Biogeochemistry in marine plankton communities is strongly influenced by the activities of microbial species. Understanding the composition and dynamics of these assemblages is essential for modeling emergent community-level processes, yet few studies have examined all of the biological assemblages present in the plankton, and benchmark data of this sort from time-series studies are rare. Abundance and biomass of the entire microbial assemblage and mesozooplankton (>200µm) were determined vertically, monthly and seasonally over a 3-year period at a coastal time-series station in the San Pedro Basin off the southwestern coast of the USA. All compartments of the planktonic community were enumerated (viruses in the femtoplankton size range [0.02–0.2µm], bacteria + archaea and cyanobacteria in the picoplankton size range [0.2–2.0µm], phototrophic and heterotrophic protists in the nanoplanktonic [2–20µm] and microplanktonic [20–200µm] size ranges, and mesozooplankton [>200µm]. Carbon biomass of each category was estimated using standard conversion factors. Plankton abundances varied over seven orders of magnitude across all categories, and total carbon biomass averaged approximately 60µgCl−1 in surface waters of the 890m water column over the study period. Bacteria + archaea comprised the single largest component of biomass (>1/3 of the total), with the sum of phototrophic protistan biomass making up a similar proportion. Temporal variability at this subtropical station was not dramatic. Monthly depth-specific and depth-integrated biomass varied 2-fold at the station, while seasonal variances were generally <50%. This study provides benchmark information for investigating long-term environmental forcing on the composition and dynamics of the microbes that dominate food web structure and function at this coastal observatory.