Diatoms are important contributors to phytoplankton biomass in many temperate coastal systems where they exhibit substantial taxonomic and morphological diversity. While we have a general understanding of the factors that contribute to coastal blooms, little is known about regulation of community structure and bloom dynamics within diatom assemblages. To address this issue, we have begun to acquire taxonomically-resolved time series of phytoplankton abundance and biomass at the Marthas Vineyard Coastal Observatory, a cabled site on the New England inner shelf. Observations have been made near continuously for over 3 years with Imaging FlowCytobot, an automated submersible imaging-in-flow cytometer optimized for analysis of cells and chains with widths ~10-150 [mu]m. Images identified to genus or species level reveal striking seasonal and inter-annual variability in the timing, amplitude, and taxonomic structure of diatom blooms. Several genera (Guinardia, Leptocylindrus, Eucampia) repeatedly occur in bloom events where they contribute over half the diatom biomass, but their relative importance is strongly seasonal and varies from year to year. Other biomass peaks of comparable magnitude and frequency are characterized by greater taxonomic diversity. Wintertime diatom bloom events are common, as noted in historical records from this region; however, our results also show that particular taxa (Eucampia, Leptocylindrus) can occur in summertime blooms that rival the winter ones. Preliminary analysis of interannual variability in the amplitude of blooms suggests that patterns are taxon-specific, with the most extreme blooms often occurring in different years for different groups. A longer record is necessary to explore this in more detail.