vitamins influence plankton species succession, however has never been assessed. In addition no study has quantified vitamin uptake rates by marine plankton communities. My dissertation examined the role of vitamin B1 and B12 in the ecology of phytoplankton communities from the disparate ecosystems of the Gulf of Alaska and coastal waters of Long Island, NY. In the Gulf of Alaska, picoplankton (0.2-2 ??m) were responsible for the majority of vitamin B12 uptake in both coastal and high nutrient low chlorophyll (HNLC) regions and B12 concentrations and B12 uptake rates were higher in HNLC regions compared to coastal regions with higher iron (Fe) concentrations. During vitamin amendment experiments, B12 alone and in conjunction with other limiting nutrients (N or Fe) significantly enhanced algal biomass and stimulated the growth of multiple groups of larger (> 2 ??m) phytoplankton. Vitamin utilization in two contrasting NY estuaries was highest in the more eutrophic systems and positively correlated with primary production. Similar to the Gulf of Alaska, vitamin B1 and B12 uptake were dominated by the picoplankton with multiple lines of evidence suggesting that heterotrophic bacteria were the main utilizers. Carbon-specific uptake vitamin uptake rates showed a much higher uptake by picoplankton compared to microplankton and were higher in the more eutrophic system. Combined with prior studies, these findings suggest that picoplankton are both the primary producers and users of B-vitamins in coastal ecosystems and that rapid microbial cycling of B-vitamins may sometimes deprive larger phytoplankton of these micronutrients and thus influence phytoplankton species succession. During a brown tide, the harmful algae A. anophagefferens and the associated microbial community rapidly utilized vitamins B1 and B12, drawing down ambient concentrations from >100 pM to <7 pM over the course of a bloom. At the peak of the bloom the majority of B1 uptake occurred in the brown tide size class (1-5 ??m) while the <1 ??m and 1-5 ??m community utilized equal parts of the B12 pool. Culture work evidenced the ability of A. anophagefferens to adapt to lower vitamin concentrations although vitamin amendment experiments demonstrated that vitamin B12 can limit the growth of this organism during the peak and demise of brown tides. Similar to the brown tide, the presence of the red tide forming dinoflagellate, Cochlodinium polykrikoides, significantly increased nutrient utilization compared to non-bloom water, with ten-fold and five-fold higher uptake rates for nitrogen and vitamin B12, respectively, during blooms while B1 uptake was unchanged. In addition the heterotrophic bacterial community associated with blooms was more abundant and was comprised of unique species with the lower carbon-specific vitamin B12 uptake rates. In a manner consistent with brown tides, the enrichment of bloom water with vitamin B12, but not B1, significantly enhanced the growth of C. polykrikoides in 60% of experiments performed. In summary, this dissertation has revealed the strong effects of B-vitamins on planktonic species succession, the importance of picoplankton as vitamin consumers, and the role of vitamins in the occurrence of HABs while making the first measurements of vitamin uptake by pelagic plankton, These findings collectively demonstrate that, like more actively researched macronutrients (N and P) and micronutrients (Fe), B-vitamins can play a central role in ecology of the ocean plankton. As such, more research is required to clarify this role.