ABSTRACT: Phytoplankton are essential components of marine food webs and biogeochemical cycles; thus, elucidating the factors that mediate their growth and composition are essential for understanding the marine environment. Vitamins have been found to be a fundamental requirement for the growth of many marine phytoplankton. Here, 20 incubation experiments were conducted over the course of 1 yr in a coastal ecosystem to examine the addition of thiamin (B1) and its components 4-methyl-5-thiazoleethanol (HET) and 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) on net phytoplankton growth rates. While vitamin addition periodically resulted in significant changes in growth rates, no uniform compound-specific or temporal patterns in rate changes were observed. Additionally, no significant changes in rates were found to correlate with starting phytoplankton abundance. Together, these findings suggest that the composition of the starting phytoplankton community likely influenced the population level response to vitamin addition and highlights the complexity of understanding the ramifications of vitamin fluxes in the marine environment.